Polyphenism predicts actuarial senescence and lifespan in tiger salamanders

Actuarial senescence (called 'senescence' hereafter) often shows broad variation at the intraspecific level. Phenotypic plasticity likely plays a central role in among-individual heterogeneity in senescence rate (i.e. the rate of increase in mortality with age), although our knowledge on this subject is still very fragmentary. Polyphenism-the unique sub-type of phenotypic plasticity where several discrete phenotypes are produced by the same genotype-may provide excellent study systems to investigate if and how plasticity affects the rate of senescence in nature. In this study, we investigated whether facultative paedomorphosis influences the rate of senescence in a salamander, Ambystoma mavortium nebulosum. Facultative paedomorphosis, a unique form of polyphenism found in dozens of urodele species worldwide, leads to the production of two discrete, environmentally induced phenotypes: metamorphic and paedomorphic individuals. We leveraged an extensive set of capture-recapture data (8948 individuals, 24 years of monitoring) that were analysed using multistate capture-recapture models and Bayesian age-dependent survival models. Multistate models revealed that paedomorphosis was the most common developmental pathway used by salamanders in our study system. Bayesian age-dependent survival models then showed that paedomorphs have accelerated senescence in both sexes and shorter adult lifespan (in females only) compared to metamorphs. In paedomorphs, senescence rate and adult lifespan also varied among ponds and individuals. Females with good body condition and high lifetime reproductive success had slower senescence and longer lifespan. Late-breeding females also lived longer but showed a senescence rate similar to that of early-breeding females. Moreover, males with good condition had longer lifespan than males with poor body condition, although they had similar senescence rates. In addition, late-breeding males lived longer but, unexpectedly, had higher senescence than early-breeding males. Overall, our work provides one of the few empirical cases suggesting that environmentally cued polyphenism could affect the senescence of a vertebrate in nature, thus providing insights on the ecological and evolutionary consequences of developmental plasticity on ageing.

Assessing the evolutionary lability of insulin signalling in the regulation of nutritional plasticity across traits and species of horned dung beetles

Nutrition-dependent growth of sexual traits is a major contributor to phenotypic diversity, and a large body of research documents insulin signalling as a major regulator of nutritional plasticity. However, findings across studies raise the possibility that the role of individual components within the insulin signalling pathway diverges in function among traits and taxa. Here, we use RNAi-mediated transcript depletion in the gazelle dung beetle to investigate the functions of forkhead box O (Foxo) and two paralogs of the insulin receptor (InR1 and InR2) in shaping nutritional plasticity in polyphenic male head horns, exaggerated fore legs, and weakly nutrition-responsive genitalia. Our functional genetic manipulations led to three main findings: FoxoRNAi reduced the length of exaggerated head horns in large males, while neither InR1 nor InR2 knock-downs resulted in measurable horn phenotypes. These results are similar to those documented previously for another dung beetle (Onthophagus taurus), but in stark contrast to findings in rhinoceros beetles. Secondly, knockdown of Foxo, InR1, and InR2 led to an increase in the intercept or slope of the scaling relationship of genitalia size. These findings are in contrast even to results documented previously for O. taurus. Lastly, while FoxoRNAi reduces male forelegs in D. gazella and O. taurus, the effects of InR1 and InR2 knockdowns diverged across dung beetle species. Our results add to the growing body of literature indicating that despite insulin signalling's conserved role as a regulator of nutritional plasticity, the functions of its components may diversify among traits and species, potentially fuelling the evolution of scaling relationships.

Body color plasticity of Diaphorina citri reflects a response to environmental stress

Body color polyphenism is common in Diaphorina citri. Previous studies compared physiological characteristics in D. citri, but the ecological and biological significance of its body color polyphenism remains poorly understood. We studied the ecological and molecular effects of stressors related to body color in D. citri. Crowding or low temperature induced a high proportion of gray morphs, which had smaller bodies, lower body weight, and greater susceptibility to the insecticide dinotefuran. We performed transcriptomic and metabolomics analysiis of 2 color morphs in D. citri. Gene expression dynamics revealed that the differentially expressed genes were predominantly involved in energy metabolism, including fatty acid metabolism, amino acid metabolism, and carbohydrate metabolism. Among these genes, plexin, glycosidase, phospholipase, take out, trypsin, and triacylglycerol lipase were differentially expressed in 2 color morphs, and 6 hsps (3 hsp70, hsp83, hsp90, hsp68) were upregulated in gray morphs. The metabolome data showed that blue morphs exhibited a higher abundance of fatty acid and amino acid, whereas the content of carbohydrates was elevated in gray morphs. This study partly explains the body color polyphenism of D. citri and provides insights into the molecular changes of stress response of D. citri.

The role of phenotypic plasticity in shaping ecological networks

Plasticity-mediated changes in interaction dynamics and structure may scale up and affect the ecological network in which the plastic species are embedded. Despite their potential relevance for understanding the effects of plasticity on ecological communities, these effects have seldom been analysed. We argue here that, by boosting the magnitude of intra-individual phenotypic variation, plasticity may have three possible direct effects on the interactions that the plastic species maintains with other species in the community: may expand the interaction niche, may cause a shift from one interaction niche to another or may even cause the colonization of a new niche. The combined action of these three factors can scale to the community level and eventually expresses itself as a modification in the topology and functionality of the entire ecological network. We propose that this causal pathway can be more widespread than previously thought and may explain how interaction niches evolve quickly in response to rapid changes in environmental conditions. The implication of this idea is not solely eco-evolutionary but may also help to understand how ecological interactions rewire and evolve in response to global change.

Aphid male wing polymorphisms are transient and have evolved repeatedly

Polymorphic phenotypes have long been used to examine the maintenance of genetic variation within and between species. Most studies have focused on persistent polymorphisms, which are retained across species boundaries, and their positive effects on speciation rates. Far less is known about the macroevolutionary impacts of more transient polymorphisms, which are also common. Here we investigated male wing polymorphisms in aphids. We estimated the phylogenetic history of wing states across species, along with several other traits that could affect wing evolution. We found that male wing polymorphisms are transient: they are found in only ~4% of extant species, but have likely evolved repeatedly across the phylogeny. We reason that the repeated evolution of transient polymorphisms might be facilitated by the existence of the asexual female wing plasticity, which is common across aphids, and would maintain the wing development program even in species with wingless males. We also discovered that male wingedness correlates positively with host plant alternation and host plant breadth, and that winged morphs and wing polymorphisms may be associated with higher speciation rates. Our results provide new evolutionary insights into this well-studied group and suggest that even transient polymorphisms may impact species diversification rates.

One genome, multiple phenotypes: decoding the evolution and mechanisms of environmentally induced developmental plasticity in insects

Plasticity in developmental processes gives rise to remarkable environmentally induced phenotypes. Some of the most striking and well-studied examples of developmental plasticity are seen in insects. For example, beetle horn size responds to nutritional state, butterfly eyespots are enlarged in response to temperature and humidity, and environmental cues also give rise to the queen and worker castes of eusocial insects. These phenotypes arise from essentially identical genomes in response to an environmental cue during development. Developmental plasticity is taxonomically widespread, affects individual fitness, and may act as a rapid-response mechanism allowing individuals to adapt to changing environments. Despite the importance and prevalence of developmental plasticity, there remains scant mechanistic understanding of how it works or evolves. In this review, we use key examples to discuss what is known about developmental plasticity in insects and identify fundamental gaps in the current knowledge. We highlight the importance of working towards a fully integrated understanding of developmental plasticity in a diverse range of species. Furthermore, we advocate for the use of comparative studies in an evo-devo framework to address how developmental plasticity works and how it evolves.

Transcriptomic analysis of mosaic brain differentiation underlying complex division of labor in a social insect

Concerted developmental programming may constrain changes in component structures of the brain, thus limiting the ability of selection to form an adaptive mosaic of size-variable brain compartments independent of total brain size or body size. Measuring patterns of gene expression underpinning brain scaling in conjunction with anatomical brain atlases can aid in identifying influences of concerted and/or mosaic evolution. Species exhibiting exceptional size and behavioral polyphenisms provide excellent systems to test predictions of brain evolution models by quantifying brain gene expression. We examined patterns of brain gene expression in a remarkably polymorphic and behaviorally complex social insect, the leafcutter ant Atta cephalotes. The majority of significant differential gene expression observed among three morphologically, behaviorally, and neuroanatomically differentiated worker size groups was attributable to body size. However, we also found evidence of differential brain gene expression unexplained by worker morphological variation and transcriptomic analysis identified patterns not linearly correlated with worker size but sometimes mirroring neuropil scaling. Additionally, we identified enriched gene ontology terms associated with nucleic acid regulation, metabolism, neurotransmission, and sensory perception, further supporting a relationship between brain gene expression, brain mosaicism, and worker labor role. These findings demonstrate that differential brain gene expression among polymorphic workers underpins behavioral and neuroanatomical differentiation associated with complex agrarian division of labor in A. cephalotes.

Detection and pH-Thermal Characterization of Proteinases Exclusive of Honeybee Worker-Fate Larvae (Apis mellifera L.)

The occurrence of the honeybee caste polyphenism arises when a change in diet is transduced into cellular metabolic responses, resulting in a developmental shift mediated by gene expression. The aim of this investigation was to detect and describe the expression profile of water-soluble proteases during the ontogenesis of honeybee worker-fate larvae. The extraction of insect homogenates was followed by the electrophoretic separation of the protein extract in polyacrylamide gels under semi-denaturing condition, precast with gelatin, pollen, or royal jelly protein extracts. The worker-fate honeybee larva showed a proteolytic pattern that varied with aging, and a protease with the highest activity at 72 h after hatching was named PS4. PS4 has a molecular weight of 45 kDa, it remained active until cell sealing, and its enzymatic properties suggest a serine-proteinase nature. To define the process that originates a queen-fate larvae, royal jelly and pollen were analysed, but PS4 was not detected in either of them. The effect of food on the PS4 was investigated by mixing crude extracts of queen and worker-fate larvae with pollen and royal jelly, respectively. Only royal jelly inhibited PS4 in worker-fate larvae. Taken together, our data suggest that PS4 could be involved in caste differentiation.

The phenotypic plasticity of an evolving digital organism

Climate change will fundamentally reshape life on Earth in the coming decades. Therefore, understanding the extent to which species will cope with rising temperatures is of paramount importance. Phenotypic plasticity is the ability of an organism to change the morphological and functional traits encoded by its genome in response to the environment. I show here that plasticity pervades not only natural but also artificial systems that mimic the developmental process of biological organisms, such as self-replicating and evolving computer programs-digital organisms. Specifically, the environment can modify the sequence of instructions executed from a digital organism's genome (i.e. its transcriptome), which results in changes in its phenotype (i.e. the ability of the digital organism to perform Boolean logic operations). This genetic-based pathway for plasticity comes at a fitness cost to an organism's viability and generation time: the longer the transcriptome (higher fitness cost), the more chances for the environment to modify the genetic execution flow control, and the higher the likelihood for the genome to encode novel phenotypes. By studying to what extent a digital organism's phenotype is influenced by both its genome and the environment, I make a parallelism between natural and artificial evolving systems on how natural selection might slide trait regulation anywhere along a continuum from total environmental control to total genomic control, which harbours lessons not only for designing evolvable artificial systems, but also for synthetic biology.

Fluctuating Asymmetry in the Polymorphic Sand Cricket (Gryllus firmus): Are More Functionally Important Structures Always More Symmetric?

Simple Summary

Asymmetry in bilateral structures occurs when animals experience perturbations during development. This fluctuating asymmetry (FA) may serve as a reliable indicator of the functional importance of a structure. For example, locomotor structures often display lower levels of FA than other paired structures, highlighting that selection can maintain symmetry in traits important for survival or reproduction. Species that have multiple distinct morphs with unique behaviors and morphologies represent an attractive model for studying the relationship between symmetry and function. The sand field cricket (Gryllus firmus) has two separate morphs that allow us to directly test whether individuals maintain higher levels of symmetry in the structures most vital for maximizing fitness based on their specific life strategy. Longwing (LW) individuals can fly but postpone reproduction until after a dispersal event, whereas shortwing (SW) individuals cannot fly but begin reproducing in early adulthood. We quantified FA across a suite of key morphological structures indicative of investment in growth, reproduction, and flight capability for males and females across the morphs. Although we did not find significant differences in FA across traits, as predicted, locomotor compensation strategies may reduce selective pressures on symmetry or developmental patterns may limit the optimization between trait form and function.

Abstract

Fluctuating asymmetry (FA) may serve as a reliable indicator of the functional importance of structures within an organism. Primary locomotor structures often display lower levels of FA than other paired structures, highlighting that selection can maintain symmetry in fitness-enhancing traits. Polyphenic species represent an attractive model for studying the fine-scale relationship between trait form and function, because multiple morphs exhibit unique life history adaptations that rely on different traits to maximize fitness. Here, we investigated whether individuals of the wing polyphenic sand field cricket (Gryllus firmus) maintain higher levels of symmetry in the bilateral structures most vital for maximizing fitness based on their specific life history strategy. We quantified FA and directional asymmetry (DA) across a suite of key morphological structures indicative of investment in somatic growth, reproduction, and flight capability for males and females across the flight-capable longwing (LW) and flight-incapable shortwing (SW) morphs. Although we did not find significant differences in FA across traits, hindwings lacked DA that was found in all other structures. We predicted that functionally important traits should maintain a higher level of symmetry; however, locomotor compensation strategies may reduce the selective pressures on symmetry or developmental constraints may limit the optimization between trait form and function.

Spatially and Temporally Distributed Complexity-A Refreshed Framework for the Study of GRN Evolution

Irrespective of the heuristic value of interpretations of developmental processes in terms of gene regulatory networks (GRNs), larger-angle views often suffer from: (i) an inadequate understanding of the relationship between genotype and phenotype; (ii) a predominantly zoocentric vision; and (iii) overconfidence in a putatively hierarchical organization of animal body plans. Here, we constructively criticize these assumptions. First, developmental biology is pervaded by adultocentrism, but development is not necessarily egg to adult. Second, during development, many unicells undergo transcriptomic profile transitions that are comparable to those recorded in pluricellular organisms; thus, their study should not be neglected from the GRN perspective. Third, the putatively hierarchical nature of the animal body is mirrored in the GRN logic, but in relating genotype to phenotype, independent assessments of the dynamics of the regulatory machinery and the animal’s architecture are required, better served by a combinatorial than by a hierarchical approach. The trade-offs between spatial and temporal aspects of regulation, as well as their evolutionary consequences, are also discussed. Multicellularity may derive from a unicell’s sequential phenotypes turned into different but coexisting, spatially arranged cell types. In turn, polyphenism may have been a crucial mechanism involved in the origin of complex life cycles.

Photoperiod controls wing polyphenism in a water strider independently of insulin receptor signalling

Insect wing polyphenism has evolved as an adaptation to changing environments and a growing body of research suggests that the nutrient-sensing insulin receptor signalling pathway is a hot spot for the evolution of polyphenisms, as it provides a direct link between growth and available nutrients in the environment. However, little is known about the potential role of insulin receptor signalling in polyphenisms which are controlled by seasonal variation in photoperiod. Here, we demonstrate that wing length polyphenism in the water strider Gerris buenoi is determined by photoperiod and nymphal density, but is not directly affected by nutrient availability. Exposure to a long-day photoperiod is highly inducive of the short-winged morph whereas high nymphal densities moderately promote the development of long wings. Using RNA interference we demonstrate that, unlike in several other species where wing polyphenism is controlled by nutrition, there is no detectable role of insulin receptor signalling in wing morph induction. Our results indicate that the multitude of possible cues that trigger wing polyphenism can be mediated through different genetic pathways and that there are multiple genetic origins to wing polyphenism in insects.

Comparative Transcriptome Analysis Reveals bmo-miR-6497-3p Regulate Circadian Clock Genes during the Embryonic Diapause Induction Process in Bivoltine Silkworm

Diapause is one of the survival strategies of insects for confronting adverse environmental conditions. Bombyx mori displays typical embryonic diapause, and offspring diapause depends on the incubation environment of the maternal embryo in the bivoltine strains of the silkworm. However, the molecular mechanisms of the diapause induction process are still poorly understood. In this study, we compared the differentially expressed miRNAs (DEmiRs) in bivoltine silkworm embryos incubated at diapause- (25 °C) and non-diapause (15 °C)-inducing temperatures during the blastokinesis (BK) and head pigmentation (HP) phases using transcriptome sequencing. There were 411 known miRNAs and 71 novel miRNAs identified during the two phases. Among those miRNAs, there were 108 and 74 DEmiRs in the BK and HP groups, respectively. By the Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis of the predicted target genes of the DEmiRs, we found that aside from metabolism, the targets were also enriched in phototransduction-fly and insect hormone biosynthesis in the BK group and the HP group, respectively. Dual luciferase reporter assay illustrated that bmo-miR-6497-3p directly regulated Bmcycle and subsequently regulated the expression of circadian genes. These results imply that microRNAs, as vitally important regulators, respond to different temperatures and participate in the diapause induction process across species.

The role of epigenetics, particularly DNA methylation, in the evolution of caste in insect societies

Eusocial insects can be defined as those that live in colonies and have distinct queens and workers. For most species, queens and workers arise from a common genome, and so caste-specific developmental trajectories must arise from epigenetic processes. In this review, we examine the epigenetic mechanisms that may be involved in the regulation of caste dimorphism. Early work on honeybees suggested that DNA methylation plays a causal role in the divergent development of queen and worker castes. This view has now been challenged by studies that did not find consistent associations between methylation and caste in honeybees and other species. Evidence for the involvement of methylation in modulating behaviour of adult workers is also inconsistent. Thus, the functional significance of DNA methylation in social insects remains equivocal. This article is part of the theme issue 'How does epigenetics influence the course of evolution?'

Proteomic Polyphenism in Color Morphotypes of Diaphorina citri, Insect Vector of Citrus Greening Disease

Diaphorina citri is a vector of "Candidatus Liberibacter asiaticus" (CLas), associated with citrus greening disease. D. citri exhibit at least two color morphotypes, blue and non-blue, the latter including gray and yellow morphs. Blue morphs have a greater capacity for long-distance flight and transmit CLas less efficiently as compared to non-blue morphs. Differences in physiology and immunity between color morphs of the insect vector may influence disease epidemiology and biological control strategies. We evaluated the effect of CLas infection on color morph and sex-specific proteomic profiles of D. citri. Immunity-associated proteins were more abundant in blue morphs as compared to non-blue morphs but were upregulated at a higher magnitude in response to CLas infection in non-blue insects. To test for differences in color morph immunity, we measured two phenotypes: (1) survival of D. citri when challenged with the entomopathogenic fungus Beauveria bassiana and (2) microbial load of the surface and internal microbial communities. Non-blue color morphs showed higher mortality at four doses of B. bassinana, but no differences in microbial load were observed. Thus, color morph polyphenism is associated with two distinct proteomic immunity phenotypes in D. citri that may impact transmission of CLas and resistance to B. bassiana under some conditions.

The European Map Butterfly Araschnia levana as a Model to Study the Molecular Basis and Evolutionary Ecology of Seasonal Polyphenism

The European map butterfly Araschnia levana is a well-known example of seasonal polyphenism. Spring and summer imagoes exhibit distinct morphological phenotypes. Key environmental factors responsible for the expression of different morphs are day length and temperature. Larval exposure to light for more than 16 h per day entails direct development and results in the adult f. prorsa summer phenotype. Less than 15.5 h per day increasingly promotes diapause and the adult f. levana spring phenotype. The phenotype depends on the timing of the release of 20-hydroxyecdysone in pupae. Release within the first days after pupation potentially inhibits the default "levana-gene-expression-profile" because pre-pupae destined for diapause or subitaneous development have unique transcriptomic programs. Moreover, multiple microRNAs and their targets are differentially regulated during the larval and pupal stages, and candidates for diapause maintenance, duration, and phenotype determination have been identified. However, the complete pathway from photoreception to timekeeping and diapause or subitaneous development remains unclear. Beside the wing polyphenism, the hormonal and epigenetic modifications of the two phenotypes also include differences in biomechanical design and immunocompetence. Here, we discuss research on the physiological and molecular basis of polyphenism in A. levana, including hormonal control, epigenetic regulation, and the effect of ecological parameters on developmental fate.

Novel brain gene-expression patterns are associated with a novel predaceous behaviour in tadpoles

Novel behaviours can spur evolutionary change and sometimes even precede morphological innovation, but the evolutionary and developmental contexts for their origins can be elusive. One proposed mechanism to generate behavioural innovation is a shift in the developmental timing of gene-expression patterns underlying an ancestral behaviour, or molecular heterochrony. Alternatively, novel suites of gene expression, which could provide new contexts for signalling pathways with conserved behavioural functions, could promote novel behavioural variation. To determine the relative contributions of these alternatives to behavioural innovation, I used a species of spadefoot toad, Spea bombifrons. Based on environmental cues, Spea larvae develop as either of two morphs: 'omnivores' that, like their ancestors, feed on detritus, or 'carnivores' that are predaceous and cannibalistic. Because all anuran larvae undergo a natural transition to obligate carnivory during metamorphosis, it has been proposed that the novel, predaceous behaviour in Spea larvae represents the accelerated activation of gene networks influencing post-metamorphic behaviours. Based on comparisons of brain transcriptional profiles, my results reject widespread heterochrony as a mechanism promoting the expression of predaceous larval behaviour. They instead suggest that the evolution of this trait relied on novel patterns of gene expression that include components of pathways with conserved behavioural functions.

Exposure to glucocorticoids alters life history strategies in a facultatively paedomorphic salamander

Polyphenisms, where two or more alternative, environmentally-cued phenotypes are produced from the same genotype, arise through variability in the developmental rate and timing of phenotypic traits. Many of these developmental processes are controlled or influenced by endogenous hormones, such as glucocorticoids, which are known to regulate a wide array of vertebrate ontogenetic transitions. Using the mole salamander, Ambystoma talpoideum, as a model, we investigated the role of glucocorticoids in regulating facultative paedomorphosis, an ontogenetic polyphenism where individuals may delay metamorphosis into terrestrial adults. Instead, individuals reproduce as aquatic paedomorphic adults. Paedomorphosis often occurs when aquatic conditions remain favorable, while metamorphosis typically occurs in response to deteriorating or "stressful" aquatic conditions. Since glucocorticoids are central to the vertebrate stress response and are known to play a central role in regulating obligate metamorphosis in amphibians, we hypothesized that they are key regulators of paedomorphic life history strategies. To test this hypothesis, we compared development of larvae in outdoor mesocosms exposed to Low, Medium, and High exogenous doses of corticosterone (CORT). Results revealed that body size and the proportion of paedomorphs were both inversely proportional to exogenous CORT doses and whole-body CORT content. Consistent with known effects of CORT on obligate metamorphosis in amphibians, our results link glucocorticoids to ontogenetic transitions in facultatively paedomorphic salamanders. We discuss our results in the context of theoretical models and the suite of environmental cues known to influence facultative paedomorphosis.

Extreme developmental instability associated with wing plasticity in pea aphids

A key focus of evolutionary developmental biology is on how phenotypic diversity is generated. In particular, both plasticity and developmental instability contribute to phenotypic variation among genetically identical individuals, but the interactions between the two phenomena and their general fitness impacts are unclear. We discovered a striking example of asymmetry in pea aphids: the presence of wings on one side and the complete or partial absence of wings on the opposite side. We used this asymmetric phenotype to study the connection between plasticity, developmental instability and fitness. We found that this asymmetric wing development (i) occurred equally on both sides and thus is a developmental instability; (ii) is present in some genetically unique lines but not others, and thus has a genetic basis; and (iii) has intermediate levels of fecundity, and thus does not necessarily have negative fitness consequences. We conclude that this dramatic asymmetry may arise from incomplete switching between developmental targets, linking plasticity and developmental instability. We suspect that what we have observed may be a more widespread phenomenon, occurring across species that routinely produce distinct, alternative phenotypes.

Epigenetic Molecular Mechanisms in Insects

Insects are the largest animal group on Earth both in biomass and diversity. Their outstanding success has inspired genetics and developmental research, allowing the discovery of dynamic process explaining extreme phenotypic plasticity and canalization. Epigenetic molecular mechanisms (EMMs) are vital for several housekeeping functions in multicellular organisms, regulating developmental, ontogenetic trajectories and environmental adaptations. In Insecta, EMMs are involved in the development of extreme phenotypic divergences such as polyphenisms and eusocial castes. Here, we review the history of this research field and how the main EMMs found in insects help to understand their biological processes and diversity. EMMs in insects confer them rapid response capacity allowing insect either to change with plastic divergence or to keep constant when facing different stressors or stimuli. EMMs function both at intra as well as transgenerational scales, playing important roles in insect ecology and evolution. We discuss on how EMMs pervasive influences in Insecta require not only the control of gene expression but also the dynamic interplay of EMMs with further regulatory levels, including genetic, physiological, behavioral, and environmental among others, as was earlier proposed by the Probabilistic Epigenesis model and Developmental System Theory.

First draft genome assembly of the desert locust, Schistocerca gregaria

Background: At the time of publication, the most devastating desert locust crisis in decades is affecting East Africa, the Arabian Peninsula and South-West Asia. The situation is extremely alarming in East Africa, where Kenya, Ethiopia and Somalia face an unprecedented threat to food security and livelihoods. Most of the time, however, locusts do not occur in swarms, but live as relatively harmless solitary insects. The phenotypically distinct solitarious and gregarious locust phases differ markedly in many aspects of behaviour, physiology and morphology, making them an excellent model to study how environmental factors shape behaviour and development. A better understanding of the extreme phenotypic plasticity in desert locusts will offer new, more environmentally sustainable ways of fighting devastating swarms. Methods: High molecular weight DNA derived from two adult males was used for Mate Pair and Paired End Illumina sequencing and PacBio sequencing. A reliable reference genome of Schistocerca gregaria was assembled using the ABySS pipeline, scaffolding was improved using LINKS. Results: In total, 1,316 Gb Illumina reads and 112 Gb PacBio reads were produced and assembled. The resulting draft genome consists of 8,817,834,205 bp organised in 955,015 scaffolds with an N50 of 157,705 bp, making the desert locust genome the largest insect genome sequenced and assembled to date. In total, 18,815 protein-encoding genes are predicted in the desert locust genome, of which 13,646 (72.53%) obtained at least one functional assignment based on similarity to known proteins. Conclusions: The desert locust genome data will contribute greatly to studies of phenotypic plasticity, physiology, neurobiology, molecular ecology, evolutionary genetics and comparative genomics, and will promote the desert locust's use as a model system. The data will also facilitate the development of novel, more sustainable strategies for preventing or combating swarms of these infamous insects.

First draft genome assembly of the desert locust, Schistocerca gregaria

Background: At the time of publication, the most devastating desert locust crisis in decades is affecting East Africa, the Arabian Peninsula and South-West Asia. The situation is extremely alarming in East Africa, where Kenya, Ethiopia and Somalia face an unprecedented threat to food security and livelihoods. Most of the time, however, locusts do not occur in swarms, but live as relatively harmless solitary insects. The phenotypically distinct solitarious and gregarious locust phases differ markedly in many aspects of behaviour, physiology and morphology, making them an excellent model to study how environmental factors shape behaviour and development. A better understanding of the extreme phenotypic plasticity in desert locusts will offer new, more environmentally sustainable ways of fighting devastating swarms. Methods: High molecular weight DNA derived from two adult males was used for Mate Pair and Paired End Illumina sequencing and PacBio sequencing. A reliable reference genome of Schistocerca gregaria was assembled using the ABySS pipeline, scaffolding was improved using LINKS. Results: In total, 1,316 Gb Illumina reads and 112 Gb PacBio reads were produced and assembled. The resulting draft genome consists of 8,817,834,205 bp organised in 955,015 scaffolds with an N50 of 157,705 bp, making the desert locust genome the largest insect genome sequenced and assembled to date. In total, 18,815 protein-encoding genes are predicted in the desert locust genome, of which 13,646 (72.53%) obtained at least one functional assignment based on similarity to known proteins. Conclusions: The desert locust genome data will contribute greatly to studies of phenotypic plasticity, physiology, neurobiology, molecular ecology, evolutionary genetics and comparative genomics, and will promote the desert locust's use as a model system. The data will also facilitate the development of novel, more sustainable strategies for preventing or combating swarms of these infamous insects.

James Petiver's 1717 Papilionum Britanniae: an analysis of the first comprehensive account of British butterflies (Lepidoptera: Papilionoidea)

Although the contributions of James Petiver to the early development of systematic natural history are widely acknowledged, he is often criticized for scientific, curatorial and even social shortcomings. This rather dubious reputation is at odds with his standing among entomologists as 'the father of British butterflies'. Shortly before his death in 1718, Petiver published a densely packed eight-page pamphlet entitled Papilionum Britanniae. Analysis of this work, which at first sight makes an apparently exaggerated claim of accounting for 'above eighty English butterflies', reveals that Petiver was an original, perceptive and truly systematic entomologist, in several important respects ahead of his time.

Trait evolution is reversible, repeatable, and decoupled in the soldier caste of turtle ants

The scope of adaptive phenotypic change within a lineage is shaped by how functional traits evolve. Castes are defining functional traits of adaptive phenotypic change in complex insect societies, and caste evolution is expected to be phylogenetically conserved and developmentally constrained at broad phylogenetic scales. Yet how castes evolve at the species level has remained largely unaddressed. Turtle ant soldiers (genus Cephalotes), an iconic example of caste specialization, defend nest entrances by using their elaborately armored heads as living barricades. Across species, soldier morphotype determines entrance specialization and defensive strategy, while head size sets the specific size of defended entrances. Our species-level comparative analyses of morphotype and head size evolution reveal that these key ecomorphological traits are extensively reversible, repeatable, and decoupled within soldiers and between soldier and queen castes. Repeated evolutionary gains and losses of the four morphotypes were reconstructed consistently across multiple analyses. In addition, morphotype did not predict mean head size across the three most common morphotypes, and head size distributions overlapped broadly across all morphotypes. Concordantly, multiple model-fitting approaches suggested that soldier head size evolution is best explained by a process of divergent pulses of change. Finally, while soldier and queen head size were broadly coupled across species, the level of head size disparity between castes was decoupled from both queen head size and soldier morphotype. These findings demonstrate that caste evolution can be highly dynamic at the species level, reshaping our understanding of adaptive morphological change in complex social lineages.

Expression profiling of winged- and wingless-destined pea aphid embryos implicates insulin/insulin growth factor signaling in morph differences

Developmental plasticity allows the matching of adult phenotypes to different environments. Although considerable effort has gone into understanding the evolution and ecology of plasticity, less is known about its developmental genetic basis. We focused on the pea aphid wing polyphenism, in which high- or low-density environments cause viviparous aphid mothers to produce winged or wingless offspring, respectively. Maternally provided ecdysone signals to embryos to be winged or wingless, but it is unknown how embryos respond to that signal. We used transcriptional profiling to investigate the gene expression state of winged-destined (WD) and wingless-destined (WLD) embryos at two developmental stages. We found that embryos differed in a small number of genes, and that gene sets were enriched for the insulin-signaling portion of the FoxO pathway. To look for a global signature of insulin signaling, we examined the size and stage of WD and WLD embryos but found no differences. These data suggest the hypothesis that FoxO signaling is important for morph development in a tissue-specific manner. We posit that maternally supplied ecdysone affects embryonic FoxO signaling, which ultimately plays a role in alternative morph development. Our study is one of an increasing number that implicate insulin signaling in the generation of alternative environmentally induced morphologies.

A Laterally Transferred Viral Gene Modifies Aphid Wing Plasticity

Organisms often respond to changing environments by altering development of particular traits. These plastic traits exhibit genetic variation; i.e., genotypes respond differently to the same environmental cues. Theoretical studies have demonstrated the importance of this variation, which is targeted by natural selection, in adapting plastic responses to maximize fitness [1, 2]. However, little is known about the underlying genetic mechanisms. We identify two laterally transferred genes that contribute to variation in a classic example of phenotypic plasticity: the pea aphid's ability to produce winged offspring in response to crowding. We discovered that aphid genotypes vary extensively for this trait and that aphid genes of viral origin are upregulated in response to crowding solely in highly inducible genotypes. We knocked down expression of these genes to demonstrate their functional role in wing plasticity. Through phylogenetic analysis, we found that these genes likely originated from a virus that infects rosy apple aphids and causes their hosts to produce winged offspring [3]. The function of these genes has therefore been retained following transfer to pea aphids. Our results uncover a novel role for co-opted viral genes, demonstrating that they are used to modulate ecologically relevant, plastic phenotypes. Our findings also address a critical question about the evolution of environmentally sensitive traits: whether the genes that control the expression of plastic traits also underlie variation in plasticity. The genes we identify originated from outside aphids themselves, and thus, our work shows that genes formerly unrelated to plasticity can fine-tune the strength of plastic responses to the environment.

Morph-specific artificial selection reveals a constraint on the evolution of polyphenisms

Theory predicts that the evolution of polyphenic variation is facilitated where morphs are genetically uncoupled and free to evolve towards their phenotypic optima. However, the assumption that developmentally plastic morphs can evolve independently has not been tested directly. Using morph-specific artificial selection, we investigated correlated evolution between the sexes and male morphs of the bulb mite Rhizoglyphus echinopus Large 'fighter' males have a thick and sharply terminating pair of legs used to kill rival males, while small 'scrambler' males have unmodified legs, and search for unguarded females, avoiding fights. We selected on the relative leg width of only the fighter male morph, tracked the evolutionary responses in fighters and the correlated evolutionary responses in scramblers and females that were untouched by direct selection. Fighters diverged in relative leg thickness after six generations; assaying scramblers and females at the ninth generation we observed correlated responses in relative leg width in both. Our results represent strong evidence for the evolution of intraspecific phenotypic diversity despite correlated evolution between morphs and sexes, challenging the idea that male morphs are genetically uncoupled and free to independently respond to selection. We therefore question the perceived necessity for genetic independence in traits with extreme phenotypic plasticity.

Polyphenism - A Window Into Gene-Environment Interactions and Phenotypic Plasticity

Phenotypic plasticity describes the capacity of a single genotype to exhibit a variety of phenotypes as well as the mechanisms that translate environmental variation into reproducible phenotypic modifications. Polyphenism describes the unique sub-type of phenotypic plasticity where the outputs are not continuous, but rather discrete and multi-stable, resulting in several distinct phenotypes on the same genetic background. Epigenetic regulation underpins the stable phenotypic divergences that exemplify polyphenism and their evolutionary origin. Here, we briefly summarize the apparent ubiquity and diversity of polyphenisms across the animal kingdom. We briefly review the best characterized models across taxa and highlight the consistent themes both in their epidemiology and what little we know about molecular mechanisms. Finally, we highlight work that supports the possibility that humans may have a subtle polyphenism at the level of metabolism.

Epidermal Remodeling in Caenorhabditis elegans Dauers Requires the Nidogen Domain Protein DEX-1

Phenotypic plasticity is a critical component of an organism's ability to thrive in a changing environment. The free-living nematode Caenorhabditis elegans adapts to unfavorable environmental conditions by pausing reproductive development and entering a stress-resistant larval stage known as dauer. The transition into dauer is marked by vast morphological changes, including remodeling of epidermis, neurons, and muscle. Although many of these dauer-specific traits have been described, the molecular basis of dauer-specific remodeling is still poorly understood. Here we show that the nidogen domain-containing protein DEX-1 facilitates stage-specific tissue remodeling during dauer morphogenesis. DEX-1 was previously shown to regulate sensory dendrite formation during embryogenesis. We find that DEX-1 is also required for proper remodeling of the stem cell-like epidermal seam cells. dex-1 mutant dauers lack distinct lateral cuticular alae during dauer and have increased sensitivity to sodium dodecyl sulfate. Furthermore, we find that DEX-1 is required for proper dauer mobility. We show that DEX-1 is secreted from the seam cells during dauer, but acts locally in a cell-autonomous manner. We find that dex-1 expression during dauer is regulated through DAF-16/FOXO-mediated transcriptional activation. Finally, we show that dex-1 acts with a family of zona pellucida domain-encoding genes to regulate dauer-specific epidermal remodeling. Taken together, our data indicate that DEX-1 is an extracellular matrix component that plays a central role in C. elegans epidermal remodeling during dauer.

Juvenile hormone as a physiological regulator mediating phenotypic plasticity in pancrustaceans

Phenotypic plasticity and polyphenism, in which phenotypes can be changed depending on environmental conditions, are common in insects. Several studies focusing on physiological, developmental, and molecular processes underlying the plastic responses have revealed that similar endocrine mechanisms using juvenile hormone (JH) are used to coordinate the flexible developmental processes. This review discusses accumulated knowledge on the caste polyphenism in social insects (especially termites), the wing and the reproductive polyphenisms in aphids, and the nutritional polyphenism and sexual dimorphism in stag beetles. For the comparison with non-insect arthropods, extensive studies on the inducible defense (and reproductive polyphenism) in daphnids (crustacean) are also addressed. In all the cases, JH (and methyl farnesoate in daphnids) plays a central role in mediating environmental stimuli with morphogenetic processes. Since the synthetic pathways for juvenoids, i.e., the mevalonate pathway and downstream pathways to sesquiterpenoids, are conserved across pancrustacean lineages (crustaceans and hexapods including insects), the evolution of developmental regulation by juvenoids that control molting (ecdysis) and metamorphosis is suggested to have occurred in the ancestral arthropods. The discontinuous postembryonic development (i.e., molting) and the regulatory physiological factors (juvenoids) would have enabled plastic developmental systems observed in many arthropod lineages.

Extensive Differential Splicing Underlies Phenotypically Plastic Aphid Morphs

Phenotypic plasticity results in a diversity of phenotypes from a single genotype in response to environmental cues. To understand the molecular basis of phenotypic plasticity, studies have focused on differential gene expression levels between environmentally determined phenotypes. The extent of alternative splicing differences among environmentally determined phenotypes has largely been understudied. Here, we study alternative splicing differences among plastically produced morphs of the pea aphid using RNA-sequence data. Pea aphids express two separate polyphenisms (plasticity with discrete phenotypes): a wing polyphenism consisting of winged and wingless females and a reproduction polyphenism consisting of asexual and sexual females. We find that pea aphids alternatively splice 34% of their genes, a high percentage for invertebrates. We also find that there is extensive use of differential spliced events between genetically identical, polyphenic females. These differentially spliced events are enriched for exon skipping and mutually exclusive exon events that maintain the open reading frame, suggesting that polyphenic morphs use alternative splicing to produce phenotype-biased proteins. Many genes that are differentially spliced between polyphenic morphs have putative functions associated with their respective phenotypes. We find that the majority of differentially spliced genes is not differentially expressed genes. Our results provide a rich candidate gene list for future functional studies that would not have been previously considered based solely on gene expression studies, such as ensconsin in the reproductive polyphenism, and CAKI in the wing polyphenism. Overall, this study suggests an important role for alternative splicing in the expression of environmentally determined phenotypes.

Immunological larval polyphenism in the map butterfly Araschnia levana reveals the photoperiodic modulation of immunity

The bivoltine European map butterfly (Araschnia levana) displays seasonal polyphenism characterized by the formation of two remarkably distinct dorsal wing phenotypes: The spring generation (A. levana levana) is predominantly orange with black spots and develops from diapause pupae, whereas the summer generation (A. levana prorsa) has black, white, and orange bands and develops from subitaneous pupae. The choice between spring or summer imagoes is regulated by the photoperiod during larval and prepupal development, but polyphenism in the larvae has not been investigated before. Recently, it has been found that the prepupae of A. levana display differences in immunity-related gene expression, so we tested whether larvae destined to become spring (short-day) or summer (long-day) morphs also display differences in innate immunity. We measured larval survival following the injection of a bacterial entomopathogen (Pseudomonas entomophila), the antimicrobial activity in their hemolymph and the induced expression of selected genes encoding antimicrobial peptides (AMPs). Larvae of the short-day generation died significantly later, exhibited higher antibacterial activity in the hemolymph, and displayed higher induced expression levels of AMPs than those of the long-day generation. Our study expands the seasonal polyphenism of A. levana beyond the morphologically distinct spring and summer imagoes to include immunological larval polyphenism that reveals the photoperiodic modulation of immunity. This may reflect life-history traits that manifest as trade-offs between immunity and fecundity.

Intra-specific structural variation among Hawaiian Hoplothrips (Thysanoptera, Phlaeothripidae), with ten new synonymies and one new species

Most of the 16 fungus-feeding species described from the Hawaiian Islands and now placed in the genus Hoplothrips were based on very few and incomplete specimens. The descriptions were published long before any studies on the biology and structural variation of fungus-feeding Phlaeothripinae. Ten of these species are here placed into synonymy, and doubts are expressed concerning the identity of some others. One new polymorphic species is described and compared to a species known only from Florida. In the absence of comprehensive studies on the Hoplothrips fauna of North America, there is little evidence of endemicity or radiation on Hawaii within this genus.

What Kills the Hindgut Flagellates of Lower Termites during the Host Molting Cycle?

Subsocial wood feeding cockroaches in the genus Cryptocercus, the sister group of termites, retain their symbiotic gut flagellates during the host molting cycle, but in lower termites, closely related flagellates die prior to host ecdysis. Although the prevalent view is that termite flagellates die because of conditions of starvation and desiccation in the gut during the host molting cycle, the work of L.R. Cleveland in the 1930s through the 1960s provides a strong alternate hypothesis: it was the changed hormonal environment associated with the origin of eusociality and its concomitant shift in termite developmental ontogeny that instigates the death of the flagellates in termites. Although the research on termite gut microbial communities has exploded since the advent of modern molecular techniques, the role of the host hormonal environment on the life cycle of its gut flagellates has been neglected. Here Cleveland’s studies are revisited to provide a basis for re-examination of the problem, and the results framed in the context of two alternate hypotheses: the flagellate symbionts are victims of the change in host social status, or the flagellates have become incorporated into the life cycle of the eusocial termite colony. Recent work on parasitic protists suggests clear paths for exploring these hypotheses and for resolving long standing issues regarding sexual-encystment cycles in flagellates of the Cryptocercus-termite lineage using molecular methodologies, bringing the problem into the modern era.

doublesex alters aggressiveness as a function of social context and sex in the polyphenic beetle Onthophagus taurus

Despite sharing nearly the same genome, individuals within the same species can vary drastically in both morphology and behaviour as a function of developmental stage, sex or developmental plasticity. Thus, regulatory processes must exist that enable the stage-, sex- or environment-specific expression of traits and their integration during ontogeny, yet exactly how trait complexes are co-regulated and integrated is poorly understood. In this study, we explore the developmental genetic basis of the regulation and integration of environment-dependent sexual dimorphism in behaviour and morphology in the horn-polyphenic dung beetle Onthophagus taurus through the experimental manipulation of the transcription factor doublesex (dsx). The gene dsx plays a profound role in the developmental regulation of morphological differences between sexes as well as alternative male morphs by inhibiting horn formation in females but enabling nutrition-responsive horn growth in males. Specifically, we investigated whether experimental downregulation of dsx expression affects male and female aggressive and courtship behaviours in two social contexts: interactions between individuals of the same sex and interactions between males and females. We find that dsx downregulation significantly alters aggressiveness in both males and females, yet does so differently for both sexes as a function of social context: dsx^RNAi males exhibited elevated aggression towards males but showed reduced aggression towards females, whereas dsx^RNAi females became more aggressive towards males, while their aggressiveness towards other females was unaffected. Moreover, we document unexpectedly high levels of female aggression independent of dsx treatment in both wild-type and control-injected individuals. Lastly, we found no effects of dsx^RNAi on courtship and mating behaviours. We discuss the role of dsx in the regulation of sex-specific and plastic behaviours, the unexpectedly high levels of aggression of hornless dsx^RNAi males in relation to the well-established description of the hornless sneaker phenotype and the potential ecological function of high female aggression.

Copidosoma floridanum (Hymenoptera: Encyrtidae) Rapidly Alters Production of Soldier Embryos in Response to Competition

Most social insects are free living and produce castes that develop in response to extrinsic environmental cues. Caste-forming polyembryonic insects, in contrast, are all endoparasitoids that form social groups inside the bodies of host insects. The best studied polyembryonic wasp is Copidosoma floridanum (Ashmead), which produces ∼3,000 clonal offspring that develop into two castes named reproductive and soldier larvae. Caste determination in this species is mediated by whether or not embryos inherit primary germ cells (PGCs). Prior results showed that C. floridanum increases the proportion of female soldier larvae it produces per host in response to other parasitoids like Microplitis demolitor. Here we show that caste ratio shifts occur through increased formation of embryos lacking PGCs. Our results further indicated that increased soldier production was a specific response to multiparasitism elicited by the chorion of M. demolitor eggs.

Aphid specialism as an example of ecological-evolutionary divergence

Debate still continues around the definition of generalism and specialism in nature. To some, generalism is equated solely with polyphagy, but this cannot be readily divorced from other essential biological factors, such as morphology, behaviour, genetics, biochemistry, chemistry and ecology, including chemical ecology. Viewed in this light, and accepting that when living organisms evolve to fill new ecological-evolutionary niches, this is the primal act of specialisation, then perhaps all living organisms are specialist in the broadest sense. To illustrate the levels of specialisation that may be found in a group of animals, we here provide an overview of those displayed by a subfamily of hemipteran insects, the Aphididae, which comprises some 1600 species/subspecies in Europe alone and whose members are specialised in a variety of lifestyle traits. These include life cycle, host adaptation, dispersal and migration, associations with bacterial symbionts (in turn related to host adaptation and resistance to hymenopterous wasp parasitoids), mutualisms with ants, and resistance to insecticides. As with polyphagy, these traits cannot easily be separated from one another, but rather, are interconnected, often highly so, which makes the Aphididae a fascinating animal group to study, providing an informative, perhaps unique, model to illustrate the complexities of defining generalism versus specialism.

KIN-MEDIATED MORPHOGENESIS IN FACULTATIVELY CANNIBALISTIC TADPOLES

Inclusive fitness theory predicts that organisms can increase their fitness by helping or not harming relatives, and many animals modify their behavior toward kin in a manner consistent with this prediction. Morphogenesis also may be sensitive to kinship environment, particularly in species where certain individuals facultatively develop structures that can be used against conspecifics as weaponry. We tested this hypothesis by examining whether and how consanguinity affected the probability that a structurally distinctive carnivore phenotype, which is opportunistically cannibalistic, would be produced in plains spadefoot toad tadpoles (Spea bombifrons) and southern spadefoot toad tadpoles (S. multiplicata). For tadpoles of S. multiplicata, individuals were significantly more likely to express the carnivore phenotype in mixed sibship groups than in pure sibship groups. For tadpoles of S. bombifrons, individuals were significantly more likely to express the carnivore phenotype when reared alone than in pure sibship groups. Both outcomes were independent of food availability or sibship specific differences in size or growth rate, and waterborne chemical signals from nonkin were sufficient to trigger expression of the carnivore phenotype. Our results suggest that morphogenesis may be responsive to kinship environment in any species or population that occurs as multiple, environmentally induced forms (polyphenism) that differ in their ability to help or to harm others.

POLYPHENISM IN SPADEFOOT TOAD TADPOLES AS A LOCALLY ADJUSTED EVOLUTIONARILY STABLE STRATEGY

I examined the evolutionary factors maintaining two environmentally induced morphs in ponds of variable duration. Larvae of New Mexico spadefoot toads (Scaphiopus multiplicatus) often occur in the same pond as a large, rapidly developing carnivorous morph and as a smaller, more slowly developing omnivorous morph. Previous studies revealed that carnivores can be induced by feeding tadpoles live fairy shrimp and that morph determination is reversible. Field and laboratory experiments indicated that the ability of an individual to become a carnivore or an omnivore is maintained evolutionarily as a response to variability in pond longevity and food abundance. Carnivores survived better in highly ephemeral artificial ponds, because they developed faster. Omnivores survived better in longer-duration artificial ponds, because their larger fat reserves enhanced postmetamorphic survival. The two morphs also occupy different trophic niches. Experimental manipulations of morph frequency in ponds of intermediate duration revealed that increased competition for food among individuals of the more common morph made the rarer form more successful. Morph frequency within each pond was stabilized at an equilibrium by frequency-dependent morph reversal, which reflected frequency-dependent natural selection on size at metamorphosis: larger metamorphs had higher survival, and individuals reared at a frequency above the pond's equilibrium frequency were smaller at metamorphosis than were individuals of that morph reared at a frequency below the pond's equilibrium. Because neighboring ponds often differed in pond longevity and food abundance, each pond possessed a unique equilibrium morph frequency. This implies that morph determination in Scaphiopus is a locally adjusted evolutionarily stable strategy (ESS).

Signals of selection in conditionally expressed genes in the diversification of three horned beetle species

Species radiations may be facilitated by phenotypic differences already present within populations, such as those arising through sex-specific development or developmental processes biased towards particular reproductive or trophic morphs. We sought to test this hypothesis by utilizing a comparative transcriptomic approach to contrast among- and within-species differentiation using three horned beetle species in the genus Onthophagus. These three species exhibit differences along three phenotypic axes reflective of much of the interspecific diversity present within the genus: horn location, polarity of sexual dimorphism and degree of nutritional sensitivity. Our approach combined de novo transcript assembly, assessment of amino acid substitutions (dN/dS) across orthologous gene pairs and integration of gene function and conditional gene expression data. We identified 17 genes across the three species pairs related to axis patterning, development and metabolism with dN/dS > 1 and detected elevated dN/dS in genes related to metabolism and biosynthesis in the most closely related species pair, which is characterized by a loss of nutritional polyphenism and a reversal of sexual dimorphism. Further, we found that genes that are conditionally expressed (i.e. as a function of sex, nutrition or body region) within one of our focal species also showed significantly stronger signals of positive or relaxed purifying selection between species divergent along the same morphological axis (i.e. polarity of sexual dimorphism, degree of nutritional sensitivity or location of horns). Our findings thus reveal a positive relationship between intraspecific differentiation due to condition-specific development and genetic divergences among species.

Mixed evidence for the erosion of intertactical genetic correlations through intralocus tactical conflict

Alternative reproductive tactics, whereby members of the same sex use different tactics to secure matings, are often associated with conditional intrasexual dimorphisms. Given the different selective pressures on males adopting each mating tactic, intrasexual dimorphism is more likely to arise if phenotypes are genetically uncoupled and free to evolve towards their phenotypic optima. However, in this context, genetic correlations between male morphs could result in intralocus tactical conflict (ITC). We investigated the genetic architecture of male dimorphism in bulb mites (Rhizoglyphus echinopus) and earwigs (Forficula auricularia). We used half-sibling breeding designs to assess the heritability and intra/intersexual genetic correlations of dimorphic and monomorphic traits in each species. We found two contrasting patterns; F. auricularia exhibited low intrasexual genetic correlations for the dimorphic trait, suggesting that the ITC is moving towards a resolution. Meanwhile, R. echinopus exhibited high and significant intrasexual genetic correlations for most traits, suggesting that morphs in the bulb mite may be limited in evolving to their optima. This also shows that intrasexual dimorphisms can evolve despite strong genetic constraints, contrary to current predictions. We discuss the implications of this genetic constraint and emphasize the potential importance of ITC for our understanding of intrasexual dimorphisms.

Life-history strategy determines constraints on immune function

Determining the factors governing investment in immunity is critical to understanding host-pathogen ecological and evolutionary dynamics. Studies often consider disease resistance in the context of life-history theory, with the expectation that investment in immunity will be optimized in anticipation of disease risk. Immunity, however, is constrained by context-dependent fitness costs. How the costs of immunity vary across life-history strategies has yet to be considered. Pea aphids are typically unwinged but produce winged offspring in response to high population densities and deteriorating conditions. This is an example of polyphenism, a strategy used by many organisms to adjust to environmental cues. The goal of this study was to examine the relationship between the fitness costs of immunity, pathogen resistance and the strength of an immune response across aphid morphs that differ in life-history strategy but are genetically identical. We measured fecundity of winged and unwinged aphids challenged with a heat-inactivated fungal pathogen, and found that immune costs are limited to winged aphids. We hypothesized that these costs reflect stronger investment in immunity in anticipation of higher disease risk, and that winged aphids would be more resistant due to a stronger immune response. However, producing wings is energetically expensive. This guided an alternative hypothesis - that investing resources into wings could lead to a reduced capacity to resist infection. We measured survival and pathogen load after live fungal infection, and we characterized the aphid immune response to fungi by measuring immune cell concentration and gene expression. We found that winged aphids are less resistant and mount a weaker immune response than unwinged aphids, demonstrating that winged aphids pay higher costs for a less effective immune response. Our results show that polyphenism is an understudied factor influencing the expression of immune costs. More generally, our work shows that in addition to disease resistance, the costs of immunity vary between individuals with different life-history strategies. We discuss the implications of these findings for understanding how organisms invest optimally in immunity in the light of context-dependent constraints.

Transcriptomics of Intralocus Sexual Conflict: Gene Expression Patterns in Females Change in Response to Selection on a Male Secondary Sexual Trait in the Bulb Mite

Intralocus sexual conflict (IASC) prevents males and females from reaching their disparate phenotypic optima and is widespread, but little is known about its genetic underpinnings. In Rhizoglyphus robini, a mite species with alternative male morphs, elevated sexual dimorphism of the armored fighter males (compared to more feminized scramblers males) was previously reported to be associated with increased IASC. Because IASC persists if gene expression patterns are correlated between sexes, we compared gene expression patterns of males and females from the replicate lines selected for increased proportion of fighter or scrambler males (F- and S-lines, respectively). Specifically, we tested the prediction that selection for fighter morph caused correlated changes in gene expression patterns in females. We identified 532 differentially expressed genes (FDR < 0.05) between the F-line and S-line males. Consistent with the prediction, expression levels of these genes also differed between females from respective lines. Thus, significant proportion of genes differentially expressed between sexually selected male phenotypes showed correlated expression levels in females, likely contributing to elevated IASC in F-lines reported in a previous study.

Seasonal phenotype-specific transcriptional reprogramming during metamorphosis in the European map butterfly Araschnia levana

The European map butterfly (Araschnia levana) is a classic example of seasonal polyphenism because the spring and summer imagoes display two distinct morphological phenotypes. The light regime and temperature during larval and prepupal development determine whether or not the pupae commit to diapause and overwintering and thus whether spring or summer imagoes emerge. We used suppression subtractive hybridization to experimentally screen for genes that are differentially expressed in prepupae committed either to accelerated metamorphosis and egg production or diapause and overwintering. The range and ontology of the differentially expressed genes in prepupae developing from larvae exposed either to long‐day (LD) or short‐day (SD) conditions revealed fundamental differences. The SD prepupae preferentially expressed genes related to cuticle formation and immunity, reflecting the formation of a robust pupal exoskeleton and the upregulation of antimicrobial peptides as preparations for overwintering. One protein preferentially expressed in SD prepupae has a counterpart in Bombyx mori that functions as a diapause duration clock. The differentially expressed genes in LD prepupae included several members of the dusky and osiris families. We also observed the strong induction of different yellow‐like genes under SD and LD conditions which suggest a role in the developmental choice between seasonal phenotypes. Our transcriptomic data will facilitate the more detailed analysis of molecular mechanisms underlying seasonal polyphenism.

Ecological Trade-offs between Migration and Reproduction Are Mediated by the Nutrition-Sensitive Insulin-Signaling Pathway

Crowding and changes in food availability are two critical environmental conditions that impact an animal's trajectory toward either migration or reproduction. Many insects facing this challenge have evolved wing polyphenisms. When conditions favor reproduction, wing polyphenic species produce adults that either have no wings or short, non-functional wings. Facultative wing growth reflects a physiological and evolutionary trade-off between migration and reproduction, triggered by environmental conditions. How environmental cues are transduced to produce these alternative forms, and their associated ecological shift from migration to reproduction, remains an important unsolved problem in evolutionary ecology. The brown planthopper, a wing polymorphic insect exhibiting strong trade-offs in investment between migration and reproduction, is one of the most serious rice pests in Asia. In this study, we investigated the function of four genes in the insulin-signaling pathway known to couple nutrition with growth, PI3 Kinase (PI3K), PDK1, Akt (Protein Kinase B), and the forkhead gene FOXO. Using a combination of RNA interference and pharmacological inhibitor treatment, we show that all four genes contribute to tissue level regulation of wing polymorphic development in this insect. As predicted, silencing of the NlPI3K, NlAkt and NlPDK1 through dsRNA and with the pharmacological inhibitor Perifosine resulted in short-winged brown planthoppers, whereas knockdown of NlFOXO resulted in long-winged planthoppers. Morphometric analyses confirm that phenotypes from our manipulations mimic what would be found in nature, i.e., major parameters such as bristle number, wing area and body weight are not significantly different from non-experimental animals. Taken together, these data implicate the insulin-signaling pathway in the transduction of environmental factors into condition-dependent patterns of wing growth in insects.

Cribrilina mutabilis n. sp., an Eelgrass-Associated Bryozoan (Gymnolaemata: Cheilostomata) with Large Variationin Zooid Morphology Related to Life History

We describe the cribrimorph cheilostome bryozoan Cribrilina mutabilis n. sp., which we detected as an epibiont on eelgrass (Zostera marina) at Akkeshi, Hokkaido, northern Japan. This species shows three distinct zooid types during summer: the R (rib), I (intermediate), and S (shield) types. Evidence indicates that zooids commit to development as a given type, rather than transform from one type to another with age. Differences in the frontal spinocyst among the types appear to be mediated by a simple developmental mechanism, acceleration or retardation in the production of lateral costal fusions as the costae elongate during ontogeny. Colonies of all three types were identical, or nearly so, in partial nucleotide sequences of the mitochondrial COI gene (555-631 bp), suggesting that they represent a single species. Zooid types varied temporally in overall frequency in the population: colonies contained nearly exclusively R-type zooids in mid-June; predominantly I-type, or both R- and I-type, zooids in mid-July; and I-type, S-type, or both I- and S-type zooids (interspersed or in discrete bands) in mid- to late August. Reproduction occurred throughout the season, but peaked in July, with only R- and I-type zooids reproducing. Reproductive zooids bear a vestigial compound (tripartite) ooecium and brood internally; S-type zooids, first appearing in August, were non-reproductive, which suggests that they may serve as an overwintering stage. As this species is easily accessible, common, and simple in form, it is potentially useful as a model system for studying polyphenism at multiple levels (zooid, colony, and population) in the context of life-history adaptations.

The transcriptomic basis of tissue- and nutrition-dependent sexual dimorphism in the beetle Onthophagus taurus

Sexual dimorphism accounts for a large fraction of intraspecific diversity. However, not all traits are equally sexually dimorphic; instead, individuals are mosaics of tissues that vary in their ability to exhibit dimorphism. Furthermore, the degree of a trait's sexual dimorphism is frequently environment‐dependent, with elaborate sexual dimorphism commonly being restricted to high nutritional conditions. Understanding the developmental basis and evolution of condition‐dependent sexual dimorphism can be critically informed by determining – across tissues and nutritional conditions – what sex‐biased genes are deployed and how they interact and translate into functional processes. Indeed, key theories concerning the evolution of condition‐dependent sexually dimorphic traits rest on assumptions regarding their developmental genetic underpinnings, yet, have largely gone unexamined by empirical studies. Here, we provide such evidence by investigating the transcriptomic basis of tissue‐ and nutrition‐dependent sexual dimorphism in the bull‐headed dung beetle Onthophagus taurus. Our findings suggest (1) that generating morphological sexual dimorphism requires sex‐biased gene expression in and developmental remodeling of both sexes, regardless of which sex exhibits externally visible trait exaggeration, (2) that although sexually dimorphic phenotypes are comprised of traits underlain by independent repertoires of sex‐biased gene expression, they act similarly at a functional level, and (3) that sexual dimorphism and condition‐dependence share common genetic underpinnings specifically in sexually‐selected traits.

Investigating the genetic architecture of conditional strategies using the environmental threshold model

The threshold expression of dichotomous phenotypes that are environmentally cued or induced comprise the vast majority of phenotypic dimorphisms in colour, morphology, behaviour and life history. Modelled as conditional strategies under the framework of evolutionary game theory, the quantitative genetic basis of these traits is a challenge to estimate. The challenge exists firstly because the phenotypic expression of the trait is dichotomous and secondly because the apparent environmental cue is separate from the biological signal pathway that induces the switch between phenotypes. It is the cryptic variation underlying the translation of cue to phenotype that we address here. With a 'half-sib common environment' and a 'family-level split environment' experiment, we examine the environmental and genetic influences that underlie male dimorphism in the earwig Forficula auricularia. From the conceptual framework of the latent environmental threshold (LET) model, we use pedigree information to dissect the genetic architecture of the threshold expression of forceps length. We investigate for the first time the strength of the correlation between observable and cryptic 'proximate' cues. Furthermore, in support of the environmental threshold model, we found no evidence for a genetic correlation between cue and the threshold between phenotypes. Our results show strong correlations between observable and proximate cues and less genetic variation for thresholds than previous studies have suggested. We discuss the importance of generating better estimates of the genetic variation for thresholds when investigating the genetic architecture and heritability of threshold traits. By investigating genetic architecture by means of the LET model, our study supports several key evolutionary ideas related to conditional strategies and improves our understanding of environmentally cued decisions.

Adaptive value of a predatory mouth-form in a dimorphic nematode

Polyphenisms can be adaptations to environments that are heterogeneous in space and time, but to persist they require conditional-specific advantages. The nematode Pristionchus pacificus is a facultative predator that displays an evolutionarily conserved polyphenism of its mouthparts. During development, P. pacificus irreversibly executes either a eurystomatous (Eu) or stenostomatous (St) mouth-form, which differ in the shape and number of movable teeth. The Eu form, which has an additional tooth, is more complex than the St form and is thus more highly derived relative to species lacking teeth. Here, we investigate a putative fitness trade-off for the alternative feeding-structures of P. pacificus. We show that the complex Eu form confers a greater ability to kill prey. When adults were provided with a prey diet, Eu nematodes exhibited greater fitness than St nematodes by several measures, including longevity, offspring survival and fecundity when followed by bacterial feeding. However, the two mouth-forms had similar fecundity when fed ad libitum on bacteria, a condition that would confer benefit on the more rapidly developing St form. Thus, the two forms show conditional fitness advantages in different environments. This study provides, to our knowledge, the first functional context for dimorphism in a model for the genetics of plasticity.

The nutritionally responsive transcriptome of the polyphenic beetle Onthophagus taurus and the importance of sexual dimorphism and body region

Developmental responses to nutritional variation represent one of the ecologically most important classes of adaptive plasticity. However, knowledge of genome-wide patterns of nutrition-responsive gene expression is limited. Here, we studied genome-wide transcriptional responses to nutritional variation and their dependency on trait and sex in the beetle Onthophagus taurus. We find that averaged across the transcriptome, nutrition contributes less to overall variation in gene expression than do sex or body region, but that for a modest subset of genes nutrition is by far the most important determinant of expression variation. Furthermore, our results reject the hypothesis that a common machinery may underlie nutrition-sensitive development across body regions. Instead, we find that magnitude (measured by number of differentially expressed contigs), composition (measured by functional enrichment) and evolutionary consequences (measured by patterns of sequence variation) are heavily dependent on exactly which body region is considered and the degree of sexual dimorphism observed on a morphological level. More generally, our findings illustrate that studies into the developmental mechanisms and evolutionary consequences of nutrition-biased gene expression must take into account the dynamics and complexities imposed by other sources of variation in gene expression such as sexual dimorphism and trait type.