Emerging model systems in eco-evo-devo: the environmentally responsive spadefoot toad

Comparative transcriptomics reveals that a novel form of phenotypic plasticity evolved via lineage-specific changes in gene expression

Novel forms of phenotypic plasticity may evolve by lineage-specific changes or by co-opting mechanisms from more general forms of plasticity. Here, we evaluated whether a novel resource polyphenism in New World spadefoot toads (genus Spea) evolved by co-opting mechanisms from an ancestral form of plasticity common in anurans-accelerating larval development rate in response to pond drying. We compared overlap in differentially expressed genes between alternative trophic morphs constituting the polyphenism in Spea versus those found between tadpoles of Old World spadefoot toads (genus Pelobates) when experiencing different pond-drying regimes. Specifically, we (1) generated a de novo transcriptome and conducted differential gene expression analysis in Spea multiplicata, (2) utilized existing gene expression data and a recently published transcriptome for Pelobates cultripes when exposed to different drying regimes, and (3) identified unique and overlapping differentially expressed transcripts. We found thousands of differentially expressed genes between S. multiplicata morphs that were involved in major developmental reorganization, but the vast majority of these were not differentially expressed in P. cultripes. Thus, S. multiplicata's novel polyphenism appears to have arisen primarily through lineage-specific changes in gene expression and not by co-opting existing patterns of gene expression involved in pond-drying plasticity. Therefore, although ancestral stress responses might jump-start evolutionary innovation, substantial lineage-specific modification might be needed to refine these responses into more complex forms of plasticity.

Characterization of Two Mitogenomes of Hyla sanchiangensis (Anura: Hylidae), with Phylogenetic Relationships and Selection Pressure Analyses of Hylidae

Hyla sanchiangensis (Anura: Hylidae) is endemic to China and is distributed across Anhui, Zhejiang, Fujian, Guangdong, Guangxi, Hunan, and Guizhou provinces. The mitogenomes of H. sanchiangensis from two different sites (Jinxiu, Guangxi, and Wencheng, Zhejiang) were sequenced. Phylogenetic analyses were conducted, including 38 mitogenomes of Hylidae from the NCBI database, and assessed the phylogenetic relationship of H. sanchiangensis within the analyzed dataset. Two mitogenomes of H. sanchiangensis showed the typical mitochondrial gene arrangement with 13 protein-coding genes (PCGs), two ribosomal RNA genes (12S rRNA and 16S rRNA), 22 transfer RNA (tRNA) genes, and one non-coding control region (D-loop). The lengths of the 12S rRNA and 16S rRNA genes from both samples (Jinxiu and Wencheng) were 933 bp and 1604 bp, respectively. The genetic distance (p-distance transformed into percent) on the basis of the mitogenomes (excluding the control region) of the two samples was calculated as 4.4%. Hyla sanchiangensis showed a close phylogenetic relationship with the clade of (H. annectans + H. tsinlingensis), which was supported by ML and BI analyses. In the branch-site model, five positive selection sites were found in the clade of Hyla and Dryophytes: Cytb protein (at position 316), ND3 protein (at position 85), and ND5 protein (at position 400) have one site, respectively, and two sites in ND4 protein (at positions 47 and 200). Based on the results, we hypothesized that the positive selection of Hyla and Dryophytes was due to their experience of cold stress in historical events, but more evidence is needed to support this conclusion.

Activational vs. organizational effects of sex steroids and their role in the evolution of reproductive behavior: Looking to foot-flagging frogs and beyond

Sex steroids play an important role in regulation of the vertebrate reproductive phenotype. This is because sex steroids not only activate sexual behaviors that mediate copulation, courtship, and aggression, but they also help guide the development of neural and muscular systems that underlie these traits. Many biologists have therefore described the effects of sex steroid action on reproductive behavior as both "activational" and "organizational," respectively. Here, we focus on these phenomena from an evolutionary standpoint, highlighting that we know relatively little about the way that organizational effects evolve in the natural world to support the adaptation and diversification of reproductive behavior. We first review the evidence that such effects do in fact evolve to mediate the evolution of sexual behavior. We then introduce an emerging animal model - the foot-flagging frog, Staurois parvus - that will be useful to study how sex hormones shape neuromotor development necessary for sexual displays. The foot flag is nothing more than a waving display that males use to compete for access to female mates, and thus the neural circuits that control its production are likely laid down when limb control systems arise during the developmental transition from tadpole to frog. We provide data that highlights how sex steroids might organize foot-flagging behavior through its putative underlying mechanisms. Overall, we anticipate that future studies of foot-flagging frogs will open a powerful window from which to see how sex steroids influence the neuromotor systems to help germinate circuits that drive signaling behavior. In this way, our aim is to bring attention to the important frontier of endocrinological regulation of evolutionary developmental biology (endo-evo-devo) and its relationship to behavior.

Disentangling the developmental origins of a novel phenotype: enhancement versus reversal of environmentally induced gene expression

Increasing evidence suggests that many novel traits might have originated via plasticity-led evolution (PLE). Yet, little is known of the developmental processes that underpin PLE, especially in its early stages. One such process is 'phenotypic accommodation', which occurs when, in response to a change in the environment, an organism experiences adjustments across variable parts of its phenotype that improve its fitness. Here, we asked if environmentally induced changes in gene expression are enhanced or reversed during phenotypic accommodation of a novel, complex phenotype in spadefoot toad tadpoles (Spea multiplicata). More genes than expected were affected by both the environment and phenotypic accommodation in the liver and brain. However, although phenotypic accommodation primarily reversed environmentally induced changes in gene expression in liver tissue, it enhanced these changes in brain tissue. Thus, depending on the tissue, phenotypic accommodation may either minimize functional disruption via reversal of gene expression patterns or promote novelty via enhancement of existing expression patterns. Our study thereby provides insights into the developmental origins of a novel phenotype and the incipient stages of PLE.

Modelling migration in birds: competition's role in maintaining individual variation

Animals exhibit extensive intraspecific variation in behaviour. Causes of such variation are less well understood. Here, we ask when competition leads to the maintenance of multiple behavioural strategies. We model variability using the timing of bird migration as an example. Birds often vary in when they return from non-breeding grounds to establish breeding territories. We assume that early-arriving birds (counting permanent residents as 'earliest') select the best territories. But arriving before the optimal (frequency-independent) breeding date incurs a fitness penalty. Using simulations, we find stable sets of return dates. When year-round residency is viable, the greatest between-individual variation occurs when a small proportion of permanent residents is favoured, and the rest of the population varies in their return times. However, when fitness losses due to year-round residency exceed the benefits of breeding in the worst territory, all individuals migrate, although their return dates often vary continuously. In that case, individual variation is inversely related to fitness risks and positively related to territory inequality. This result is applicable across many systems: when there is more to gain through competition, or when its risks are small, a diversity of individual strategies prevails. Additionally, stability can depend upon the distribution of resources.

A novel larval diet interacts with nutritional stress to modify juvenile behaviors and glucocorticoid responses

Developmental plasticity can allow the exploitation of alternative diets. While such flexibility during early life is often adaptive, it can leave a legacy in later life that alters the overall health and fitness of an individual. Species of the spadefoot toad genus Spea are uniquely poised to address such carryover effects because their larvae can consume drastically different diets: their ancestral diet of detritus or a derived shrimp diet. Here, we use Spea bombifrons to assess the effects of developmental plasticity in response to larval diet type and nutritional stress on juvenile behaviors and stress axis reactivity. We find that, in an open-field assay, juveniles fed shrimp as larvae have longer latencies to move, avoid prey items more often, and have poorer prey-capture abilities. While juveniles fed shrimp as larvae are more exploratory, this effect disappears if they also experienced a temporary nutritional stressor during early life. The larval shrimp diet additionally impairs juvenile jumping performance. Finally, larvae that were fed shrimp under normal nutritional conditions produce juveniles with higher overall glucocorticoid levels, and larvae that were fed shrimp and experienced a temporary nutritional stressor produce juveniles with higher stress-induced glucocorticoid levels. Thus, while it has been demonstrated that consuming the novel, alternative diet can be adaptive for larvae in nature, doing so has marked effects on juvenile phenotypes that may recalibrate an individual's overall fitness. Given that organisms often utilize diverse diets in nature, our study underscores the importance of considering how diet type interacts with early-life nutritional adversity to influence subsequent life stages.

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.

Geographic variation in shape and size of anuran tadpoles: Interpopulation comparisons in Scinax fuscovarius (Anura, Hylidae)

Among anuran species with biphasic life cycle, the occurrence of intraspecific larval morphotypes has been related to variations in developmental time, diet, geographical variation, or response to predators. Here, we evaluated the external morphological variation of larvae among three populations, located more than 270 km apart, of the anuran hylid Scinax fuscovarius by linear and geometric methods, to elucidate the presence of geographically different morphs. Comparisons targeted development, growth, and external morphology. Studied populations exhibited differences in reproductive seasonality, growth rate, timing of development, shape, and size. Shape and size comparisons revealed two well-differentiated morphs, one of them shared by the two closest populations. Morphological differences evidenced a smaller and depressed form of the entire body plan in the most distant population, which showed continuous reproduction throughout the rainy season and under more unpredictable conditions. We interpret the occurrence of the two different larval morphs in S. fuscovarius as a by-product of local geographical conditions, and discuss on possible associations with biotic and abiotic factors cues.

Evolution and plasticity of morph-specific integration in the bull-headed dung beetle Onthophagus taurus

Developmental and evolutionary processes underlying phenotypic variation frequently target several traits simultaneously, thereby causing covariation, or integration, among phenotypes. While phenotypic integration can be neutral, correlational selection can drive adaptive covariation. Especially, the evolution and development of exaggerated secondary sexual traits may require the adjustment of other traits that support, compensate for, or otherwise function in a concerted manner. Although phenotypic integration is ubiquitous, the interplay between genetic, developmental, and ecological conditions in shaping integration and its evolution remains poorly understood. Here, we study the evolution and plasticity of trait integration in the bull-headed dung beetle Onthophagus taurus which is characterized by the polyphenic expression of horned ('major') and hornless ('minor') male morphs. By comparing populations subject to divergent intensities of mate competition, we tested whether mating system shifts affect integration of traits predicted to function in a morph-specific manner. We focussed on fore and hind tibia morphology as these appendages are used to stabilize major males during fights, and on wings, as they are thought to contribute to morph-based differences in dispersal behavior. We found phenotypic integration between fore and hind tibia length and horn length that was stronger in major males, suggesting phenotypic plasticity in integration and potentially secondary sexual trait compensation. Similarly, we observed that fore tibia shape was also integrated with relative horn length. However, although we found population differentiation in wing and tibia shape and allometry, populations did not differ in integration. Lastly, we detected little evidence for morph differences in integration in either tibia or wing shape, although wing allometries differed between morphs. This contrasts with previous studies documenting intraspecific differentiation in morphology, behavior, and allometry as a response to varying levels of mate competition across O. taurus populations. We discuss how sexual selection may shape morph-specific integration, compensation, and allometry across populations.

A way forward with eco evo devo: an extended theory of resource polymorphism with postglacial fishes as model systems

A major goal of evolutionary science is to understand how biological diversity is generated and altered. Despite considerable advances, we still have limited insight into how phenotypic variation arises and is sorted by natural selection. Here we argue that an integrated view, which merges ecology, evolution and developmental biology (eco evo devo) on an equal footing, is needed to understand the multifaceted role of the environment in simultaneously determining the development of the phenotype and the nature of the selective environment, and how organisms in turn affect the environment through eco evo and eco devo feedbacks. To illustrate the usefulness of an integrated eco evo devo perspective, we connect it with the theory of resource polymorphism (i.e. the phenotypic and genetic diversification that occurs in response to variation in available resources). In so doing, we highlight fishes from recently glaciated freshwater systems as exceptionally well-suited model systems for testing predictions of an eco evo devo framework in studies of diversification. Studies on these fishes show that intraspecific diversity can evolve rapidly, and that this process is jointly facilitated by (i) the availability of diverse environments promoting divergent natural selection; (ii) dynamic developmental processes sensitive to environmental and genetic signals; and (iii) eco evo and eco devo feedbacks influencing the selective and developmental environments of the phenotype. We highlight empirical examples and present a conceptual model for the generation of resource polymorphism - emphasizing eco evo devo, and identify current gaps in knowledge.

Use of field-portable ultrasonography reveals differences in developmental phenology and maternal egg provisioning in two sympatric viviparous snakes

A thorough understanding of the life cycles underlying the demography of wild species is limited by the difficulty of observing hidden life‐history traits, such as embryonic development. Major aspects of embryonic development, such as the rate and timing of development, and maternal–fetal interactions can be critical features of early‐life fitness and may impact population trends via effects on individual survival. While information on development in wild snakes and lizards is particularly limited, the repeated evolution of viviparity and diversity of reproductive mode in this clade make it a valuable subject of study. We used field‐portable ultrasonography to investigate embryonic development in two sympatric garter snake species, Thamnophis sirtalis and Thamnophis elegans in the Sierra Nevada mountains of California. This approach allowed us to examine previously hidden reproductive traits including the timing and annual variation in development and differences in parental investment in young. Both species are viviparous, occupy similar ecological niches, and experience the same annual environmental conditions. We found that T. sirtalis embryos were more developmentally advanced than T. elegans embryos during June of three consecutive years. We also found that eggs increased in volume more substantially across developmental stages in T. elegans than in T. sirtalis, indicating differences in maternal provisioning of embryos via placental transfer of water. These findings shed light on interspecific differences in parental investment and timing of development within the same environmental context and demonstrate the value of field ultrasonography for pursuing questions relating to the evolution of reproductive modes, and the ecology of development.

Plasticity and regeneration of gonads in the annelid Pristina leidyi

Background

Gonads are specialized gamete-producing structures that, despite their functional importance, are generated by diverse mechanisms across groups of animals and can be among the most plastic organs of the body. Annelids, the segmented worms, are a group in which gonads have been documented to be plastic and to be able to regenerate, but little is known about what factors influence gonad development or how these structures regenerate. In this study, we aimed to identify factors that influence the presence and size of gonads and to investigate gonad regeneration in the small asexually reproducing annelid, Pristina leidyi.

Results

We found that gonad presence and size in asexual adult P. leidyi are highly variable across individuals and identified several factors that influence these structures. An extrinsic factor, food availability, and two intrinsic factors, individual age and parental age, strongly influence the presence and size of gonads in P. leidyi. We also found that following head amputation in this species, gonads can develop by morphallactic regeneration in previously non-gonadal segments. We also identified a sexually mature individual from our laboratory culture that demonstrates that, although our laboratory strain reproduces only asexually, it retains the potential to become fully sexual.

Conclusions

Our findings demonstrate that gonads in P. leidyi display high phenotypic plasticity and flexibility with respect to their presence, their size, and the segments in which they can form. Considering our findings along with relevant data from other species, we find that, as a group, clitellate annelids can form gonads in at least four different contexts: post-starvation refeeding, fission, morphallactic regeneration, and epimorphic regeneration. This group is thus particularly useful for investigating the mechanisms involved in gonad formation and the evolution of post-embryonic phenotypic plasticity.

Electronic supplementary material

The online version of this article (doi:10.1186/s13227-016-0059-1) contains supplementary material, which is available to authorized users.

Sexual selection's impacts on ecological specialization: an experimental test

In many species, individuals specialize on different resources, thereby reducing competition. Such ecological specialization can promote the evolution of alternative ecomorphs-distinct phenotypes adapted for particular resources. Elucidating whether and how this process is influenced by sexual selection is crucial for understanding how ecological specialization promotes the evolution of novel traits and, potentially, speciation between ecomorphs. We evaluated the population-level effects of sexual selection (as mediated by mate choice) on ecological specialization in spadefoot toad tadpoles that express alternative ecomorphs. We manipulated whether sexual selection was present or reversed by mating females to their preferred versus non-preferred males, respectively. We then exposed their tadpoles to resource competition in experimental mesocosms. The resulting distribution of ecomorphs was similar between treatments, but sexual selection generated poorer trait integration in, and lower fitness of, the more specialized carnivore morph. Moreover, disruptive and directional natural selection were weaker in the sexual selection present treatment. Nevertheless, this effect on disruptive selection was smaller than previously documented effects of ecological opportunity and competitor density. Thus, sexual selection can inhibit adaptation to resource competition and thereby hinder ecological specialization, particularly when females obtain fitness benefits from mate choice that offset the cost of producing competitively inferior offspring.

Living organisms influence on environmental conditions: pH modulation by amphibian embryos versus aluminum toxicity

The LC10, 50 and 90/24h of aluminum for Rhinella arenarum embryos at complete operculum stage were 0.55, 0.75 and 1mgAl(3+)/L respectively. Those values did not change significantly by expanding the exposure period till 168h. The aluminum toxicity was evaluated in different pH conditions by means of a citrate buffer resulting for instance, 1mgAl(3+)/L at pH 4, 4.1, 5 and 6 in 100%, 70%, 35% and 0% of lethality respectively. As an outstanding feature, the embryos changed the pH of the maintaining media both in the case of Al(3+) or citrate buffer treatments toward neutral. 10 embryos in 40mL of AMPHITOX solution were able to increase the pH from 4.2 to 7.05, a fact related with a metabolic shift resulting in an increase in nitrogen loss as ammonia. Our study point out the natural selection of the most resistant amphibian embryos both for pH or aluminum as well as the capacity of living organisms (as a population) to alter their chemical environment toward optimal conditions for their survival. As these facts occur at early life stages, it expand the concept that living organisms at ontogenic stages are biomarker of environmental signatures of the evolutionary process (Herkovits, 2006) to a global Onto-Evo concept which imply also the feedback mechanisms from living organisms to shape environmental conditions in a way that benefits them.

Budgett's frog (Lepidobatrachus laevis): A new amphibian embryo for developmental biology

The large size and rapid development of amphibian embryos has facilitated ground-breaking discoveries in developmental biology. Here, we describe the embryogenesis of the Budgett's frog (Lepidobatrachus laevis), an unusual species with eggs that are over twice the diameter of laboratory Xenopus, and embryos that can tolerate higher temperatures to develop into a tadpole four times more rapidly. In addition to detailing their early development, we demonstrate that, like Xenopus, these embryos are amenable to explant culture assays and can express exogenous transcripts in a tissue-specific manner. Moreover, the steep developmental trajectory and large scale of Lepidobatrachus make it exceptionally well-suited for morphogenesis research. For example, the developing organs of the Budgett's frog are massive compared to those of most model species, and are composed of larger individual cells, thereby affording increased subcellular resolution of early vertebrate organogenesis. Furthermore, we found that complete limb regeneration, which typically requires months to achieve in most vertebrate models, occurs in a matter of days in the Budgett's tadpole, which substantially accelerates the pace of experimentation. Thus, the unusual combination of the greater size and speed of the Budgett's frog model provides inimitable advantages for developmental studies-and a novel inroad to address the mechanisms of spatiotemporal scaling during evolution.

Evolution of rapid development in spadefoot toads is unrelated to arid environments

The extent to which species' life histories evolve to match climatic conditions is a critical question in evolutionary biology and ecology and as human activities rapidly modify global climate. GIS-based climatic data offer new opportunities to rigorously test this question. Superficially, the spadefoot toads of North America (Scaphiopodidae) seem to offer a classic example of adaptive life-history evolution: some species occur in extremely dry deserts and have evolved the shortest aquatic larval periods known among anurans. However, the relationships between the climatic conditions where spadefoots occur and the relevant life-history traits have not been explicitly tested. Here, we analyzed these relationships using GIS-based climatic data, published life-history data, and a time-calibrated phylogeny for pelobatoid frogs. Surprisingly, we find no significant relationships between life-history variables and precipitation or aridity levels where these species occur. Instead, rapid development in pelobatoids is strongly related to their small genome sizes and to phylogeny.

Eco-evo-devo: the time has come

The major goal of ecological evolutionary developmental biology, also known as "eco-evo-devo," is to uncover the rules that underlie the interactions between an organism's environment, genes, and development and to incorporate these rules into evolutionary theory. In this chapter, we discuss some key and emerging concepts within eco-evo-devo. These concepts show that the environment is a source and inducer of genotypic and phenotypic variation at multiple levels of biological organization, while development acts as a regulator that can mask, release, or create new combinations of variation. Natural selection can subsequently fix this variation, giving rise to novel phenotypes. Combining the approaches of eco-evo-devo and ecological genomics will mutually enrich these fields in a way that will not only enhance our understanding of evolution, but also of the genetic mechanisms underlying the responses of organisms to their natural environments.

Assaying environmental nickel toxicity using model nematodes

Although nickel exposure results in allergic reactions, respiratory conditions, and cancer in humans and rodents, the ramifications of excess nickel in the environment for animal and human health remain largely undescribed. Nickel and other cationic metals travel through waterways and bind to soils and sediments. To evaluate the potential toxic effects of nickel at environmental contaminant levels (8.9-7,600 µg Ni/g dry weight of sediment and 50-800 µg NiCl₂/L of water), we conducted assays using two cosmopolitan nematodes, Caenorhabditis elegans and Pristionchus pacificus. We assayed the effects of both sediment-bound and aqueous nickel upon animal growth, developmental survival, lifespan, and fecundity. Uncontaminated sediments were collected from sites in the Midwestern United States and spiked with a range of nickel concentrations. We found that nickel-spiked sediment substantially impairs both survival from larval to adult stages and adult longevity in a concentration-dependent manner. Further, while aqueous nickel showed no adverse effects on either survivorship or longevity, we observed a significant decrease in fecundity, indicating that aqueous nickel could have a negative impact on nematode physiology. Intriguingly, C. elegans and P. pacificus exhibit similar, but not identical, responses to nickel exposure. Moreover, P. pacificus could be tested successfully in sediments inhospitable to C. elegans. Our results add to a growing body of literature documenting the impact of nickel on animal physiology, and suggest that environmental toxicological studies could gain an advantage by widening their repertoire of nematode species.

Ecological epigenetics: an introduction to the symposium

Phenotypic variation arises from interactions between environmental and genetic variation, and the emergence of such variation is, in part, mediated by epigenetic mechanisms: factors that modify gene expression but do not change the gene sequence, per se. The role of epigenetic variation and inheritance in natural populations, however, remains poorly understood. The budding field of Ecological Epigenetics seeks to extend our knowledge of epigenetic mechanisms and processes to natural populations, and recent conceptual and technical advances have made progress toward this goal more feasible. In light of these breakthroughs, now is a particularly opportune time to develop a framework that will guide and facilitate exceptional studies in Ecological Epigenetics. Toward this goal, the Ecological Epigenetics symposium brought together researchers with diverse strengths in theory, developmental genetics, ecology, and evolution, and the proceedings from their talks are presented in this issue. By characterizing environmentally dependent epigenetic variation in natural populations, we will enhance our understanding of developmental, ecological, and evolutionary phenomena. In particular, ecological epigenetics has the potential to explain how populations endure (or fail to endure) profound and rapid environmental change. Here, my goal is to introduce some of the common goals and challenges shared by those pursuing this critical field.

Widespread disruptive selection in the wild is associated with intense resource competition

Background

Disruptive selection has been documented in a growing number of natural populations. Yet, its prevalence within individual systems remains unclear. Furthermore, few studies have sought to identify the ecological factors that promote disruptive selection in the wild. To address these issues, we surveyed 15 populations of Mexican spadefoot toad tadpoles, Spea multiplicata, and measured the prevalence of disruptive selection acting on resource-use phenotypes. We also evaluated the relationship between the strength of disruptive selection and the intensity of intraspecific competition—an ecological agent hypothesized to be an important driver of disruptive selection.

Results

Disruptive selection was the predominant mode of quadratic selection across all populations. However, a directional component of selection favoring an extreme ecomorph—a distinctive carnivore morph—was also common. Disruptive selection was strongest in populations experiencing the most intense intraspecific competition, whereas stabilizing selection was only found in populations experiencing relatively weak intraspecific competition.

Conclusions

Disruptive selection can be common in natural populations. Intraspecific competition for resources may be a key driver of such selection.

Developmental reversal in neuropeptide Y action on feeding in an amphibian

Neuropeptide Y (NPY) is expressed in the hypothalamus where it exerts orexigenic actions within the feeding control circuit. While NPY stimulates feeding in juvenile and adult animals, it is not known whether NPY influences food intake at earlier life stages. We investigated a role for NPY in regulating feeding at two stages of the life cycle of an amphibian, the Western spadefoot toad Spea hammondii. We administered NPY by intracerebroventricular (i.c.v.) injection to juvenile toads or prometamorphic tadpoles, and monitored locomotion, feeding behavior and/or food intake. Injection of NPY (20 or 200 ng/g BW) into juvenile toads decreased the latency to, and increased the number of strikes at prey, and the number of crickets eaten compared to uninjected or vehicle-injected controls. By contrast, injection of NPY (0.02-20 ng/g BW) into prometamorphic tadpoles caused a dose-dependent decrease in time spent foraging compared to controls. Blocking NPY signaling in the prometamorphic tadpole brain by i.c.v. injection of a general NPY receptor antagonist increased foraging, and partly blocked the action of exogenous NPY on foraging. Taken together, our findings show a developmental reversal in NPY actions on feeding in an amphibian, with the peptide having a characteristic orexigenic action in the juvenile toad, but an inhibitory action on foraging in the prometamorphic tadpole. The anorexigenic action of NPY in the tadpole correlates with a decrease in feeding that occurs at metamorphic climax when the tadpole's gut and cranium remodels for the transition to a carnivorous diet.

A suite of molecular markers for identifying species, detecting introgression and describing population structure in spadefoot toads (Spea spp.)

Two congeneric species of spadefoot toad, Spea multiplicata and Spea bombifrons, have been the focus of hybridization studies since the 1970s. Because complex hybrids are not readily distinguished phenotypically, genetic markers are needed to identify introgressed individuals. We therefore developed a set of molecular markers (amplified fragment length polymorphism, polymerase chain reaction-restriction fragment length polymorphism and single nucleotide polymorphism) for identifying pure-species, F1 hybrids and more complex introgressed types. To do so, we tested a series of markers across both species and known hybrids using populations in both allopatry and sympatry. We retained those markers that differentiated the two pure-species and also consistently identified known species hybrids. These markers are well suited for identifying hybrids between these species. Moreover, those markers that show variation within each species can be used in conjunction with existing molecular markers in studies of population structure and gene flow.

Relaxed genetic constraint is ancestral to the evolution of phenotypic plasticity

Phenotypic plasticity--the capacity of a single genotype to produce different phenotypes in response to varying environmental conditions--is widespread. Yet, whether, and how, plasticity impacts evolutionary diversification is unclear. According to a widely discussed hypothesis, plasticity promotes rapid evolution because genes expressed differentially across different environments (i.e., genes with "biased" expression) experience relaxed genetic constraint and thereby accumulate variation faster than do genes with unbiased expression. Indeed, empirical studies confirm that biased genes evolve faster than unbiased genes in the same genome. An alternative hypothesis holds, however, that the relaxed constraint and faster evolutionary rates of biased genes may be a precondition for, rather than a consequence of, plasticity's evolution. Here, we evaluated these alternative hypotheses by characterizing evolutionary rates of biased and unbiased genes in two species of frogs that exhibit a striking form of phenotypic plasticity. We also characterized orthologs of these genes in four species of frogs that had diverged from the two plastic species before the plasticity evolved. We found that the faster evolutionary rates of biased genes predated the evolution of the plasticity. Furthermore, biased genes showed greater expression variance than did unbiased genes, suggesting that they may be more dispensable. Phenotypic plasticity may therefore evolve when dispensable genes are co-opted for novel function in environmentally induced phenotypes. Thus, relaxed genetic constraint may be a cause--not a consequence--of the evolution of phenotypic plasticity, and thereby contribute to the evolution of novel traits.