Pupal Remodeling and the Development and Evolution of Sexual Dimorphism in Horned Beetles

Adhesion and shrinkage transform the rounded pupal horn into an angular adult horn in Japanese rhinoceros beetle

Clarifying the mechanisms underlying shape alterations during insect metamorphosis is important for understanding exoskeletal morphogenesis. The large horn of the Japanese rhinoceros beetle Trypoxylus dichotomus is the result of drastic metamorphosis, wherein it appears as a rounded shape during pupation and then undergoes remodeling into an angular adult shape. However, the mechanical mechanisms underlying this remodeling process remain unknown. In this study, we investigated the remodeling mechanisms of the Japanese rhinoceros beetle horn by developing a physical simulation. We identified three factors contributing to remodeling by biological experiments - ventral adhesion, uneven shrinkage, and volume reduction - which were demonstrated to be crucial for transformation using a physical simulation. Furthermore, we corroborated our findings by applying the simulation to the mandibular remodeling of stag beetles. These results indicated that physical simulation applies to pupal remodeling in other beetles, and the morphogenic mechanism could explain various exoskeletal shapes.

Histone deacetylases regulate organ-specific growth in a horned beetle

BACKGROUND

Nutrient availability is among the most widespread means by which environmental variability affects developmental outcomes. Because almost all cells within an individual organism share the same genome, structure-specific growth responses must result from changes in gene regulation. Earlier work suggested that histone deacetylases (HDACs) may serve as epigenetic regulators linking nutritional conditions to trait-specific development. Here we expand on this work by assessing the function of diverse HDACs in the structure-specific growth of both sex-shared and sex-specific traits including evolutionarily novel structures in the horned dung beetle Onthophagus taurus.

RESULTS

We identified five HDAC members whose downregulation yielded highly variable mortality depending on which HDAC member was targeted. We then show that HDAC1, 3, and 4 operate in both a gene- and trait-specific manner in the regulation of nutrition-responsiveness of appendage size and shape. Specifically, HDAC 1, 3, or 4 knockdown diminished wing size similarly while leg development was differentially affected by RNAi targeting HDAC3 and HDAC4. In addition, depletion of HDAC3 transcript resulted in a more rounded shape of genitalia at the pupal stage and decreased the length of adult aedeagus across all body sizes. Most importantly, we find that HDAC3 and HDAC4 pattern the morphology and regulate the scaling of evolutionarily novel head and thoracic horns as a function of nutritional variation.

CONCLUSION

Collectively, our results suggest that both functional overlap and division of labor among HDAC members contribute to morphological diversification of both conventional and recently evolved appendages. More generally, our work raises the possibility that HDAC-mediated scaling relationships and their evolution may underpin morphological diversification within and across insect species broadly.

Sexually transmitted mutualist nematodes shape host growth across dung beetle species

Many symbionts are sexually transmitted and impact their host's development, ecology, and evolution. While the significance of symbionts that cause sexually transmitted diseases (STDs) is relatively well understood, the prevalence and potential significance of the sexual transmission of mutualists remain elusive. Here, we study the effects of sexually transmitted mutualist nematodes on their dung beetle hosts. Symbiotic Diplogastrellus monhysteroides nematodes are present on the genitalia of male and female Onthophagus beetles and are horizontally transmitted during mating and vertically passed on to offspring during oviposition. A previous study indicates that the presence of nematodes benefits larval development and life history in a single host species, Onthophagus taurus. However, Diplogastrellus nematodes can be found in association with a variety of beetle species. Here, we replicate these previous experiments, assess whether the beneficial effects extend to other host species, and test whether nematode-mediated effects differ between male and female host beetles. Rearing three relatively distantly related dung beetle species with and without nematodes, we find that the presence of nematodes benefits body size, but not development time or survival across all three species. Likewise, we found no difference in the benefit of nematodes to male compared to female beetles. These findings highlight the role of sexually transmitted mutualists in the evolution and ecology of dung beetles.

Sex-specific regulation of development, growth and metabolism

Adult females and males of most species differ in many aspects of their morphology, physiology and behavior, in response to sex-specific selective pressures that maximize fitness. While we have an increasingly good understanding of the genetic mechanisms that initiate these differences, the sex-specific developmental trajectories that generate them are much less well understood. Here we review recent advances in the sex-specific regulation of development focusing on two models where this development is increasingly well understood: Sexual dimorphism of body size in the fruit fly Drosophila melanogaster and sexual dimorphism of horns in the horned beetle Onthophagus taurus. Because growth and development are also supported by metabolism, the regulation of sex-specific metabolism during and after development is an important aspect of the generation of female and male phenotypes. Hitherto, the study of sex-specific development has largely been independent of the study of sex-specific metabolism. Nevertheless, as we discuss in this review, recent research has begun to reveal considerable overlap in the cellular and physiological mechanisms that regulate sex-specific development and metabolism.

Unraveling the heritage of lost traits

We synthesize ontogenetic work spanning the past century that show evolutionarily lost structures are rarely entirely absent from earlier developmental stages. We discuss morphological and genetic insights from developmental studies reveal about the evolution of trait loss and regain.

Sexual differences in human cranial morphology: Is one sex more variable or one region more dimorphic?

The quantification of cranial sexual dimorphism (CSD) among modern humans is relevant in evolutionary studies of morphological variation and in a forensic context. Despite the abundance of quantitative studies of CSD, few have specifically examined intra-sex variability. Here we quantify CSD in a geographically homogeneous sample of adult crania, which includes Italian individuals from the 19th and 20th centuries. Cranial morphology is described with 92 3D landmarks analyzed using Procrustean geometric morphometrics (PGMM). Size and shape variables are used to compare morphological variance between sexes in the whole cranium and four individual regions. The same variables, plus Procrustes form, are used to quantify average sex differences and explore classification accuracy. Our results indicate that: (a) as predicted by Wainer's rule, males present overall more variance in size and shape, albeit this is statistically significant only for total cranial size; (b) differences between sexes are dominated by size and to a lesser extent by Procrustes form; (c) shape only accounts for a minor proportion of variance; (d) the cranial base shows almost no dimorphism for shape; and (e) facial Procrustes form is the most accurate predictor of skeletal sex. Overall, this study suggests developmental factors underlying differences in CSD among cranial regions; stresses the need for population-specific models that describe craniofacial variation as the basis for models that facilitate the estimation of sex in unidentified skeletal remains; and provides one of the first confirmations of "Wainer's rule" in relation to sexual dimorphism in mammals specific to the human cranium.

Evidence of phenotypic plasticity along an altitudinal gradient in the dung beetle Onthophagus proteus

Background

High altitude insects are an ecologically specialized group and possess a suite of adaptions which allow persistence in the inhospitable conditions often associated with mountain tops. Changes in body coloration and reductions or increases in body size are thought to be examples of such adaptions. Melanic individuals, or individuals containing high levels of eumelanin, possess several traits which increase resistance to ultraviolet radiation and desiccation, while aiding thermoregulation. Trait variation is often observed in dung beetles and is associated with dimorphism and sexual selection. In this study, we identified trait changes which occur across an altitudinal gradient by measuring morphological color and body size traits in a montane insect.

Methods

Using standard digital photography and Image J, we examined individuals of Afromontane dung beetle Onthophagus proteus. Individuals were classified according to sex and color morph to identify intrasexual variance. Nine morphometric traits were measured per beetle to identify patterns of morphology across discrete 500 m altitude segments.

Results

The results of this study provide one of the first descriptions of trait changes associated with elevation in an African dung beetle. We suggest that color polymorphism in Onthophagus proteus might be at least partly driven by environmental factors as there is significantly increased melanism with increasing elevation and significant differences in color hues between altitude bands. We also suggest changes in horn length are density dependent, as we observed an increase in cephalic horn length at high elevations where O. proteus is the most abundant species.

Maternal and larval niche construction interact to shape development, survival, and population divergence in the dung beetle Onthophagus taurus

Through niche construction, organisms modify their environments in ways that can alter how selection acts on themselves and their offspring. However, the role of niche construction in shaping developmental and evolutionary trajectories, and its importance for population divergences and local adaptation, remains largely unclear. In this study, we manipulated both maternal and larval niche construction and measured the effects on fitness-relevant traits in two rapidly diverging populations of the bull-headed dung beetle, Onthophagus taurus. We find that both types of niche construction enhance adult size, peak larval mass, and pupal mass, which when compromised lead to a synergistic decrease in survival. Furthermore, for one measure, duration of larval development, we find that the two populations have diverged in their reliance on niche construction: larval niche construction appears to buffer against compromised maternal niche construction only in beetles from Western Australia, but not in beetles from the Eastern United States. We discuss our results in the context of rapid adaptation to novel conditions and the role of niche construction therein.

Don't stand so close to me: Microbiota-facilitated enemy release dynamics in introduced Onthophagus taurus dung beetles

Microbial symbionts can influence their hosts in stunningly diverse ways. Emerging research suggests that an underappreciated facet of these relationships is the influence microbes can have on their host's responses to novel, or stressful, environmental conditions. We sought to address these and related questions in populations resulting from the recent introduction and subsequent rapid range expansion of Onthophagus taurus dung beetles. Specifically, we manipulated both microbial communities and rearing temperature to detect signatures of developmental and life history differentiation in response to the local thermal conditions in two populations derived from the southern most (Florida) and northern most (Michigan) extremes of the exotic Eastern U.S. range of O. taurus. We then sought to determine the contributions, if any, of host-associated microbiota to this differentiation. We found that when reared under common garden conditions individuals from Florida and Michigan populations differed significantly in developmental performance measures and life history traits, consistent with population divergence. At the same time, and contrary to our predictions, we failed to find support for the hypothesis that animals perform better if reared at temperatures that match their location of origin and that performance differences may be mediated by host-associated microbiota. Instead, we found that microbiome swapping across host populations improved developmental performance in both populations, consistent with enemy release dynamics. We discuss the implications of our results for our understanding of the rapid spread of exotic O. taurus through the Eastern United States and the significance of symbiosis in host responses to novel environmental conditions more broadly.

Beetle horns evolved from wing serial homologs

Understanding how novel complex traits originate is a foundational challenge in evolutionary biology. We investigated the origin of prothoracic horns in scarabaeine beetles, one of the most pronounced examples of secondary sexual traits in the animal kingdom. We show that prothoracic horns derive from bilateral source tissues; that diverse wing genes are functionally required for instructing this process; and that, in the absence of Hox input, prothoracic horn primordia transform to contribute to ectopic wings. Once induced, however, the transcriptional profile of prothoracic horns diverges markedly from that of wings and other wing serial homologs. Our results substantiate the serial homology between prothoracic horns and insects wings and suggest that other insect innovations may derive similarly from wing serial homologs and the concomitant establishment of structure-specific transcriptional landscapes.

Sexual dimorphism and heightened conditional expression in a sexually selected weapon in the Asian rhinoceros beetle

Among the most dramatic examples of sexual selection are the weapons used in battles between rival males over access to females. As with ornaments of female choice, the most "exaggerated" sexually selected weapons vary from male to male more widely than other body parts (hypervariability), and their growth tends to be more sensitive to nutritional state or physiological condition compared with growth of other body parts ("heightened" conditional expression). Here, we use RNAseq analysis to build on recent work exploring these mechanisms in the exaggerated weapons of beetles, by examining patterns of differential gene expression in exaggerated (head and thorax horns) and non-exaggerated (wings, genitalia) traits in the Asian rhinoceros beetle, Trypoxylus dichotomus. Our results suggest that sexually dimorphic expression of weaponry involves large-scale changes in gene expression, relative to other traits, while nutrition-driven changes in gene expression in these same weapons are less pronounced. However, although fewer genes overall were differentially expressed in high- vs. low-nutrition individuals, the number of differentially expressed genes varied predictably according to a trait's degree of condition dependence (head horn > thorax horn > wings > genitalia). Finally, we observed a high degree of similarity in direction of effects (vectors) for subsets of differentially expressed genes across both sexually dimorphic and nutritionally responsive growth. Our results are consistent with a common set of mechanisms governing sexual size dimorphism and condition dependence.

Rhinoceros beetle horn development reveals deep parallels with dung beetles

Beetle horns are attractive models for studying the evolution of novel traits, as they display diverse shapes, sizes, and numbers among closely related species within the family Scarabaeidae. Horns radiated prolifically and independently in two distant subfamilies of scarabs, the dung beetles (Scarabaeinae), and the rhinoceros beetles (Dynastinae). However, current knowledge of the mechanisms underlying horn diversification remains limited to a single genus of dung beetles, Onthophagus. Here we unveil 11 horn formation genes in a rhinoceros beetle, Trypoxylus dichotomus. These 11 genes are mostly categorized as larval head- and appendage-patterning genes that also are involved in Onthophagus horn formation, suggesting the same suite of genes was recruited in each lineage during horn evolution. Although our RNAi analyses reveal interesting differences in the functions of a few of these genes, the overwhelming conclusion is that both head and thoracic horns develop similarly in Trypoxylus and Onthophagus, originating in the same developmental regions and deploying similar portions of appendage patterning networks during their growth. Our findings highlight deep parallels in the development of rhinoceros and dung beetle horns, suggesting either that both horn types arose in the common ancestor of all scarabs, a surprising reconstruction of horn evolution that would mean the majority of scarab species (~35,000) actively repress horn growth, or that parallel origins of these extravagant structures resulted from repeated co-option of the same underlying developmental processes.

Goliath and Hercules beetles include some of the largest insects known, and the horns they wield are spectacular. These ‘rhinoceros’ beetles form a subfamily within the Scarabaeidae, a clade containing ~35,000 primarily hornless species. The other subfamily of horned scarabs, dung beetles, is distantly related and their horns are considered a separate origin and parallel radiation. We characterize horn development in a rhinoceros beetle and show that the details are surprisingly similar to the horns of dung beetles. Our results reveal exciting parallels at the level of underlying developmental mechanism. The superficial similarity of these two types of beetle horns mirrors an even deeper similarity in the pathways and genes responsible for their construction.

Neofunctionalization of embryonic head patterning genes facilitates the positioning of novel traits on the dorsal head of adult beetles

The origin and integration of novel traits are fundamental processes during the developmental evolution of complex organisms. Yet how novel traits integrate into pre-existing contexts remains poorly understood. Beetle horns represent a spectacular evolutionary novelty integrated within the context of the adult dorsal head, a highly conserved trait complex present since the origin of insects. We investigated whether otd1/2 and six3, members of a highly conserved gene network that instructs the formation of the anterior end of most bilaterians, also play roles in patterning more recently evolved traits. Using ablation-based fate-mapping, comparative larval RNA interference (RNAi) and transcript sequencing, we found that otd1/2, but not six3, play a fundamental role in the post-embryonic formation of the adult dorsal head and head horns of Onthophagus beetles. By contrast, neither gene appears to pattern the adult head of Tribolium flour beetles even though all are expressed in the dorsal head epidermis of both Onthophagus and Tribolium We propose that, at least in beetles, the roles of otd genes during post-embryonic development are decoupled from their embryonic functions, and that potentially non-functional post-embryonic expression in the dorsal head facilitated their co-option into a novel horn-patterning network during Onthophagus evolution.

Evidence of developmental niche construction in dung beetles: effects on growth, scaling and reproductive success

Niche construction occurs when organisms modify their environments and alter selective conditions through their physiology and behaviours. Such modifications can bias phenotypic variation and enhance organism-environment fit. Yet few studies exist that experimentally assess the degree to which environmental modifications shape developmental and fitness outcomes, how their influences may differ among species and identify the underlying proximate mechanisms. Here, we experimentally eliminate environmental modifications from the developmental environment of Onthophagus dung beetles. We show that these modifications (1) differentially influence growth among species, (2) consistently shape scaling relationships in fitness-related traits, (3) are necessary for the maintenance of sexual dimorphism, (4) influence reproductive success among females of at least one species and (5) implicate larval cultivation of an external rumen as a possible mechanism for environmental modification. Our results present evidence that Onthophagus larvae engage in niche construction, and that this is a fundamental component of beetle development and fitness.

Developmental regulation and evolution of scaling: novel insights through the study of Onthophagus beetles

Scaling relationships play critical roles in defining biological shape, trait functionality, and species characteristics, yet the developmental basis of scaling and its evolution remain poorly resolved in most taxa. In the horned beetle genus Onthophagus, scaling relationships of most traits are largely comparable across many species, however, the morphology and scaling of horns, a recent evolutionary invention, has diversified dramatically, ranging from modestly to highly positively linear to more complex sigmoidal allometries. Through a series of transcriptomic screens and gene function assays, the doublesex, hedgehog, insulin, and serotonin signaling pathways have recently been implicated in the regulation of amplitude, slope, and threshold location of the highly sigmoidal horn allometry in O. taurus. These and other findings suggest that co-option of these pathways into the regulation of horn development may have been critical in the evolutionary transitions from isometric to positively allometric to sigmoidal allometries in Onthophagus, thereby contributing to the extraordinary diversification of one of the most species-rich genera in the animal kingdom.

Hedgehog signaling enables nutrition-responsive inhibition of an alternative morph in a polyphenic beetle

The recruitment of modular developmental genetic components into new developmental contexts has been proposed as a central mechanism enabling the origin of novel traits and trait functions without necessitating the origin of novel pathways. Here, we investigate the function of the hedgehog (Hh) signaling pathway, a highly conserved pathway best understood for its role in patterning anterior/posterior (A/P) polarity of diverse traits, in the developmental evolution of beetle horns, an evolutionary novelty, and horn polyphenisms, a highly derived form of environment-responsive trait induction. We show that interactions among pathway members are conserved during development of Onthophagus horned beetles and have retained the ability to regulate A/P polarity in traditional appendages, such as legs. At the same time, the Hh signaling pathway has acquired a novel and highly unusual role in the nutrition-dependent regulation of horn polyphenisms by actively suppressing horn formation in low-nutrition males. Down-regulation of Hh signaling lifts this inhibition and returns a highly derived sigmoid horn body size allometry to its presumed ancestral, linear state. Our results suggest that recruitment of the Hh signaling pathway may have been a key step in the evolution of trait thresholds, such as those involved in horn polyphenisms and the corresponding origin of alternative phenotypes and complex allometries.

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.

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.

A general mechanism for conditional expression of exaggerated sexually-selected traits

Sexually-selected exaggerated traits tend to be unusually reliable signals of individual condition, as their expression tends to be more sensitive to nutritional history and physiological circumstance than that of other phenotypes. As such, these traits are the foundation for many models of sexual selection and animal communication, such as "handicap" and "good genes" models. Exactly how expression of these traits is linked to the bearer's condition has been a central yet unresolved question, in part because the underlying physiological mechanisms regulating their development have remained largely unknown. Recent discoveries across animals as diverse as deer, beetles, and flies now implicate the widely conserved insulin-like signaling pathway, as a common physiological mechanism regulating condition-sensitive structures with extreme growth. This raises the exciting possibility that one highly conserved pathway may underlie the evolution of trait exaggeration in a multitude of sexually-selected signal traits across the animal kingdom.

Polyandry and sex-specific gene expression

Polyandry is widespread in nature, and has important evolutionary consequences for the evolution of sexual dimorphism and sexual conflict. Although many of the phenotypic consequences of polyandry have been elucidated, our understanding of the impacts of polyandry and mating systems on the genome is in its infancy. Polyandry can intensify selection on sexual characters and generate more intense sexual conflict. This has consequences for sequence evolution, but also for sex-biased gene expression, which acts as a link between mating systems, sex-specific selection and the evolution of sexual dimorphism. We discuss this and the remarkable confluence of sexual-conflict theory and patterns of gene expression, while also making predictions about transcription patterns, mating systems and sexual conflict. Gene expression is a key link in the genotype-phenotype chain, and although in its early stages, understanding the sexual selection-transcription relationship will provide significant insights into this critical association.

Diversification of doublesex function underlies morph-, sex-, and species-specific development of beetle horns

Sex-specific trait expression is frequently associated with highly variable, condition-dependent expression within sexes and rapid divergence among closely related species. Horned beetles are an excellent example for studying the molecular basis of these phenomena because horn morphology varies markedly among species, between sexes, and among alternative, nutritionally-cued morphs within sexes. In addition, horns lack obvious homology to other insect traits and provide a good opportunity to explore the molecular basis of the rapid diversification of a novel trait within and between species. Here we show that the sex-determination gene doublesex (dsx) underlies important aspects of horn development, including differences between sexes, morphs, and species. In male Onthophagus taurus, dsx transcripts were preferentially expressed in the horns of the large, horned morph, and RNAi-mediated knockdown of dsx dramatically altered male horn allometry by massively reducing horn development in large males, but not in smaller males. Conversely, dsx RNAi induced ectopic, nutrition-sensitive horn development in otherwise hornless females. Finally, in a closely related species (Onthophagus sagittarius) that has recently evolved a rare reversed sexual dimorphism, dsx RNAi revealed reversed as well as novel dsx functions despite an overall conservation of dsx expression. This suggests that rapid evolution of dsx functions has facilitated the transition from a regular sexual dimorphism to a reversed sexual dimorphism in this species. Our findings add beetle horns to existing examples of a close relationship between dsx and sexual trait development, and suggest that dsx function has been coopted to facilitate both the evolution of environmentally-cued intrasexual dimorphisms and rapid species divergences in a novel trait.

Beetle horns and horned beetles: emerging models in developmental evolution and ecology

Many important questions in developmental biology increasingly interface with related questions in other biological disciplines such as evolutionary biology and ecology. In this article, we review and summarize recent progress in the development of horned beetles and beetle horns as study systems amenable to the integration of a wide range of approaches, from gene function analysis in the laboratory to population ecological and behavioral studies in the field. Specifically, we focus on three key questions at the current interface of developmental biology, evolutionary biology and ecology: (1) the developmental mechanisms underlying the origin and diversification of novel, complex traits, (2) the relationship between phenotypic diversification and the diversification of genes and transcriptomes, and (3) the role of behavior as a leader or follower in developmental evolution. For each question we discuss how work on horned beetles is contributing to our current understanding of key issues, as well as highlight challenges and opportunities for future studies.

Insulin signaling as a mechanism underlying developmental plasticity: the role of FOXO in a nutritional polyphenism

We investigated whether insulin signaling, known to mediate physiological plasticity in response to changes in nutrition, also facilitates discrete phenotypic responses such as polyphenisms. We test the hypothesis that the gene FOXO--which regulates growth arrest under nutrient stress--mediates a nutritional polyphenism in the horned beetle, Onthophagus nigriventris. Male beetles in the genus Onthophagus vary their mating strategy with body size: large males express horns and fight for access to females while small males invest heavily in genitalia and sneak copulations with females. Given that body size and larval nutrition are linked, we predicted that 1) FOXO expression would differentially scale with body size (nutritional status) between males and females, and 2) manipulation of FOXO expression would affect the nutritional polyphenism in horns and genitalia. First, we found that FOXO expression varied with body size in a tissue- and sex-specific manner, being more highly expressed in the abdominal tissue of large (horned) males, in particular in regions associated with genitalia development. Second, we found that knockdown of FOXO through RNA-interference resulted in the growth of relatively larger copulatory organs compared to control-injected individuals and significant, albeit modest, increases in relative horn length. Our results support the hypothesis that FOXO expression in the abdominal tissue limits genitalia growth, and provides limited support for the hypothesis that FOXO regulates relative horn length through direct suppression of horn growth. Both results support the idea that tissue-specific FOXO expression may play a general role in regulating scaling relationships in nutritional polyphenisms by signaling traits to be relatively smaller.

Sex, war, and disease: the role of parasite infection on weapon development and mating success in a horned beetle (Gnatocerus cornutus)

While parasites and immunity are widely believed to play important roles in the evolution of male ornaments, their potential influence on systems where male weaponry is the object of sexual selection is poorly understood. We experimentally infect larval broad-horned flour beetles with a tapeworm and study the consequent effects on: 1) adult male morphology 2) male-male contests for mating opportunities, and 3) induction of the innate immune system. We find that infection significantly reduces adult male size in ways that are expected to reduce mating opportunities in nature. The sum of our morphological, competition, and immunological data indicate that during a life history stage where no new resources are acquired, males allocate their finite resources in a way that increases future mating potential.

Pangolin expression influences the development of a morphological novelty: beetle horns

Morphological diversity arises during development through the actions and interactions of diverse developmental pathways. Among those, the Wnt pathway is known to contribute to diverse developmental processes such as segmentation and the morphogenesis of appendages. Here, we characterize a transcription factor in the Wnt pathway, pangolin (pan), to investigate the role of Wnt signaling in the development of evolutionarily novel body structures: the horns of beetles. Beetle horns are highly diverse in size, shape, and number and develop principally from two major body regions: the head and prothorax. We investigate horns in two species of the genus Onthophagus using comparative in situ hybridization, larval RNA interference, and allometric measurements to analyze whether horn formation is regulated by pan and by extension the Wnt pathway. Our results illustrate that pan expression affects beetle horn growth in a species-, sex-, and location-specific manner in two morphologically distinct, yet closely-related, Onthophagus species.

Decapentaplegic (dpp) regulates the growth of a morphological novelty, beetle horns

Studies focusing on the development of morphological novelties suggest that patterning genes underlying traditional appendage development (i.e. mouthparts, legs, and wings) also play important roles in patterning novel morphological structures. In this study, we examine whether the expression and function of a member of the TGF-β signaling pathway, decapentaplegic (dpp), promotes development of a morphologically novel structure: beetle horns. Beetle horns are complex secondary sexual structures that develop in the head and/or prothorax, lack obvious homology to other insect outgrowths, and vary remarkably between species and sexes. We studied dpp expression through in situ hybridization, performed functional analyses with RNA interference, and gathered allometric measurements to determine the role of dpp during both pronotal and head horn development in both sexes of two morphologically dissimilar species in the Onthophagus genus, Onthophagus binodis and Onthophagus sagittarius. Our findings show that in addition to affecting growth and patterning of traditional appendages, dpp regulates beetle horn growth and remodeling.

Programed cell death shapes the expression of horns within and between species of horned beetles

Holometabolous insects provide an excellent opportunity to study both the properties of development as well as their evolution and diversification across taxa. Here we investigate the developmental basis and evolutionary diversification of secondary trait loss during development in the expression of beetle horns, a novel and highly diverse class of secondary sexual traits. In many species, horn growth during late larval development is followed by a period of dramatic remodeling during the pupal stage, including the complete resorption of horns in many cases. Here we show that programed cell death plays an important and dynamic role in the secondary resorption of pupal horn primordia during pupal development. Surprisingly, the degree of cell death mediated horn resorption depended on species, sex, and body region, suggesting the existence of regulatory mechanisms that can diversify quickly over short phylogenetic distances. More generally, our results illustrate that secondary, differential loss of structures during development can be a powerful mechanism for generating considerable morphological diversity both within and between species.

Developmental decoupling of alternative phenotypes: insights from the transcriptomes of horn-polyphenic beetles

Developmental mechanisms play an important role in determining the costs, limits, and evolutionary consequences of phenotypic plasticity. One issue central to these claims is the hypothesis of developmental decoupling, where alternate morphs result from evolutionarily independent developmental pathways. We address this assumption through a microarray study that tests whether differences in gene expression between alternate morphs are as divergent as those between sexes, a classic example of developmental decoupling. We then examine whether genes with morph-biased expression are less conserved than genes with shared expression between morphs, as predicted if developmental decoupling relaxes pleiotropic constraints on divergence. We focus on the developing horns and brains of two species of horned beetles with impressive sexual- and morph-dimorphism in the expression of horns and fighting behavior. We find that patterns of gene expression were as divergent between morphs as they were between sexes. However, overall patterns of gene expression were also highly correlated across morphs and sexes. Morph-biased genes were more evolutionarily divergent, suggesting a role of relaxed pleiotropic constraints or relaxed selection. Together these results suggest that alternate morphs are to some extent developmentally decoupled, and that this decoupling has significant evolutionary consequences. However, alternative morphs may not be as developmentally decoupled as sometimes assumed and such hypotheses of development should be revisited and refined.

EST and microarray analysis of horn development in Onthophagus beetles

Background

The origin of novel traits and their subsequent diversification represent central themes in evo-devo and evolutionary ecology. Here we explore the genetic and genomic basis of a class of traits that is both novel and highly diverse, in a group of organisms that is ecologically complex and experimentally tractable: horned beetles.

Results

We developed two high quality, normalized cDNA libraries for larval and pupal Onthophagus taurus and sequenced 3,488 ESTs that assembled into 451 contigs and 2,330 singletons. We present the annotation and a comparative analysis of the conservation of the sequences. Microarrays developed from the combined libraries were then used to contrast the transcriptome of developing primordia of head horns, prothoracic horns, and legs. Our experiments identify a first comprehensive list of candidate genes for the evolution and diversification of beetle horns. We find that developing horns and legs show many similarities as well as important differences in their transcription profiles, suggesting that the origin of horns was mediated partly, but not entirely, by the recruitment of genes involved in the formation of more traditional appendages such as legs. Furthermore, we find that horns developing from the head and prothorax differ in their transcription profiles to a degree that suggests that head and prothoracic horns are not serial homologs, but instead may have evolved independently from each other.

Conclusion

We have laid the foundation for a systematic analysis of the genetic basis of horned beetle development and diversification with the potential to contribute significantly to several major frontiers in evolutionary developmental biology.

Ontogeny of a sexual dimorphism in tiger salamanders

Sexual dimorphism results when the sexes differ in the degree to which trait elaboration confers a reproductive or survival advantage. Trait size dimorphism is often reported in terms of allometry, typically using adults of varying ages (static allometry). A static allometric analysis of tail length in breeding tiger salamanders ( Ambystoma tigrinum (Green, 1825)) revealed that tail length is a positive allometric trait in both sexes, as well as a sexually dimorphic trait. Although static analyses are common in the literature, ontogenetic allometric analyses in which individuals are measured through time are preferred because they provide insight into the heterochronic process underlying trait divergence between the sexes and which sex is diverging from its earlier growth trajectory. I reared 91 individuals from the zygote stage to sexual maturity. An ontogenetic analysis revealed that tail length was isometric in larvae and young metamorphs of both sexes; however, tail length became allometric in males but not in females prior to sexual maturation. I also present static allometric analyses and show how conclusions differ from those of ontogenetic analyses. Lastly, I discuss how sex differences in selection gradients, as well as resource allocation costs, might influence differences between the sexes in the duration and rate of trait growth.

Differential recruitment of limb patterning genes during development and diversification of beetle horns

The origins of novel complex phenotypes represent one of the most fundamental, yet largely unresolved, issues in evolutionary biology. Here we explore the developmental genetic regulation of beetle horns, a class of traits that lacks obvious homology to traits in other insects. Furthermore, beetle horns are remarkably diverse in their expression, including sexual dimorphisms, male dimorphisms, and interspecific differences in location of horn expression. At the same time, beetle horns share aspects of their development with that of more traditional appendages. We used larval RNA interference-mediated gene function analysis of 3 cardinal insect appendage patterning genes, dachshund, homothorax, and Distal-less, to investigate their role in development and diversification of beetle horns within and between species. Transcript depletion of all 3 patterning genes generated phenotypic effects very similar to those documented in previous studies that focused on general insect development. In addition, we found that Distal-less and homothorax, but not dachshund, regulate horn expression in a species-, sex-, body region-, and body size-dependent manner. Our results demonstrate differential co-option of appendage patterning genes during the evolution and radiation of beetle horns. Furthermore, our results illustrate that regulatory genes whose functions are otherwise highly conserved nevertheless retain the capacity to acquire additional functions, and that little phylogenetic distance appears necessary for the evolution of sex- and species-specific differences in these functions.

Chapter 6. The origin and diversification of complex traits through micro- and macroevolution of development: insights from horned beetles

Understanding how development and ecology shape organismal evolution is a central goal of evolutionary developmental biology. This chapter highlights a class of traits and organisms that are emerging as new models in evo-devo and eco-devo research: beetle horns and horned beetles. Horned beetles are morphologically diverse, ecologically rich, and developmentally and genetically increasingly accessible. Recent studies have begun to take advantage of these attributes and are starting to link the microevolution of horned beetle development to the macroevolution of novel features, and to identify the genetic, developmental, and ecological mechanisms, and the interactions between them, that mediate organismal innovation and diversification in natural populations. Here, I review the most significant recent findings and their contributions to current frontiers in evolutionary developmental biology.

On the origins of novelty in development and evolution

The origin of novel traits is what draws many to evolutionary biology, yet our understanding of the mechanisms that underlie the genesis of novelty remains limited. Here I review definitions of novelty including its relationship to homology. I then discuss how ontogenetic perspectives may allow us to move beyond current roadblocks in our understanding of the mechanics of innovation. Specifically, I explore the roles of canalization, plasticity and threshold responses during development in generating a reservoir of cryptic genetic variation free to drift and accumulate in natural populations. Environmental or genetic perturbations that exceed the buffering capacity of development can then release this variation, and, through evolution by genetic accommodation, result in rapid diversification, recurrence of lost phenotypes as well as the origins of novel features. I conclude that, in our quest to understand the nature of innovation, the nature of development deserves to take center stage.

Emerging model systems in evo-devo: horned beetles and the origins of diversity

Horned beetles and beetle horns are emerging as a model system suited to address fundamental questions in evolutionary developmental biology. Here we briefly review the biology of horned beetles and highlight the unusual opportunities they provide for evo-devo research. We then summarize recent advances in the development of new approaches and techniques that are now available to scientists interested in working with these organisms. We end by discussing ways to implement and combine these new approaches to explore new frontiers in evo-devo research previously unavailable to reseachers working outside traditional model organisms.

Juvenile hormone mediates sexual dimorphism in horned beetles

The causes and consequences of sexual dimorphism are major themes in biology. Here we explore the endocrine regulation of sexual dimorphism in horned beetles. Specifically, we explore the role of juvenile hormone (JH) in regulating horn expression in females of two species with regular sexual dimorphism for pronotal horns (females have much shorter horns than males) and a third species with a rare reversed sexual dimorphism for both pronotal and head horns (females have much larger horns in both body regions compared with males). Applications of the JH analog methoprene caused females of the two more typical species to grow significantly shorter pronotal horns than control females, whereas no consistent effect on pronotal horn development was detected in the third, sex-reversed species. Instead, females in this species showed an unexpected and significant increase in head horn expression in response to methoprene treatment. Lastly, horn shape was also affected in females of one of the regularly sexually dimorphic species, but in the opposite direction than horn length. Although methoprene exerted a feminizing effect on female horn length in this species, it significantly masculinized horn shape by inducing a peculiar shape change observed naturally only in males. Our results suggest that JH influences both overall size and shape of female horns, but does so flexibly and as a function of species, sex and horn location. We use our results to review current models on the role of endocrine mechanisms in development and evolution of horned beetle diversity.

Pupal remodeling and the evolution and development of alternative male morphologies in horned beetles

BACKGROUND

How novel morphological traits originate and diversify represents a major frontier in evolutionary biology. Horned beetles are emerging as an increasingly popular model system to explore the genetic, developmental, and ecological mechanisms, as well as the interplay between them, in the genesis of novelty and diversity. The horns of beetles originate during a rapid growth phase during the prepupal stage of larval development. Differential growth during this period is either implicitly or explicitly assumed to be the sole mechanism underlying differences in horn expression within and between species. Here I focus on male horn dimorphisms, a phenomenon at the center of many studies in behavioral ecology and evolutionary development, and quantify the relative contributions of a previously ignored developmental process, pupal remodeling, to the expression of male dimorphism in three horned beetle species.

RESULTS

Prepupal growth is not the only determinant of differences in male horn expression. Instead, following their initial prepupal growth phase, beetles may be extensively remodeled during the subsequent pupal stage in a sex and size-dependent manner. Specifically, male dimorphism in the three Onthophagus species studied here was shaped not at all, partly or entirely by such pupal remodeling rather than differential growth, suggesting that pupal remodeling is phylogenetically widespread, evolutionarily labile, and developmentally flexible.

CONCLUSION

This study is the first to document that male dimorphism in horned beetles is the product of two developmentaly dissociated processes: prepupal growth and pupal remodeling. More generally, adult morphology alone appears to provide few clues, if any, as to the relative contributions of both processes to the expression of alternative male morphs, underscoring the importance of developmental studies in efforts aimed at understanding the evolution of adult diversity patterns.

On the origin and evolutionary diversification of beetle horns

Many scarab beetles produce rigid projections from the body called horns. The exaggerated sizes of these structures and the staggering diversity of their forms have impressed biologists for centuries. Recent comparative studies using DNA sequence-based phylogenies have begun to reconstruct the historical patterns of beetle horn evolution. At the same time, developmental genetic experiments have begun to elucidate how beetle horns grow and how horn growth is modulated in response to environmental variables, such as nutrition. We bring together these two perspectives to show that they converge on very similar conclusions regarding beetle evolution. Horns do not appear to be difficult structures to gain or lose, and they can diverge both dramatically and rapidly in form. Although much of this work is still preliminary, we use available information to propose a conceptual developmental model for the major trajectories of beetle horn evolution. We illustrate putative mechanisms underlying the evolutionary origin of horns and the evolution of horn location, shape, allometry, and dimorphism.

Integrating micro- and macroevolution of development through the study of horned beetles

A major challenge in evolutionary developmental biology is to understand how developmental evolution on the level of populations and closely related species relates to macroevolutionary transitions and the origin of evolutionary novelty. Here, I review the genetic, developmental, endocrine, and ecological basis of beetle horns, a morphological novelty that exhibits remarkable diversity both below and above the species level. Integrating from a variety of approaches three major insights emerge: the origin of beetle horns relied at least in part on the redeployment of already existing genetic, developmental and endocrine mechanisms. At the same time little to no phylogenetic distance appeared to have been necessary for the evolution of diverse modifier mechanisms that permit substantial modulation of trait expression at different time points during development in different species, sexes, alternative male morphs or even different tissue regions of the same individual. Lastly, at least a subset of these modifier mechanisms can evolve rapidly in geographically isolated populations, apparently driven by relatively simple, and probably ubiquitous, changes in ecological conditions. I discuss the implications of these results for our understanding of the genesis of morphological novelty and diversity.