Evolutionary trade-off between weapons and testes

Ferocious fighting between male grasshoppers

Contests among individuals over mating opportunities are common across diverse taxa, yet physical conflict is relatively rare. Due to the potentially fatal consequences of physical fighting, most animals employ mechanisms of conflict resolution involving signalling and ritualistic assessment. Here we provide the first evidence of ubiquitous escalated fighting in grasshoppers. The chameleon grasshopper (Kosciuscola tristis) is an Australian alpine specialist, in which males engage in highly aggressive combat over ovipositing females. We describe discrete agonistic behaviours including mandible flaring, mounting, grappling, kicking and biting, and their use depending on the individual's role as challenger or defender. We show that male role predicts damage, with challengers being more heavily damaged than males defending females (defenders). Challengers also possess wider mandibles than defenders, but are similar in other metrics of body size. Our data suggest that fights escalate between males matched in body size and that mandibles are used as weapons in this species. This system represents an exciting opportunity for future research into the evolution of costly fighting behaviour in an otherwise placid group.

Sperm traits negatively covary with size and asymmetry of a secondary sexual trait in a freshwater crayfish

In species where females mate promiscuously, the reproductive success of males depends both on their ability to acquire mates (pre-copulatory sexual selection) and ability of their ejaculates to outcompete those of other males (post-copulatory sexual selection). Sperm competition theory predicts a negative relationship between investment in body traits favouring mate acquisition (secondary sexual characters, SSCs) and investment in ejaculate size or quality, due to the inherent costs of sperm production. In contrast, the phenotype-linked fertility hypothesis posits that male fertilizing efficiency is reliably reflected by the phenotypic expression of male SSCs, allowing females to obtain direct benefits by selecting more ornamented males as copulation partners. In this study, we investigated the relationships between male SSCs and size and quality (viability and longevity) of ejaculates allocated to females in mating trials of the freshwater crayfish Austropotamobius italicus. We showed that the relative size of male weapons, the chelae, was negatively related to ejaculate size, and that chelae asymmetry, resulting from regeneration of lost chelipeds, negatively covaried with sperm longevity. Moreover, males allocated more viable sperm to mates from their own rather than different stream of origin. Our findings thus suggest that, according to sperm competition theory, pre-copulatory sexual selection for large weapons used in male fighting may counteract post-copulatory sperm competition in this crayfish species, and that investment in cheliped regeneration may impair ejaculate quality.

Sperm competition games: a general model for precopulatory male-male competition

Reproductive males face a trade-off between expenditure on precopulatory male-male competition--increasing the number of females that they secure as mates--and sperm competition--increasing their fertilization success with those females. Previous sperm allocation models have focused on scramble competition in which males compete by searching for mates and the number of matings rises linearly with precopulatory expenditure. However, recent studies have emphasized contest competition involving precopulatory expenditure on armaments, where winning contests may be highly dependent on marginal increases in relative armament level. Here, we develop a general model of sperm allocation that allows us to examine the effect of all forms of precopulatory competition on sperm allocation patterns. The model predicts that sperm allocation decreases if either the "mate-competition loading,"a, or the number of males competing for each mating, M, increases. Other predictions remain unchanged from previous models: (i) expenditure per ejaculate should increase and then decrease, and (ii) total postcopulatory expenditure should increase, as the level of sperm competition increases. A negative correlation between a and M is biologically plausible, and may buffer deviations from the previous models. There is some support for our predictions from comparative analyses across dung beetle species and frog populations.

The genetics of primary and secondary sexual character trade-offs in a horned beetle

When structures compete for shared resources, this may lead to acquisition and allocation trade-offs so that the enlargement of one structure occurs at the expense of another. Among the studies of morphological trade-offs, their importance has been demonstrated primarily through experimental manipulations and comparative analyses. Relatively, a few studies have investigated the underlying genetic basis of phenotypic patterns. Here, we use a half-sibling breeding design to determine the genetic underpinnings of the phenotypic trade-off between head horns and the male copulatory organ or aedeagus that has been found in the dung beetle Onthophagus taurus. Instead of the predicted negative genetic covariance among characters that trade-off, we find positive genetic covariance between absolute horn and aedeagus length and zero genetic covariance between relative horn and aedeagus length. Therefore, although the genetic covariance between absolute horn and aedeagus length would constrain the independent evolution of primary and secondary sexual characters in this population, there was no evidence of a trade-off. We discuss alternative hypotheses for the observed patterns of genetic correlation between traits that compete for resources and the implications that these have for selection and the evolution of such traits.

Determinants of reproductive success across sequential episodes of sexual selection in a firefly

Because females often mate with multiple males, it is critical to expand our view of sexual selection to encompass pre-, peri- and post-copulatory episodes to understand how selection drives trait evolution. In Photinus fireflies, females preferentially respond to males based on their bioluminescent courtship signals, but previous work has shown that male paternity success is negatively correlated with flash attractiveness. Here, we experimentally manipulated both the attractiveness of the courtship signal visible to female Photinus greeni fireflies before mating and male nuptial gift size to determine how these traits might each influence mate acceptance and paternity share. We also measured pericopulatory behaviours to examine their influence on male reproductive success. Firefly males with larger spermatophores experienced dual benefits in terms of both higher mate acceptance and increased paternity share. We found no effect of courtship signal attractiveness or pericopulatory behaviour on male reproductive success. Taken together with previous results, this suggests a possible trade-off for males between producing an attractive courtship signal and investing in nuptial gifts. By integrating multiple episodes of sexual selection, this study extends our understanding of sexual selection in Photinus fireflies and provides insight into the evolution of male traits in other polyandrous species.

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.

Tradeoffs limit the evolution of male traits that are attractive to females

Tradeoffs occur between a variety of traits in a diversity of organisms, and these tradeoffs can have major effects on ecological and evolutionary processes. Far less is known, however, about tradeoffs between male traits that affect mate attraction than about tradeoffs between other types of traits. Previous results indicate that females of the variable field cricket, Gryllus lineaticeps, prefer male songs with higher chirp rates and longer chirp durations. In the current study, we tested the hypothesis that a tradeoff between these traits affects the evolution of male song. The two traits were negatively correlated among full-sibling families, consistent with a genetically based tradeoff, and the tradeoff was stronger when nutrients were limiting. In addition, for males from 12 populations reared in a common environment, the traits were negatively correlated within populations, the strength of the tradeoff was largely invariant across populations, and the within-population tradeoff predicted how the traits have evolved among populations. A widespread tradeoff thus affects male trait evolution. Finally, for males from four populations assayed in the field, the traits were negatively correlated within and among populations. The tradeoff is thus robust to the presence of environmental factors that might mask its effects. Together, our results indicate there is a fundamental tradeoff between male traits that: (i) limits the ability of males to produce multiple attractive traits; (ii) limits how male traits evolve; and (iii) might favour plasticity in female mating preferences.

Epigenetic mechanisms underlying developmental plasticity in horned beetles

All developmental plasticity arises through epigenetic mechanisms. In this paper we focus on the nature, origins, and consequences of these mechanisms with a focus on horned beetles, an emerging model system in evolutionary developmental genetics. Specifically, we introduce the biological significance of developmental plasticity and summarize the most important facets of horned beetle biology. We then compare and contrast the epigenetic regulation of plasticity in horned beetles to that of other organisms and discuss how epigenetic mechanisms have facilitated innovation and diversification within and among taxa. We close by highlighting opportunities for future studies on the epigenetic regulation of plastic development in these and other organisms.

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.

The role of developmental plasticity in evolutionary innovation

Explaining the origins of novel traits is central to evolutionary biology. Longstanding theory suggests that developmental plasticity, the ability of an individual to modify its development in response to environmental conditions, might facilitate the evolution of novel traits. Yet whether and how such developmental flexibility promotes innovations that persist over evolutionary time remains unclear. Here, we examine three distinct ways by which developmental plasticity can promote evolutionary innovation. First, we show how the process of genetic accommodation provides a feasible and possibly common avenue by which environmentally induced phenotypes can become subject to heritable modification. Second, we posit that the developmental underpinnings of plasticity increase the degrees of freedom by which environmental and genetic factors influence ontogeny, thereby diversifying targets for evolutionary processes to act on and increasing opportunities for the construction of novel, functional and potentially adaptive phenotypes. Finally, we examine the developmental genetic architectures of environment-dependent trait expression, and highlight their specific implications for the evolutionary origin of novel traits. We critically review the empirical evidence supporting each of these processes, and propose future experiments and tests that would further illuminate the interplay between environmental factors, condition-dependent development, and the initiation and elaboration of novel phenotypes.

Sperm competition and ejaculate economics

Sperm competition was identified in 1970 as a pervasive selective force in post-copulatory sexual selection that occurs when the ejaculates of different males compete to fertilise a given set of ova. Since then, sperm competition has been much studied both empirically and theoretically. Because sperm competition often favours large ejaculates, an important challenge has been to understand the evolution of strategies through which males invest in sperm production and economise sperm allocation to maximise reproductive success under competitive conditions. Sperm competition mechanisms vary greatly, depending on many factors including the level of sperm competition, space constraints in the sperm competition arena, male mating roles, and female influences on sperm utilisation. Consequently, theoretical models of ejaculate economics are complex and varied, often with apparently conflicting predictions. The goal of this review is to synthesise the theoretical basis of ejaculate economics under sperm competition, aiming to provide empiricists with categorised model assumptions and predictions. We show that apparent contradictions between older and newer models can often be reconciled and there is considerable consensus in the predictions generated by different models. We also discuss qualitative empirical support for some of these predictions, and detail quantitative matches between predictions and observations that exist in the yellow dung fly. We argue that ejaculate economic theory represents a powerful heuristic to explain the diversity in ejaculate traits at multiple levels: across species, across males and within individual males. Future progress requires greater understanding of sperm competition mechanisms, quantification of trade-offs between ejaculate allocation and numbers of matings gained, further knowledge of mechanisms of female sperm selection and their associated costs, further investigation of non-sperm ejaculate effects, and theoretical integration of pre- and post-copulatory episodes of sexual selection.

Why mammalian lineages respond differently to sexual selection: metabolic rate constrains the evolution of sperm size

The hypothesis that sperm competition should favour increases in sperm size, because it results in faster swimming speeds, has received support from studies on many taxa, but remains contentious for mammals. We suggest that this may be because mammalian lineages respond differently to sexual selection, owing to major differences in body size, which are associated with differences in mass-specific metabolic rate. Recent evidence suggests that cellular metabolic rate also scales with body size, so that small mammals have cells that process energy and resources from the environment at a faster rate. We develop the 'metabolic rate constraint hypothesis' which proposes that low mass-specific metabolic rate among large mammals may limit their ability to respond to sexual selection by increasing sperm size, while this constraint does not exist among small mammals. Here we show that among rodents, which have high mass-specific metabolic rates, sperm size increases under sperm competition, reaching the longest sperm sizes found in eutherian mammals. By contrast, mammalian lineages with large body sizes have small sperm, and while metabolic rate (corrected for body size) influences sperm size, sperm competition levels do not. When all eutherian mammals are analysed jointly, our results suggest that as mass-specific metabolic rate increases, so does maximum sperm size. In addition, species with low mass-specific metabolic rates produce uniformly small sperm, while species with high mass-specific metabolic rates produce a wide range of sperm sizes. These findings support the hypothesis that mass-specific metabolic rates determine the budget available for sperm production: at high levels, sperm size increases in response to sexual selection, while low levels constrain the ability to respond to sexual selection by increasing sperm size. Thus, adaptive and costly traits, such as sperm size, may only evolve under sexual selection when metabolic rate does not constrain cellular budgets.

Good genes and sexual selection in dung beetles (Onthophagus taurus): genetic variance in egg-to-adult and adult viability

Whether species exhibit significant heritable variation in fitness is central for sexual selection. According to good genes models there must be genetic variation in males leading to variation in offspring fitness if females are to obtain genetic benefits from exercising mate preferences, or by mating multiply. However, sexual selection based on genetic benefits is controversial, and there is limited unambiguous support for the notion that choosy or polyandrous females can increase the chances of producing offspring with high viability. Here we examine the levels of additive genetic variance in two fitness components in the dung beetle Onthophagus taurus. We found significant sire effects on egg-to-adult viability and on son, but not daughter, survival to sexual maturity, as well as moderate coefficients of additive variance in these traits. Moreover, we do not find evidence for sexual antagonism influencing genetic variation for fitness. Our results are consistent with good genes sexual selection, and suggest that both pre- and postcopulatory mate choice, and male competition could provide indirect benefits to females.

Sperm competition and the evolution of sperm design in mammals

Background

The influence of sperm competition upon sperm size has been a controversial issue during the last 20 years which remains unresolved for mammals. The hypothesis that, when ejaculates compete with rival males, an increase in sperm size would make sperm more competitive because it would increase sperm swimming speed, has generated contradictory results from both theoretical and empirical studies. In addition, the debate has extended to which sperm components should increase in size: the midpiece to accommodate more mitochondria and produce more energy to fuel motility, or the principal piece to generate greater propulsion forces.

Results

In this study we examined the influence of sperm competition upon sperm design in mammals using a much larger data set (226 species) than in previous analyses, and we corrected for phylogenetic effects by using a more complete and resolved phylogeny, and more robust phylogenetic control methods. Our results show that, as sperm competition increases, all sperm components increase in an integrated manner and sperm heads become more elongated. The increase in sperm length was found to be associated with enhanced swimming velocity, an adaptive trait under sperm competition.

Conclusions

We conclude that sperm competition has played an important role in the evolution of sperm design in mammals, and discuss why previous studies have failed to detect it.

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.

Gene discovery in the horned beetle Onthophagus taurus

Background

Horned beetles, in particular in the genus Onthophagus, are important models for studies on sexual selection, biological radiations, the origin of novel traits, developmental plasticity, biocontrol, conservation, and forensic biology. Despite their growing prominence as models for studying both basic and applied questions in biology, little genomic or transcriptomic data are available for this genus. We used massively parallel pyrosequencing (Roche 454-FLX platform) to produce a comprehensive EST dataset for the horned beetle Onthophagus taurus. To maximize sequence diversity, we pooled RNA extracted from a normalized library encompassing diverse developmental stages and both sexes.

Results

We used 454 pyrosequencing to sequence ESTs from all post-embryonic stages of O. taurus. Approximately 1.36 million reads assembled into 50,080 non-redundant sequences encompassing a total of 26.5 Mbp. The non-redundant sequences match over half of the genes in Tribolium castaneum, the most closely related species with a sequenced genome. Analyses of Gene Ontology annotations and biochemical pathways indicate that the O. taurus sequences reflect a wide and representative sampling of biological functions and biochemical processes. An analysis of sequence polymorphisms revealed that SNP frequency was negatively related to overall expression level and the number of tissue types in which a given gene is expressed. The most variable genes were enriched for a limited number of GO annotations whereas the least variable genes were enriched for a wide range of GO terms directly related to fitness.

Conclusions

This study provides the first large-scale EST database for horned beetles, a much-needed resource for advancing the study of these organisms. Furthermore, we identified instances of gene duplications and alternative splicing, useful for future study of gene regulation, and a large number of SNP markers that could be used in population-genetic studies of O. taurus and possibly other horned beetles.

Larger testes are associated with a higher level of polyandry, but a smaller ejaculate volume, across bushcricket species (Tettigoniidae)

While early models of ejaculate allocation predicted that both relative testes and ejaculate size should increase with sperm competition intensity across species, recent models predict that ejaculate size may actually decrease as testes size and sperm competition intensity increase, owing to the confounding effect of potential male mating rate. A recent study demonstrated that ejaculate volume decreased in relation to increased polyandry across bushcricket species, but testes mass was not measured. Here, we recorded testis mass for 21 bushcricket species, while ejaculate (ampulla) mass, nuptial gift mass, sperm number and polyandry data were largely obtained from the literature. Using phylogenetic-comparative analyses, we found that testis mass increased with the degree of polyandry, but decreased with increasing ejaculate mass. We found no significant relationship between testis mass and either sperm number or nuptial gift mass. While these results are consistent with recent models of ejaculate allocation, they could alternatively be driven by substances in the ejaculate that affect the degree of polyandry and/or by a trade-off between resources spent on testes mass versus non-sperm components of the ejaculate.

Sperm swimming velocity predicts competitive fertilization success in the green swordtail Xiphophorus helleri

Sperm competition is expected to favour the evolution of traits that influence the performance of sperm when they compete to fertilize a female's eggs. While there is considerable evidence that selection favours increases in sperm numbers, much less is known about how sperm quality contributes towards competitive fertilization success. Here, we determine whether variation in sperm quality influences competitive fertilization success in the green swordtail Xiphophorus helleri, a highly promiscuous livebearing fish. We use artificial insemination as a method of controlled sperm delivery and show that sperm swimming velocity is the primary determinant of fertilization success when ejaculates from two males compete to fertilize a female's eggs. By contrast, we found no evidence that sperm length had any effect on siring success. We also found no evidence that pre- and postcopulatory sexual traits were phenotypically integrated in this species, suggesting that the previous observation that reproductive skew favours males with high mating rates is unlikely to be due to any direct association between sperm quality and male sexual ornamentation.

Mating system drives negative associations between morphological features in Schistosomatidae

BACKGROUND

Sexual morphological features are known to be associated with the mating systems of several animal groups. However, it has been suggested that morphological features other than sexual characteristics could also be constrained by the mating system as a consequence of negative associations. Schistosomatidae are parasitic organisms that vary in mating system and can thus be used to explore links between the mating system and negative associations with morphological features.

RESULTS

A comparative analysis of Schistosomatidae morphological features revealed an association between the mating system (monogamous versus polygynandrous) and morphological characteristics of reproduction, nutrition, and locomotion.

CONCLUSIONS

The mating system drives negative associations between somatic and sexual morphological features. In monogamous species, males display a lower investment in sexual tissues and a higher commitment of resources to tissues involved in female transport, protection, and feeding assistance. In contrast, males of polygynandrous species invest to a greater extent in sexual tissues at the cost of reduced commitment to female care.

Quantitative genetic evidence that males trade attractiveness for ejaculate quality in guppies

Polyandry, where females mate with multiple males, means that a male's reproductive success will depend both on his ability to acquire mates and the ability of his sperm to compete effectively for fertilizations. But, how do males partition their reproductive investment between these two episodes of selection? Theory predicts that increases in ejaculate investment will come at a cost to investment in other reproductive traits. Although evidence revealing such trade-offs is accumulating, we know little about their genetic basis. Here, I report patterns of genetic (co)variation for a range of traits subject to pre- and post-copulatory sexual selection in the guppy Poecilia reticulata, a promiscuous livebearing fish in which males alternate between courtship and sneak matings to obtain copulations. The analyses of genetic variation and covariation for these behaviours revealed a strong genetic predisposition for one tactic over the other. Both mating tactics were also strongly genetically integrated with the level of sexual ornamentation and ejaculate quality. Males that predominantly performed sneak matings were less ornamented but had faster swimming sperm than those that predominantly used courtship. These patterns of genetic variation and covariation reveal potential evolutionary constraints on the direction of selection of pre- and post-copulatory traits, and support sperm competition theory by revealing a trade-off between sexual attractiveness and investment in ejaculates.

Ejaculatory strategies associated with experience of losing

Modulation of behaviours as a result of fighting experience has been observed in many animals and can influence pre-copulatory sexual selection. This study investigated how fighting experience affects ejaculatory strategies. In male flour beetles, Gnatocerus cornutus, experience of losing a fight decreases a male's aggressiveness for up to 4 days. We found that males losing a fight show increased ejaculatory investment, but there was no ejaculatory modulation owing to winning. However, the increase in ejaculate investment following a loss was no longer observed after 5 days. These results indicate that males adjust their investment in sperm competition according to their experience, and that fighting experience can significantly influence pre- and post-copulatory reproductive tactics.

Phenotypic plasticity and diversity in insects

Phenotypic plasticity in general and polyphenic development in particular are thought to play important roles in organismal diversification and evolutionary innovation. Focusing on the evolutionary developmental biology of insects, and specifically that of horned beetles, I explore the avenues by which phenotypic plasticity and polyphenic development have mediated the origins of novelty and diversity. Specifically, I argue that phenotypic plasticity generates novel targets for evolutionary processes to act on, as well as brings about trade-offs during development and evolution, thereby diversifying evolutionary trajectories available to natural populations. Lastly, I examine the notion that in those cases in which phenotypic plasticity is underlain by modularity in gene expression, it results in a fundamental trade-off between degree of plasticity and mutation accumulation. On one hand, this trade-off limits the extent of plasticity that can be accommodated by modularity of gene expression. On the other hand, it causes genes whose expression is specific to rare environments to accumulate greater variation within species, providing the opportunity for faster divergence and diversification between species, compared with genes expressed across environments. Phenotypic plasticity therefore contributes to organismal diversification on a variety of levels of biological organization, thereby facilitating the evolution of novel traits, new species and complex life cycles.

Dispersal and ejaculatory strategies associated with exaggeration of weapon in an armed beetle

Weapons used in male fighting can be costly to males and are often reported to trade off with other characters such as wings or spermatogenic investment. This study investigated whether increased investment into weapons can generate evolutionary changes in mating strategy for armed males. Male flour beetles, Gnatocerus cornutus, have enlarged mandibles that are used in male-male competition. We subjected these weapons to 12 generations of bidirectional selection and found trade-offs between weapons and two other male characters: wing and testis size. In addition, probably as a consequence of the observed changes in investment, dispersal ability and ejaculatory volume differ significantly between the lines. This indicates that the exaggeration of a weapon can be associated with dispersal and ejaculatory strategies. Thus, altered investment into weapons can lead to correlated changes in life-history traits.

Adaptive plasticity of mammalian sperm production in response to social experience

Sperm competition theory predicts that males should invest prudently in ejaculates according to levels of female promiscuity. Males may therefore be sensitive to cues in their social environment associated with sexual competition, and tailor investment in sperm production accordingly. We tested this idea experimentally for the first time, to our knowledge, in a mammal by comparing reproductive traits of male house mice (Mus musculus domesticus) that had experienced contrasting encounter regimes with potential sexual competitors. We found that daily sperm production and numbers of sperm in the caput epididymis were significantly higher in subjects that had experienced a high encounter rate of social cues from three other males compared to those that had experienced a low encounter rate of social cues from just one other male. Epididymal sperm counts were negatively correlated with the frequency of scent-marking behaviour across all males in our study, suggesting that investment in ejaculate production may be traded off against traits that function in gaining copulations, although there was no difference in overall levels of scent marking between treatment groups. We conclude that social experience-mediated phenotypic plasticity in mammalian spermatogenesis is likely to be adaptive under sperm competition, enabling males to balance the energetic costs and paternity-enhancing benefits of ejaculate production, and is a potentially widespread explanation for intraspecific variation in ejaculate expenditure.

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.