The complex ecology of genitalia: Gonopodium length and allometry in the Trinidadian guppy

Male genitalia present an extraordinary pattern of rapid divergence in animals with internal fertilization, which is usually attributed to sexual selection. However, the effect of ecological factors on genitalia divergence could also be important, especially so in animals with nonretractable genitalia because of their stronger interaction with the surrounding environment in comparison with animals with retractable genitalia. Here, we examine the potential of a pervasive ecological factor (predation) to influence the length and allometry of the male genitalia in guppies. We sampled guppies from pairs of low‐predation (LP) and high‐predation (HP) populations in seven rivers in Trinidad, and measured their body and gonopodium length. A key finding was that HP adult males do not have consistently longer gonopodia than do LP adult males, as had been described in previous work. However, we did find such divergence for juvenile males: HP juveniles have longer gonopodia than do LP juveniles. We therefore suggest that an evolutionary trend toward the development of longer gonopodia in HP males (as seen in the juveniles) is erased after maturity owing to the higher mortality of mature males with longer gonopodia. Beyond these generalities, gonopodium length and gonopodium allometry were remarkably variable among populations even within a predation regime, thus indicating strong context dependence to their development/evolution. Our findings highlight the complex dynamics of genitalia evolution in Trinidadian guppies.     

Precopulation Sexual Selection in Nysius huttoni White (Heteroptera: Lygaeidae) in Relation to Morphometric Traits

Nysius huttoni White is a polygamous bug, endemic to New Zealand, and an important pest of wheat and brassicas. This bug has a female-biased sexual size dimorphism but relative to body length, males have longer antennae, suggesting that the allometric scales of antennal–body relationships may be highly selective in sexual selection. Body weight and most morphometric traits measured have no effect on mating success of either sex. Males significantly preferred mating with females having thicker abdomens, more mature eggs, and longer ovipositors. This result suggests that males may select their mates on the basis of immediate reproductive benefit: fertilizing more eggs and ensuring better survival of these eggs. Males with large genital structures have mating advantages over those with small ones, suggesting that precopulation sexual selection in this species act on male genital traits rather than body weight and nonsexual traits. Finally, females significantly preferred males with greater slopes for the antennal-body relationship for mating. The allometry in the male antennal length may be an indicator of male reproductive fitness.     

Intraspecific evidence from guppies for correlated patterns of male and female genital trait diversification

The role of sexual selection in fuelling genital evolution is becoming increasingly apparent from comparative studies revealing interspecific divergence in male genitalia and evolutionary associations between male and female genital traits. Despite this, we know little about intraspecific variance in male genital morphology, or how male and female reproductive traits covary among divergent populations. Here we address both topics using natural populations of the guppy, Poecilia reticulata, a livebearing fish that exhibits divergent patterns of male sexual behaviour among populations. Initially, we performed a series of mating trials on a single population to examine the relationship between the morphology of the male's copulatory organ (the gonopodium) and the success of forced matings. Using a combination of linear measurements and geometric morphometrics, we found that variation in the length and shape of the gonopodium predicted the success of forced matings in terms of the rate of genital contacts and insemination success, respectively. We then looked for geographical divergence in these traits, since the relative frequency of forced matings tends to be greater in high-predation populations. We found consistent patterns of variation in male genital size and shape in relation to the level of predation, and corresponding patterns of (co)variation in female genital morphology. Together, these data enable us to draw tentative conclusions about the underlying selective pressures causing correlated patterns of divergence in male and female genital traits, which point to a role for sexually antagonistic selection.     

Predictors of male insemination success in the mosquitofish (Gambusia holbrooki)

Identifying targets of selection is key to understanding the evolution of sexually selected behavioral and morphological traits. Many animals have coercive mating, yet little is known about whether and how mate choice operates when these are the dominant mating tactic. Here, we use multivariate selection analysis to examine the direction and shape of selection on male insemination success in the mosquitofish (Gambusia holbrooki). We found direct selection on only one of five measured traits, but correlational selection involving all five traits. Larger males with longer gonopodia and with intermediate sperm counts were more likely to inseminate females than smaller males with shorter gonopodia and extreme sperm counts. Our results highlight the need to investigate sexual selection using a multivariate framework even in species that lack complex sexual signals. Further, female choice appears to be important in driving the evolution of male sexual traits in this species where sexual coercion is the dominant mating tactic.     

Longer exaggerated male genitalia confer defensive sperm-competitive benefits in an earwig

As evidence mounts that male genitalia can affect relative fertilisation success, the role that sexual selection has played in the rapid and divergent evolution of genitalia is becoming increasingly recognized. Unfortunately, the limited functional understanding of these complex structures and their interactions with the female reproductive tract often limit interpretation regarding their evolution. Here, we address this issue using the earwig Euborellia brunneri, where both the male intromittent organ and the female spermatheca are highly exaggerated in length yet structurally simple. In a double mating design, we use the sterile male technique to study how sperm precedence patterns are affected by male genital length, male age, and the size of the male sperm storage organ, the seminal vesicle. Relative fertilisation success exhibited considerable variation around modest last-male paternity. Only an interaction between first and second male genital length affected paternity, where males gained reduced paternity when preceded by rivals with longer genitalia. Longer genitalia confer defensive benefits in sperm competition by apparently depositing ejaculate deeper in the tubular spermatheca, safe from removal by rivals. Paternity similarly depended on an interaction between the ages of both males, likely mediated by sperm traits as testes size decreased with age. Seminal vesicle size showed positive allometry but did not affect paternity; instead, greater seminal vesicle size in last males expedited oviposition. The exaggerated yet relatively simple genitalia of E. brunneri facilitate an unusually clear example of post-copulatory selection on phenotypic variation in multiple reproductive traits.     

Sexual selection for small size in male mosquitofish (Gambusia holbrooki)

In many poeciliid fishes, large males which court females coexist with small males which sneak-copulate. It is unclear whether these two tactics represent two evolutionarily stable strategies or if sneaking is a conditional strategy adopted by small, unattractive males. We studied the success of sneaky copulation by looking for sperm in the gonoduct of females after they were kept for 48 h with a male. A logistic regression analysis showed that the probability of a female being inseminated increased with female length and decreased with male length. The length of the male relative to that of the female was the best predictor of success. This result was confirmed using virgin females, thereby excluding any possible confounding effect due to the release of sperm from previous copulations. Sperm counts suggested that large males do not compensate for their reduced copulatory success by releasing larger sperm numbers. Behavioural data indicate that the advantages to small males are twofold: they have a greater chance to approach females from behind without being detected, and manoeuvre better when inserting the gonopodium into the female''s gonoduct. The selective advantage of small size might explain male dwarfism in poeciliids. Our results also suggest that small males adopting the sneaky tactic may be as successful as large males adopting courtship, and that alternative mating strategies may be maintained by negative density-dependent selection.     

Evolution of the elaborate male intromittent organ of Xiphophorus fishes

Internally fertilizing animals show a remarkable diversity in male genital morphology that is associated with sexual selection, and these traits are thought to be evolving particularly rapidly. Male fish in some internally fertilizing species have “gonopodia,” highly modified anal fins that are putatively important for sexual selection. However, our understanding of the evolution of genital diversity remains incomplete. Contrary to the prediction that male genital traits evolve more rapidly than other traits, here we show that gonopodial traits and other nongonopodial traits exhibit similar evolutionary rates of trait change and also follow similar evolutionary models in an iconic genus of poeciliid fish (Xiphophorus spp.). Furthermore, we find that both mating and nonmating natural selection mechanisms are unlikely to be driving the diverse Xiphophorus gonopodial morphology. Putative holdfast features of the male genital organ do not appear to be influenced by water flow, a candidate selective force in aquatic habitats. Additionally, interspecific divergence in gonopodial morphology is not significantly higher between sympatric species, than between allopatric species, suggesting that male genitals have not undergone reproductive character displacement. Slower rates of evolution in gonopodial traits compared with a subset of putatively sexually selected nongenital traits suggest that different selection mechanisms may be acting on the different trait types. Further investigations of this elaborate trait are imperative to determine whether it is ultimately an important driver of speciation.     

Seasonal variation in male alternative reproductive tactics

Genetic parentage analyses reveal considerable diversity in alternative reproductive behaviours (e.g. sneaking) in many taxa. However, little is known about whether these behaviours vary seasonally and between populations. Here, we investigate seasonal variation in male reproductive behaviours in a population of two‐spotted gobies (Gobiusculus flavescens) in Norway. Male two‐spotted gobies guard nests, attract females and care for fertilized eggs. We collected clutches and nest‐guarding males early and late in the breeding season in artificial nests and used microsatellite markers to reconstruct parentage from a subset of offspring from each nest. We hypothesized that mating, reproductive success and sneaking should be more prevalent early in the breeding season when competition for mates among males is predicted to be higher. However, parentage analyses revealed similar values of mating, reproductive success and high frequencies of successful sneaking early (30% of nests) and late (27% of nests) in the season. We also found that multiple females with eggs in the same nest were fertilized by one or more sneaker males, indicating that some males in this population engage in a satellite strategy. We contrast our results to previous work that demonstrates low levels of cuckoldry in a population in Sweden. Our results demonstrate marked stability in both the genetic mating system and male alternative reproductive tactics over the breeding season. However, sneaking rates may vary geographically within a species, likely due to local selection influencing ecological factors encountered at different locations.     

Male size,mating potential and lifetime reproductive success in the field cricket,Gryllus bimaculatus (De Geer)

Male size is known to influence short-term, competitive mating success in the field cricket, Gryllus bimaculatus. Short-term measures of mating success are, however, potentially misleading when considering the influence of sexual selection on male morphology and behaviour. The lifetime mating success of males was investigated in the absence of competition to determine the effects of male size per se on reproductive potential and any effects of female behaviour on male lifetime mating success. The cost of spermatophore production was relatively greater for small males who consequently had a longer refractory period between matings, thus limiting their reproductive potential. Large males had a mating advantage resulting from a differential response of females to the courtship attempts of large and small males. Furthermore, small males were ‘sexually incompetent’ in terms of their ability to attach spermatophores once mounted. There was, therefore, an overall positive relationship between male size and lifetime reproductive success.     

Genetic evidence for the mating system and reproductive success of black sea bream (Acanthopagrus schlegelii)

Understanding the mating system and reproductive success of a species provides evidence for sexual selection. We examined the mating system and the reproductive success of captive adult black sea bream (Acanthopagrus schlegelii), using parentage assignment based on two microsatellites multiplex PCR systems, with 91.5% accuracy in a mixed family (29 sires, 25 dams, and 200 offspring). Based on the parentage result, we found that 93.1% of males and 100% of females participated in reproduction. A total of 79% of males and 92% of females mated with multiple partners (only 1 sire and 1 dam were monogamous), indicating that polygynandry best described the genetic mating system of black sea bream. For males, maximizing the reproductive success by multiple mating was accorded with the sexual selection theory while the material benefits hypothesis may contribute to explain the multiple mating for females. For both sexes, there was a significant correlation between mating success and reproductive success and the variance in reproductive success of males was higher than females. Variation in mating success is the greatest determinant to variation in reproductive success when the relationship is strongly positive. The opportunity for sexual selection of males was twice that of females, as well as the higher slope of the Bateman curve in males suggested that the intensity of intrasexual selection of males was higher than females. Thus, male–male competition would lead to the greater variation of mating success for males, which caused greater variation in reproductive success in males. The effective population number of breeders (N_b) was 33, and the N_b/N ratio was 0.61, slightly higher than the general ratio in polygynandrous fish populations which possibly because most individuals mated and had offspring with a low variance. The relatively high N_b contributes to the maintenance of genetic diversity in farmed black sea bream populations.     

Divergence in genital morphology may contribute to mechanical reproductive isolation in a millipede

Genitalia appear to evolve rapidly and divergently in taxa with internal fertilization. The current consensus is that intense directional sexual selection drives the rapid evolution of genitalia. Recent research on the millipede Antichiropus variabilis suggests that the male genitalia are currently experiencing stabilizing selection – a pattern of selection expected for lock‐and‐key structures that enforce mate recognition and reproductive isolation. Here, we investigate how divergence in genital morphology affects reproductive compatibility among isolated populations of A. variabilis. Females from a focal population were mated first to a male from their own population and, second, to a male from one of two populations with divergent genital morphology. We observed variation in mating behavior that might indicate the emergence of precopulatory reproductive barriers: males from one divergent population took significantly longer to recognize females and exhibited mechanical difficulty in genital insertion. Moreover, we observed very low paternity success for extra‐population males who were successful in copulating. Our data suggest that divergence in genital shape may be contributing to reproductive isolation, and incipient speciation among isolated populations of A. variabilis.     

Sexual selection on male body size and mating behaviour in American toads,Bufo americanus

Mating behaviour of B. americanus was observed from 1985 to 1987. The population contained 38–45 males and 11–26 females, depending on the year. The breeding season of this ‘explosive breeder’ usually encompassed less than 48 h. Male reproductive success varied from zero to two matings per season and zero to an estimated 15 126 zygotes per season. All females mated once per season and variation in their zygote production was estimated to be 4017–11 624 zygotes per season. Body length explained 76% of the variation in zygote production of females. However, male body length was only weakly correlated with mating success in two of three seasons, and with zygote production in one of three seasons. Male arm length was predicted to correlate with male mating success because longer arms should facilitate remaining clasped to females when challenged by rival males. However, mating males did not differ from non-mating males in arm length, and the relationship between arm length and body length was the same for the sexes. Various male behaviours were measured using focal-animal sampling but only call rate correlated with male mating success. The pattern of size dimorphism (females larger than males) is consistent with the observed sex-specific relationships between reproductive success and body size.     

Independent Mating Preferences for Male Body Size and Coloration in Female Trinidadian Guppies

Although females in numerous species generally prefer males with larger, brighter and more elaborate sexual traits, there is nonetheless considerable intra‐ and interpopulation variation in mating preferences amongst females that requires explanation. Such variation exists in the Trinidadian guppy, Poecilia reticulata, an important model organism for the study of sexual selection and mate choice. While female guppies tend to prefer more ornamented males as mates, particularly those with greater amounts of orange coloration, there remains variation both in male traits and female mating preferences within and between populations. Male body size is another trait that is sexually selected through female mate choice in some species, but has not been examined as extensively as body coloration in the guppy despite known intra‐ and interpopulation variation in this trait among adult males and its importance for survivorship in this species. In this study, we used a dichotomous‐choice test to quantify the mating preferences of female guppies, originating from a low‐predation population in Trinidad, for two male traits, body length and area of the body covered with orange and black pigmentation, independently of each other. We expected strong female mating preferences for both male body length and coloration in this population, given relaxation from predation and presumably relatively low cost of choice. Females indeed exhibited a strong preference for larger males as expected, but surprisingly a weaker (but nonetheless significant) preference for orange and black coloration. Interestingly, larger females demonstrated stronger preferences for larger males than did smaller females, which could potentially lead to size‐assortative mating in nature.     

The Effects of Perceived Mating Opportunities on Patterns of Reproductive Investment by Male Guppies

Males pay considerable reproductive costs in acquiring mates (precopulatory sexual selection) and in producing ejaculates that are effective at fertilising eggs in the presence of competing ejaculates (postcopulatory sexual selection). Given these costs, males must balance their reproductive investment in a given mating to optimise their future reproductive potential. Males are therefore expected to invest in reproduction prudently according to the likelihood of obtaining future matings. In this study we tested this prediction by determining whether male reproductive investment varies with expected future mating opportunities, which were experimentally manipulated by visually exposing male guppies (Poecilia reticulata) to high or low numbers of females in the absence of competing males. Our experiment did not reveal consistent effects of perceived future mating opportunity on either precopulatory (male mate choice and mating behaviour) or postcopulatory (sperm quality and quantity) investment. However, we did find that male size and female availability interacted to influence mating behaviour; large males visually deprived of females during the treatment phase became more choosy and showed greater interest in their preferred female than those given continuous visual access to females. Overall, our results suggest males tailor pre- rather than postcopulatory traits according to local female availability, but critically, these effects depend on male size.     

The Influence of Male Body Size and Social Environment on the Mating Behavior of Phallichthys quadripunctatus (Pisces: Poeciliidae)

In some poeciliid fishes, variation in male size is accompanied by differences in mating behavior. Large males are preferred by females and perform courtship displays followed by copulatory thrusts, whereas small males perform copulatory thrusts with few or no displays. This phenomenon has been described in only a few genera and little is known about mating behavior in other poeciliids. Although Phallichthys quadripunctatus males display size dimorphism that has a genetic component, mating behavior of this species has not been documented. We conducted experiments using socially experienced and socially naive males to characterize the mating behavior of this species and to evaluate potential size-dependent differences in behavior. Males were tested with postpartum (presumably receptive) and midcycle (presumably unreceptive) females in different social environments. Whereas neither size class of P. quadripunctatus males performed courtship displays or altered behavior based on female receptivity, large males performed several reproductive behaviors more frequently than small males. This trend was repeatable and occurred in all social environments examined. Some males also attempted to mate with other males, with small males showing a greater tendency to perform this behavior than large males. The manner in which differences in reproductive activity translate into differences in reproductive success must be examined before inferring sexual selection favoring large males in P. quadripunctatus.     

The Consequences of Predation Risk on the Male Territorial Behavior in a Solitary Bee

Territorial males may adopt a mating tactic that yields greater reproductive success but that at the same time increases the risk of predation. Plasticity in reproductive behavior can reflect a balance between sexual selection and natural selection. In this study, we sought to verify the effect of predation risk on territorial behavior of males of the solitary bee Ptilothrix fructifera (Apidae). Males of the species employ alternative mating tactics and can be territorial in defense of larval food sources. By manipulating predator models in the field, we tested whether (1) males avoid perch flowers containing predator models; (2) males alternate between mating tactics when their territory is associated with a predation risk; and (3) female foraging on flowers in a territory is altered in the presence of a predator model. We measured the responses of males and females in flowers containing and not containing a model of a spider or a stuffed bird. The results show that territorial males of P. fructifera alter their territorial behavior when faced with a high risk of predation. They do not abandon their territory or change to a non‐territorial mating tactic, but instead change the use of their territory, avoiding flowers containing predator models or perching on other flowers when the previous flower presented a potential predation risk. Female P. fructifera decreases the frequency of their visits to flowers and the length of time spent there in the presence of a spider model. In short, in the face of predation risk, females and males alter their behaviors at the cost of less efficient foraging and searching for partners, respectively.     

Sperm competition generates evolution of increased paternal investment in a sex role‐reversed seed beetle

When males provide females with resources at mating, they can become the limiting sex in reproduction, in extreme cases leading to the reversal of typical courtship roles. The evolution of male provisioning is thought to be driven by male reproductive competition and selection for female fecundity enhancement. We used experimental evolution under male‐ or female‐biased sex ratios and limited or unlimited food regimes to investigate the relative roles of these routes to male provisioning in a sex role‐reversed beetle, Megabruchidius tonkineus, where males provide females with nutritious ejaculates. Males evolving under male‐biased sex ratios transferred larger ejaculates than did males from female‐biased populations, demonstrating a sizeable role for reproductive competition in the evolution of male provisioning. Although larger ejaculates elevated female lifetime offspring production, we found little evidence of selection for larger ejaculates via fecundity enhancement: males evolving under resource‐limited and unlimited conditions did not differ in mean ejaculate size. Resource limitation did, however, affect the evolution of conditional ejaculate allocation. Our results suggest that the resource provisioning that underpins sex role reversal in this system is the result of male–male reproductive competition rather than of direct selection for males to enhance female fecundity.     

Evolution of male and female genitalia following release from sexual selection

Despite the key functions of the genitalia in sexual interactions and fertilization, the role of sexual selection and conflict in shaping genital traits remains poorly understood. Seed beetle (Callosobruchus maculatus) males possess spines on the intromittent organ, and females possess a thickened reproductive tract wall that also bears spines. We investigated the role of sexual selection and conflict by imposing monogamous mating on eight replicate populations of this naturally polygamous insect, while maintaining eight other populations under polygamy. To establish whether responses to mating system manipulation were robust to ecological context, we simultaneously manipulated life-history selection (early/late reproduction). Over 18-21 generations, male genital spines evolved relatively reduced length in large males (i.e., shallower static allometry) in monogamous populations. Two nonintromittent male genital appendages also evolved in response to the interaction of mating system and ecology. In contrast, no detectable evolution occurred in female genitalia, consistent with the expectation of a delayed response in defensive traits. Our results support a sexually antagonistic role for the male genital spines, and demonstrate the evolution of static allometry in response to variation in sexual selection opportunity. We argue that further advances in the study of genital coevolution will require a much more detailed understanding of the functions of male and female genital traits.     

The impact of Wolbachia,male age and mating history on cytoplasmic incompatibility and sperm transfer in Drosophila simulans

Most insects harbour a variety of maternally inherited endosymbionts, the most widespread being Wolbachia pipientis that commonly induce cytoplasmic incompatibility (CI) and reduced hatching success in crosses between infected males and uninfected females. High temperature and increasing male age are known to reduce the level of CI in a variety of insects. In Drosophila simulans, infected males have been shown to mate at a higher rate than uninfected males. By examining the impact of mating rate independent of age, this study investigates whether a high mating rate confers an advantage to infected males through restoring their compatibility with uninfected females over and above the effect of age. The impact of Wolbachia infection, male mating rate and age on the number of sperm transferred to females during copulation and how it relates to CI expression was also assessed. As predicted, we found that reproductive compatibility was restored faster in males that mate at higher rate than that of low mating and virgin males, and that the effect of mating history was over and above the effect of male age. Nonvirgin infected males transferred fewer sperm than uninfected males during copulation, and mating at a high rate resulted in the transfer of fewer sperm per mating irrespective of infection status. These results indicate that the advantage to infected males of mating at a high rate is through restoration of reproductive compatibility with uninfected females, whereas uninfected males appear to trade off the number of sperm transferred per mating with female encounter rate and success in sperm competition. This study highlights the importance Wolbachia may play in sexual selection by affecting male reproductive strategies.     

SEXUAL SELECTION ON BODY SIZE AND SHAPE IN THE WESTERN HARVESTER ANT,POGONOMYRMEX OCCIDENTALIS CRESSON

Mating in social insects has generally been studied in relation to reproductive allocation and relatedness. Despite the tremendous morphological diversity in social insects, little is known about how individual morphology affects mating success. We examined the correlation of male size and shape with mating success in the western harvester ant, Pogonomyrmex occidentalis. Larger males had significantly higher mating success in two independent collections of males at mating aggregations. We also detected significant linear and nonlinear selection on aspects of male shape that were consistent across years. These shape components are independent of size, suggesting that male mating success is a complex function of size and shape. Successful males had elongate thoraxes and short mandibles relative to males collected at random at the lek. Overall, mated males also had longer postpetioles relative to body size, but there was also evidence of nonlinear selection on relative postpetiole length in both years. We found no evidence of assortative mating based on size or multivariate shape measures in either year, but in one year we found weak assortative mating based on some univariate traits.