A test of genetic models for the evolutionary maintenance of same-sex sexual behaviour

Same-sex sociosexual behaviour is widespread and heritable in male rhesus macaques

Numerous reports have documented the occurrence of same-sex sociosexual behaviour (SSB) across animal species. However, the distribution of the behaviour within a species needs to be studied to test hypotheses describing its evolution and maintenance, in particular whether the behaviour is heritable and can therefore evolve by natural selection. Here we collected detailed observations across 3 yr of social and mounting behaviour of 236 male semi-wild rhesus macaques, which we combined with a pedigree dating back to 1938, to show that SSB is both repeatable (19.35%) and heritable (6.4%). Demographic factors (age and group structure) explained SSB variation only marginally. Furthermore, we found a positive genetic correlation between same-sex mounter and mountee activities, indicating a common basis to different forms of SSB. Finally, we found no evidence of fitness costs to SSB, but show instead that the behaviour mediated coalitionary partnerships that have been linked to improved reproductive success. Together, our results demonstrate that SSB is frequent in rhesus macaques, can evolve, and is not costly, indicating that SSB may be a common feature of primate reproductive ecology.

Indiscriminate Mating and the Coevolution of Sex Discrimination and Sexual Signals

AbstractThe presence of same-sex sexual behavior across the animal kingdom is often viewed as unexpected. One explanation for its prevalence in some taxa is indiscriminate mating-a strategy wherein an individual does not attempt to determine the sex of its potential partner before attempting copulation. Indiscriminate mating has been argued to be the ancestral mode of sexual reproduction and can also be an optimal strategy given search costs of choosiness. Less attention has been paid to the fact that sex discrimination requires not just the attempt to differentiate between the sexes but also some discernible difference (a signal or cue) that can be detected. To address this, we extend models of mating behavior to consider the coevolution of sex discrimination and sexual signals. We find that under a wide range of parameters, including some with relatively minor costs, indiscriminate mating and the absence of sexual signals will be an evolutionary end point. Furthermore, the absence of both sex discrimination and sexual signals is always evolutionarily stable. These results suggest that an observable difference between the sexes likely arose as a by-product of the evolution of different sexes, allowing discrimination to evolve.

Sex-limited chromosomes and non-reproductive traits

Sex chromosomes are typically viewed as having originated from a pair of autosomes, and differentiated as the sex-limited chromosome (e.g. Y) has degenerated by losing most genes through cessation of recombination. While often thought that degenerated sex-limited chromosomes primarily affect traits involved in sex determination and sex cell production, accumulating evidence suggests they also influence traits not sex-limited or directly involved in reproduction. Here, we provide an overview of the effects of sex-limited chromosomes on non-reproductive traits in XY, ZW or UV sex determination systems, and discuss evolutionary processes maintaining variation at sex-limited chromosomes and molecular mechanisms affecting non-reproductive traits.

Genomic evidence consistent with antagonistic pleiotropy may help explain the evolutionary maintenance of same-sex sexual behaviour in humans

Human same-sex sexual behaviour (SSB) is heritable, confers no immediately obvious direct reproductive or survival benefit and can divert mating effort from reproductive opportunities. This presents a Darwinian paradox: why has SSB been maintained despite apparent selection against it? We show that genetic effects associated with SSB may, in individuals who only engage in opposite-sex sexual behaviour (OSB individuals), confer a mating advantage. Using results from a recent genome-wide association study of SSB and a new genome-wide association study on number of opposite-sex sexual partners in 358,426 individuals, we show that, among OSB individuals, genetic effects associated with SSB are associated with having more opposite-sex sexual partners. Computer simulations suggest that such a mating advantage for alleles associated with SSB could help explain how it has been evolutionarily maintained. Caveats include the cultural specificity of our UK and US samples, the societal regulation of sexual behaviour in these populations, the difficulty of measuring mating success and the fact that measured variants capture a minority of the total genetic variation in the traits.

Same-sex sexual behaviour and selection for indiscriminate mating

The widespread presence of same-sex sexual behaviour (SSB) has long been thought to pose an evolutionary conundrum, as participants in SSB suffer the cost of failing to reproduce after expending the time and energy to find a mate. The potential for SSB to occur as part of an optimal strategy has received less attention, although indiscriminate sexual behaviour may be the ancestral mode of sexual reproduction. Here, we build a simple model of sexual reproduction and create a theoretical framework for the evolution of indiscriminate sexual behaviour. We provide strong support for the hypothesis that SSB can be maintained by selection for indiscriminate sexual behaviour, by showing that indiscriminate mating is the optimal strategy under a wide range of conditions. Further, our model suggests that the conditions that most strongly favour indiscriminate mating were probably present at the origin of sexual behaviour. These findings have implications not only for the evolutionary origins of SSB, but also for the evolution of discriminate sexual behaviour across the animal kingdom.

Offspring Production Among the Relatives of Istmo Zapotec Men and Muxes

Male androphilia (i.e., sexual attraction toward adult males) is influenced by biological factors, reliably occurs across diverse cultures, and has persisted over evolutionary time despite the fact that it reduces reproduction. One possible solution to this evolutionary paradox is the sexually antagonistic gene hypothesis (SAGH), which states that genes associated with male androphilia reduce reproduction when present in males but increase reproduction when present in their female relatives. The present study tested the SAGH among the Istmo Zapotec-a non-Euro-American culture in Oaxaca, Mexico, where transgender and cisgender androphilic males are known as muxe gunaa and muxe nguiiu, respectively. To test the SAGH, we compared offspring production by the biological relatives of muxe gunaa (n = 115), muxe nguiiu (n = 112), and gynephilic men (i.e., cisgender males who are sexually attracted to adult females; n = 171). The mothers and paternal aunts of muxe gunaa had higher offspring production than those of muxe nguiiu. Additionally, the relatives of muxe gunaa had more offspring than those of gynephilic men, whereas no such differences were found between the families of gynephilic men and muxe nguiiu. Elevated reproduction by the mothers and, particularly the aunts, of muxe gunaa is consistent with the SAGH. However, the absence of group differences between gynephilic men and muxe nguiiu, and the group differences between the two types of muxes are not predicted by the SAGH. This is the first study to demonstrate reproductive differences between kin of transgender and cisgender androphilic males within the same non-Euro-American culture.

Sexually antagonistic selection on genetic variation underlying both male and female same-sex sexual behavior

BACKGROUND

Intralocus sexual conflict, arising from selection for different alleles at the same locus in males and females, imposes a constraint on sex-specific adaptation. Intralocus sexual conflict can be alleviated by the evolution of sex-limited genetic architectures and phenotypic expression, but pleiotropic constraints may hinder this process. Here, we explored putative intralocus sexual conflict and genetic (co)variance in a poorly understood behavior with near male-limited expression. Same-sex sexual behaviors (SSBs) generally do not conform to classic evolutionary models of adaptation but are common in male animals and have been hypothesized to result from perception errors and selection for high male mating rates. However, perspectives incorporating sex-specific selection on genes shared by males and females to explain the expression and evolution of SSBs have largely been neglected.

RESULTS

We performed two parallel sex-limited artificial selection experiments on SSB in male and female seed beetles, followed by sex-specific assays of locomotor activity and male sex recognition (two traits hypothesized to be functionally related to SSB) and adult reproductive success (allowing us to assess fitness consequences of genetic variance in SSB and its correlated components). Our experiments reveal both shared and sex-limited genetic variance for SSB. Strikingly, genetically correlated responses in locomotor activity and male sex-recognition were associated with sexually antagonistic fitness effects, but these effects differed qualitatively between male and female selection lines, implicating intralocus sexual conflict at both male- and female-specific genetic components underlying SSB.

CONCLUSIONS

Our study provides experimental support for the hypothesis that widespread pleiotropy generates pervasive intralocus sexual conflict governing the expression of SSBs, suggesting that SSB in one sex can occur due to the expression of genes that carry benefits in the other sex.