Across the tree of life, species have repeatedly evolved similar phenotypes. While well-studied for ecological traits, there is also evidence for recurrent evolution of sexually selected traits. Swordtail fish (Xiphophorus) are a classic model system for studying sexual selection, and female Xiphophorus exhibit strong mate preferences for large male body size and a range of sexually dimorphic ornaments. Interestingly, sexually selected traits have also been lost multiple times in the genus. However, there has been uncertainty over the number of losses of ornamentation and large body size because phylogenetic relationships between species in this group have historically been controversial, partially due to prevalent gene flow. Here, we use whole-genome sequencing approaches to re-examine phylogenetic relationships within a Xiphophorus clade that varies in the presence and absence of sexually selected traits. Using wild-caught individuals, we determine the phylogenetic placement of a small, unornamented species, X. continens, confirming an additional loss of ornamentation and large body size in the clade. With these revised phylogenetic relationships, we analyze evidence for coevolution between body size and other sexually selected traits using phylogenetic comparative methods. These results provide insights into the evolutionary pressures driving the recurrent loss of suites of sexually selected traits.

Female fertility signals are found across taxa, and the precision of such signals may be influenced by the relative strength of different sexual selection mechanisms. Among primates, more precise signals may be found in species with stronger direct male-male competition and indirect female mate choice, and less precise signals in species with stronger indirect male-male competition (e.g. sperm competition) and direct female mate choice. We tested this hypothesis in a wild population of Kinda baboons in Zambia, combining data on female signals with reproductive hormones (estrogen and progesterone metabolites) and intra- and inter-cycle fertility. We predicted that Kinda baboons will exhibit less precise fertility signals than other baboon species, as they experience weaker direct and stronger indirect male-male competition. The frequency of copulation calls and proceptive behavior did not vary with hormones or intra- or inter-cycle fertility in almost all models. Sexual swelling size was predicted by the ratio of estrogen to progesterone metabolites, and was largest in the fertile phase, but differences in size across days were small. Additionally, there was variability in the timing of ovulation relative to the day of sexual swelling detumescence across cycles and swelling size did not vary with inter-cycle fertility. Our results suggest that female Kinda baboon sexual swellings are less precise indicators of fertility compared to other baboon species, while signals in other modalities do not reflect variation in intra- and inter-cycle fertility. Female Kinda baboon sexual signals may have evolved as a strategy to reduce male monopolizability, allowing for more female control over reproduction by direct mate choice.

Our understanding of the evolution of social mating systems is largely based on an atemporal ecological framework, whereas macroevolutionary and phylogenetic perspectives looking at the causes of mating systems variation are less developed. Here, we present analyses of the evolution of social mating systems in birds at an unprecedented scale, including 66% of the world\'s birds and using trait-dependent speciation and extinction models. We found that lekking (no social bond between the sexes) is very rarely lost, in accordance with the hypothesis that a male shifting to investing in one rather than multiple mates would suffer a severe fitness cost. In contrast, resource-defense polygamous lineages (with a weak, transient socio-sexual bond) frequently revert back to monogamy (strong, durable socio-sexual bond) and have an elevated extinction fraction. We tentatively attribute this to the impossibility for females to settle on an optimal parental care strategy under this system. Finally, we found that most gains of lekking have been directly from monogamy, rather than through an intermediate stage of resource-defense polygamy.

Background: In polygynous species, the development of secondary sexual characters is usually decisive for male reproductive success. However, our understanding about the links between the growth of these traits and reproductive efficiency is still elusive. Most research efforts in this topic have been also focused on adult males, although the development of some secondary sexual characters, like bovid horns, typically starts after birth, continues during the puberty and in some species, such as the common eland, slows or even stops during adulthood. In this study, we investigated the relationships between horn size and testicular function during sexual development in common elands using a comprehensive approach that considers both spermatogenic and sperm parameters. Methods: Twenty-two non-sexually mature common elands were used for the present study. Horn size, body mass, testes mass, and gonadosomatic index were assessed. Spermatogenic activity was determined by cytological and histological analyses. Sperm concentration, morphology, morphometry, and intramale variation in sperm size were evaluated on epididymal sperm samples. Cluster analysis was performed to explore the influence of age on relationships between horn size and reproductive function. Results: We found that bigger horns are associated with increased Sertoli cell efficiency and reduced intramale variation in sperm size. Both parameters were not related to one another while they have shown to be associated with enhanced sperm quality in ungulates. Moreover, horn size was positively linked to the testis mass, sperm concentration, and testicular investment in the seminiferous epithelium. Spiral length and basal circumference were the horn traits most strongly correlated with spermatogenic and sperm parameters as well as those responsible for the sexual dimorphism in this species. Cluster analysis rendered two groups: the first one including males ≤30 months old, while the second one those >30 months old. Horn development and reproductive function were still correlated within age groups, with the strongest relationship found between horn size and sperm size homogeneity in males >30 months old. Conclusion: Taken together, our results indicate that horn size can be regarded as a good index of male reproductive potential during sexual development and provide insights into the role of secondary sexual characters in sexual selection dynamics.

Sexual size dimorphism theory predicts biased operational sex ratios (OSRs) and an uneven distribution of males among certain females. We studied this phenomenon through a field census of the giant wood spider Nephila pilipes (family Nephilidae) in Singapore, a species where females are, on average, 6.9 times larger than males. Specifically, we tested two hypotheses concerning male distribution, given their tendency to aggregate in certain female webs. The optimal female size hypothesis predicts that males would predominantly occupy webs of intermediate-sized females. The web clustering hypothesis posits that more males would be found in webs closer together compared to those farther apart. Our snapshot census revealed a female-biased OSR (females: males = 1.85) with an uneven distribution of males in female webs. Most males were found in webs of intermediate-sized females aligning with the optimal female size hypothesis. Proximity among female webs was indicative of male presence, lending support to the web clustering hypothesis. While our study\'s limited sample size warrants caution, we conclude that in N. pilipes, male occupation of female webs is facilitated by the clustering of webs, and males prefer to cohabit with optimally sized, receptive females.

The breeder\'s equation, Δ z ¯ = G β   , allows us to understand how genetics (the genetic covariance matrix, G) and the vector of linear selection gradients β interact to generate evolutionary trajectories. Estimation of β using multiple regression of trait values on relative fitness revolutionized the way we study selection in laboratory and wild populations. However, multicollinearity, or correlation of predictors, can lead to very high variances of and covariances between elements of β, posing a challenge for the interpretation of the parameter estimates. This is particularly relevant in the era of big data, where the number of predictors may approach or exceed the number of observations. A common approach to multicollinear predictors is to discard some of them, thereby losing any information that might be gained from those traits. Using simulations, we show how, on the one hand, multicollinearity can result in inaccurate estimates of selection, and, on the other, how the removal of correlated phenotypes from the analyses can provide a misguided view of the targets of selection. We show that regularized regression, which places data-validated constraints on the magnitudes of individual elements of β, can produce more accurate estimates of the total strength and direction of multivariate selection in the presence of multicollinearity and limited data, and often has little cost when multicollinearity is low. We also compare standard and regularized regression estimates of selection in a reanalysis of three published case studies, showing that regularized regression can improve fitness predictions in independent data. Our results suggest that regularized regression is a valuable tool that can be used as an important complement to traditional least-squares estimates of selection. In some cases, its use can lead to improved predictions of individual fitness, and improved estimates of the total strength and direction of multivariate selection.

Sexual signals such as colour ornamentation and birdsong evolve independently of each other in some clades, and in others they evolve positively or negatively correlated. We rarely know why correlated evolution does or does not occur. Here, we show positively correlated evolution between plumage colour and song motor performance among canaries, goldfinches and allies, associated with species differences in body size. When controlling for body size, the pattern of correlated evolution between song performance and colour disappeared. Syllable diversity was not as strongly associated with size, and did not evolve in a correlated manner with colour. We argue that correlated evolution between song and colour was mediated by large size limiting song motor performance, likely due to constraints on the speed of moving heavier bills, and by larger species having less saturated plumage colour, possibly due to life-history traits of larger birds (e.g. longevity, stable pairs) contributing to weaker sexual selection. Results are consistent with the hypothesis that correlated evolution between sexual signals is influenced by how, in a clade, selective pressures and constraints affecting each type of signal happen to be co-distributed across species. Such contingency helps explain the diversity in clade-specific patterns of correlated evolution between sexual signals.

Intersexual differentiation is crucial for the speciation and maintenance of dioecious plants, but the underlying mechanisms, including the genes involved, are still poorly understood. Here, we focused on a typical dioicous plant Morus alba, to explore the molecular footprints relevant to sex evolution by revealing the differentially expressed genes (DEGs) between two sexes and the testing signals of selection for these DEGs. From the results, we found a total of 1543 DEGs. Interestingly, 333 and 66 genes expression were detected only in male and female inflorescences, respectively. Using comparative transcriptomics, the expression of 841 genes were found to be significantly higher in male than in female inflorescences and were mainly enriched in defense-related pathways including the biosynthesis of phenylpropanoids, cutin, suberine and waxes. Meanwhile, the expression of 702 genes was female-biased and largely enriched in pathways related to growth and development, such as carbohydrate metabolism, auxin signaling and cellular responses. In addition, 16.7% and 17.6% signals of selection were significantly detected in female- and male-biased genes, respectively, suggesting their non-negligible role in evolution. Our findings expanded the understanding of the molecular basis of intersexual differentiation and contribute to further research on sex evolution in dioecious plants.

AbstractThe strength and direction of sexual selection can vary among populations. However, spatial variability is rarely explored at the level of the social group. Here we investigate sexual selection and sex roles in the paternally mouthbrooding, socially monogamous, and site-attached pajama cardinalfish, Sphaeramia nematoptera. Females were larger and more aggressive and had a longer dorsal fin filament, indicating reversed sex roles. At the scale of social groups, we show that the Bateman gradient and reproductive variance depend on the sex ratio and size of groups. In small and medium-sized groups with balanced or male-biased sex ratios, Bateman gradients were steeper for females, whereas gradients were equally steep for both sexes in large groups or when the sex ratio was female biased. For both sexes, reproductive variance increased with group size and with a higher male-to-female sex ratio. In S. nematoptera, mating opportunities outside the socially monogamous pair appear to impact sexual selection. We conclude that strength and direction of sexual selection can be masked by social dynamics in group-living species when considering only population and large-scale demographic processes.

Socially plastic behaviours are widespread among animals and can have a significant impact on fitness. Here we investigated whether the socially plastic responses of female Drosophila melanogaster can evolve in predictable ways following long term manipulation of adult sex ratio and adult nutrient availability. Previous reports show that female D. melanogaster respond plastically to their same-sex social environment, and lay significantly fewer eggs after mating when previously exposed to other females. In this study, we tested two hypotheses, using females drawn from lines with an evolutionary history of exposure to variation in adult sex ratio (male biased, female biased or equal sex ratio) and adult nutritional environment (high or low quality). The first was that a history of elevated competition in female-biased regimes would select for increased plastic fecundity responses in comparison to females from other lines. The second was that these responses would also be magnified under poor nutritional resource regimes. Neither hypothesis was supported. Instead, we found that plastic fecundity responses were retained in females from all lines, and did not differ significantly across any of them. The lack of differences does not appear to be due to insufficient selection, as we did observe significant evolutionary responses in virgin egg laying patterns according to sex ratio and nutritional regime. The lack of variation in the magnitude of predicted plasticity is consistent with the idea that the costs of maintaining plasticity are low, benefits high, and that plasticity itself can be relatively hard wired.