Why does inbreeding reduce male paternity? Effects on sexually selected traits

Sperm of more colourful males are better adapted to ovarian fluids in lake char (Salmonidae)

Fish often spawn eggs with ovarian fluids that have been hypothesized to support the sperm of some males over others (cryptic female choice). Alternatively, sperm reactions to ovarian fluids could reveal male strategies. We used wild-caught lake char (Salvelinus umbla) to experimentally test whether sperm react differently to the presence of ovarian fluid, and whether any differential sperm reaction could be predicted by male breeding coloration, male inbreeding coefficients (based of 4150 SNPs) or the kinship coefficients between males and females. Male coloration was positively linked to body size and current health (based on lymphocytosis and thrombocytosis) but was a poor predictor of inbreeding or kinship coefficients. We found that sperm of more colourful males were faster in diluted ovarian fluids than in water only, while sperm of paler males were faster in water than in ovarian fluids. We then let equal numbers of sperm compete for fertilizations in the presence or absence of ovarian fluids and genetically assigned 1464 embryos (from 70 experimental trials) to their fathers. The presence of ovarian fluids significantly increased the success of the more colourful competitors. Sperm of less inbred competitors were more successful when tested in water only than in diluted ovarian fluids. The kinship coefficients had no significant effects on sperm traits or fertilization success in the presence of ovarian fluids, although parallel stress tests on embryos had revealed that females would profit more from mating with least related males rather than most coloured ones. We conclude that sperm of more colourful males are best adapted to ovarian fluids, and that the observed reaction norms suggest male strategies rather than cryptic female choice.

Disentangling the effects of male age and mating history: Contrasting effects of mating history on precopulatory mating behavior and paternity success

Many studies ask whether young or older males are better at acquiring mates. Even so, how age affects reproductive success is still poorly understood because male age and mating history are confounded in most studies: older males usually have more mating experience. To what extent does mating history rather than age explain variation in male mating success? And how do mating history and male age determine paternity when there is also postcopulatory sexual selection? Here, we experimentally manipulated the mating history of old and young males in the eastern mosquitofish (Gambusia holbrooki). We then recorded male mating behavior and share of paternity (1259 offspring from 232 potential sires) when they competed for mates and fertilizations. Old males, and males with no mating experience, spent significantly more time approaching females, and attempting to mate, than did young males and those with greater mating experience. Male age and mating history interacted to affect paternity: old males benefited from having previous mating experience, but young males did not. Our results highlight that the age-related changes in male reproductive traits and in paternity that have been described in many taxa may be partly attributable to male mating history and not simply to age itself.

Male age alone predicts paternity success under sperm competition when effects of age and past mating effort are experimentally separated

Older males often perform poorly under post-copulatory sexual selection. It is unclear, however, whether reproductive senescence is because of male age itself or the accumulated costs of the higher lifetime mating effort that is usually associated with male age. To date, very few studies have accounted for mating history and sperm storage when testing the effect of male age on sperm traits, and none test how age and past mating history influence paternity success under sperm competition. Here, we experimentally manipulate male mating history to tease apart its effects from that of age on ejaculate traits and paternity in the mosquitofish, Gambusia holbrooki. We found that old, naive males had more sperm than old, experienced males, while the reverse was true for young males. By contrast, neither male age nor mating history affected sperm velocity. Finally, using artificial insemination to experimentally control the number of sperm per male, we found that old males sired significantly more offspring than young males independently of their mating history. Our results highlight that the general pattern of male reproductive senescence described in many taxa may often be affected by two naturally confounding factors, male mating history and sperm age, rather than male age itself.

Male size and reproductive performance in three species of livebearing fishes (Gambusia spp.): A systematic review and meta-analysis

The genus Gambusia represents approximately 45 species of polyandrous livebearing fishes with reversed sexual size dimorphism (i.e. males smaller than females) and with copulation predominantly via male coercion. Male body size has been suggested as an important sexually selected trait, but despite abundant research, evidence for sexual selection on male body size in this genus is mixed. Studies have found that large males have an advantage in both male-male competition and female choice, but that small males perform sneaky copulations better and at higher frequency and thus may sire more offspring in this coercive mating system. Here, we synthesized this inconsistent body of evidence using pre-registered methods and hypotheses. We performed a systematic review and meta-analysis of summary and primary (raw) data combining both published (n = 19 studies, k = 106 effect sizes) and unpublished effect sizes (n = 17, k = 242) to test whether there is overall selection on male body size across studies in Gambusia. We also tested several specific hypotheses to understand the sources of heterogeneity across effects. Meta-analysis revealed an overall positive correlation between male size and reproductive performance (r = 0.23, 95% confidence interval: 0.10-0.35, n = 36, k = 348, 4,514 males, three Gambusia species). Despite high heterogeneity, the large-male advantage appeared robust across all measures studied (i.e. female choice, mating success, paternity, sperm quantity and quality), and was considerably larger for female choice (r = 0.43, 95% confidence interval: 0.28-0.59, n = 14, k = 43). Meta-regressions found several important factors explaining heterogeneity across effects, including type of sperm characteristic, male-to-female ratio, female reproductive status and environmental conditions. We found evidence of publication bias; however, its influence on our estimates was attenuated by including a substantial amount of unpublished effects, highlighting the importance of open primary data for more accurate meta-analytic estimates. In addition to positive selection on male size, our study suggests that we need to rethink the role and form of sexual selection in Gambusia and, more broadly, to consider the ecological factors that affect reproductive behaviour in livebearing fishes.

Shifts in Reproductive Investment in Response to Competitors Lower Male Reproductive Success

AbstractIn many species, males exhibit phenotypic plasticity in sexually selected traits when exposed to social cues about the intensity of sexual competition. To date, however, few studies have tested how this plasticity affects male reproductive success. We initially tested whether male mosquitofish, Gambusia holbrooki (Poeciliidae), change their investment in traits under pre- and postcopulatory sexual selection depending on the social environment. For a full spermatogenesis cycle, focal males were exposed to visual and chemical cues of rivals that were either present (competitive treatment) or absent (control). Males from the competitive treatment had significantly slower-swimming sperm but did not differ in sperm count from control males. When two males competed for a female, competitive treatment males also made significantly fewer copulation attempts and courtship displays than control males. Further, paternity analysis of 708 offspring from 148 potential sires, testing whether these changes in reproductive traits affected male reproductive success, showed that males previously exposed to cues about the presence of rivals sired significantly fewer offspring when competing with a control male. We discuss several possible explanations for these unusual findings.

Fitness benefits of male dominance behaviours depend on the degree of individual inbreeding in a polyandrous lizard

In polyandrous species, sexual selection extends beyond mating competition to selection for egg fertilization. As a result, the degree to which factors influencing mating success impact overall reproductive success becomes variable. Here, we used a longitudinal behavioural and genetic dataset for a population of eastern water dragons (Intellagama lesueurii) to investigate the degree to which male dominance, a pre-mating selection trait, influences overall reproductive success, measured as the number of surviving offspring. Moreover, we examine the interactive effects with a genetic trait, individual inbreeding, known to influence the reproductive success of males in this species. We found fitness benefits of male dominance, measured as body size and frequency of dominance behaviours displayed. However, individuals' propensity to display dominance behaviours had mixed effects, depending on the degree of inbreeding. While inbred males benefited from frequent displays, highly outbred males exhibited better reproductive outputs when displaying to a lesser extent. Given that outbred males have enhanced reproductive success in this species, the costs of displaying dominance behaviours may outweigh the benefits. Overall, our results demonstrate the fitness benefits of dominance in a polyandrous lizard, and suggest that these are modulated by an independent genetic trait. Our results may contribute to explaining the presence of alternative mating tactics in this species, owing to the variability in net fitness benefits of dominance. Our findings also reveal the challenges associated with investigating fitness traits in isolation, which may undermine the validity of results when important interactions are ignored.

Geographical and temporal variation of multiple paternity in invasive mosquitofish (Gambusia holbrooki, Gambusia affinis)

Multiple paternity (MP) increases offspring's genetic variability, which could be linked to invasive species' evolvability in novel distribution ranges. Shifts in MP can be adaptive, with greater MP in harsher/colder environments or towards the end of the reproductive season, but climate could also affect MP indirectly via its effect on reproductive life histories. We tested these hypotheses by genotyping N = 2,903 offspring from N = 306 broods of two closely related livebearing fishes, Gambusia holbrooki and Gambusia affinis. We sampled pregnant females across latitudinal gradients in their invasive ranges in Europe and China, and found more sires per brood and a greater reproductive skew towards northern sampling sites. Moreover, examining monthly sampling from two G. affinis populations, we found MP rates to vary across the reproductive season in a northern Chinese, but not in a southern Chinese population. While our results confirm an increase of MP in harsher/more unpredictable environments, path analysis indicated that, in both cases, the effects of climate are likely to be indirect, mediated by altered life histories. In both species, which rank amongst the 100 most invasive species worldwide, higher MP at the northern edge of their distribution probably increases their invasive potential and favours range expansions, especially in light of the predicted temperature increases due to global climate changes.

Why does inbreeding reduce male paternity? Effects on sexually selected traits

Mating with relatives has often been shown to negatively affect offspring fitness (inbreeding depression). There is considerable evidence for inbreeding depression due to effects on naturally selected traits, particularly those expressed early in life, but there is less evidence of it for sexually selected traits. This is surprising because sexually selected traits are expected to exhibit strong inbreeding depression. Here, we experimentally created inbred and outbred male mosquitofish (Gambusia holbrooki). Inbred males were the offspring of matings between full siblings. We then investigated how inbreeding influenced a number of sexually selected male traits, specifically: attractiveness, sperm number and velocity, as well as sperm competitiveness based on a male's share of paternity. We found no inbreeding depression for male attractiveness or sperm traits. There was, however, evidence that lower heterozygosity decreased paternity due to reduced sperm competitiveness. Our results add to the growing evidence that competitive interactions exacerbate the negative effects of the increased homozygosity that arises when there is inbreeding.