Immediate predation risk alters the relationship between potential and realised selection on male traits in the Trinidad guppy Poecilia reticulata.

Imminent predation risk affects mating behaviours in prey individuals in a multitude of ways that can theoretically impact the strength of sexual selection, as well as its operation on traits. However, empirical studies of the effects of imminent predation risk on sexual selection dynamics are still scarce. Here we explore how perceived predation affects: (1) the relationship between the opportunity for selection and the actual strength of selection on male traits; and (2) which traits contribute to male fitness and the shape of selection on these traits. We simulate two consecutive reproductive episodes, under control conditions and perceived predation risk using experimental populations of Trinidad guppies. The opportunity for selection is higher under predation risk compared to the control condition, but realised selection on traits remains unaffected. Pre- and postcopulatory traits follow complex patterns of nonlinear selection in both conditions. Differences in selection gradients deviate from predictions based on evolutionary and non-lethal effects of predation, the most notable being strong disruptive selection on courtship rate under predation risk. Our results demonstrate that sexual selection is sensitive to imminent predation risk perception and reinforce the notion that both trait-based and variance-based metrics should be employed for an informative quantification.

Female sperm storage mediates post-copulatory costs and benefits of ejaculate anticipatory plasticity in the guppy.

Males of many species evolved the capability of adjusting their ejaculate phenotype in response to social cues to match the expected mating conditions. When females store sperm for a prolonged time, the expected fitness return of plastic adjustments of ejaculate phenotype may depend on the interval between mating and fertilization. Although prolonged female sperm storage (FSS) increases the opportunity for sperm competition, as a consequence of the longer temporal overlap of ejaculates from several males, it may also create variable selective forces on ejaculate phenotype, for example by exposing trade-offs between sperm velocity and sperm survival. We evaluated the relationship between the plasticity of ejaculate quality and FSS in the guppy, Poecilia reticulata, a polyandrous live-bearing fish in which females store sperm for several months and where stored sperm contribute significantly to a male's lifelong reproductive success. In this species, males respond to the perception of future mating opportunities by increasing the quantity (number) and quality (swimming velocity) of ready-to-use sperm (an anticipatory response called 'sperm priming'). Here we investigated (a) the effect of sperm priming on in vitro sperm viability at stripping and its temporal decline (as an estimate of sperm survival), and (b) the in vivo competitive fertilization success in relation to female sperm storage using artificial insemination. As expected, sperm-primed males produced more numerous and faster sperm, but with a reduced in vitro sperm viability at stripping and after 4 hr, compared with their counterparts. Artificial insemination revealed that the small (nonsignificant) advantage of primed sperm when fertilization immediately follows insemination is reversed when eggs are fertilized by female-stored sperm, weeks after insemination. By suggesting a plastic trade-off between sperm velocity and viability, these results demonstrate that prolonged female sperm storage generates divergent selection pressures on ejaculate phenotype.

Population demography and heterozygosity-fitness correlations in natural guppy populations: An examination using sexually selected fitness traits.

Heterozygosity-fitness correlations (HFCs) have been examined in a wide diversity of contexts, and the results are often used to infer the role of inbreeding in natural populations. Although population demography, reflected in population-level genetic parameters such as allelic diversity or identity disequilibrium, is expected to play a role in the emergence and detectability of HFCs, direct comparisons of variation in HFCs across many populations of the same species, with different genetic histories, are rare. Here, we examined the relationship between individual microsatellite heterozygosity and a range of sexually selected traits in 660 male guppies from 22 natural populations in Trinidad. Similar to previous studies, observed HFCs were weak overall. However, variation in HFCs among populations was high for some traits (although these variances were not statistically different from zero). Population-level genetic parameters, specifically genetic diversity levels (number of alleles, observed/expected heterozygosity) and measures of identity disequilibrium (g2 and heterozygosity-heterozygosity correlations), were not associated with variation in population-level HFCs. This latter result indicates that these metrics do not necessarily provide a reliable predictor of HFC effect sizes across populations. Importantly, diversity and identity disequilibrium statistics were not correlated, providing empirical evidence that these metrics capture different essential characteristics of populations. A complex genetic architecture likely underpins multiple fitness traits, including those associated with male fitness, which may have reduced our ability to detect HFCs in guppy populations. Further advances in this field would benefit from additional research to determine the demographic contexts in which HFCs are most likely to occur.