Sexual selection and the evolution of sperm morphology in sharks

Abstract

Post‐copulatory sexual selection, and sperm competition in particular, is a powerful selective force shaping the evolution of sperm morphology. Although mounting evidence suggests that post‐copulatory sexual selection influences the evolution of sperm morphology among species, recent evidence also suggests that sperm competition influences variation in sperm morphology at the intraspecific level. However, contradictory empirical results and limited taxonomic scope have led to difficulty in assessing the generality of sperm morphological responses to variation in the strength of sperm competition. Here, we use phylogenetically controlled analyses to explore the effects of sperm competition on sperm morphology and variance in sharks, a basal vertebrate group characterized by wide variation in rates of multiple mating by females, and consequently sperm competition risk. Our analyses reveal that shark species experiencing greater levels of sperm competition produce sperm with longer flagella and that sperm flagellum length is less variable in species under higher sperm competition risk. In contrast, neither the length of the sperm head and midpiece nor variation in sperm head and midpiece length was associated with sperm competition risk. Our findings demonstrate that selection influences both the inter‐ and intraspecific variation in sperm morphology and suggest that the flagellum is an important target of sexual selection in sharks. These findings provide important insight into patterns of selection on the ejaculate in a basal vertebrate lineage.

Sperm competition and fertilization mode in fishes

“…The only available tests of predictions from sperm competition theory in sharks focus on sperm number (i.e. testes size) and sperm length and variance [22,45]. Specifically, male sharks invest more in relative testes mass when genetic estimates of greater sperm competition risk ( percentage of broods sired by multiple males) and intensity (number of sires per brood) are higher [22].…”

Weapons Evolve Faster Than Sperm in Bovids and Cervids

“…Low rates of phenotypic diversification may occur if postcopulatory sexual selection on sperm morphology is stabilizing rather than directional. Indeed, strong postcopulatory selection is hypothesized, and has been demonstrated, to reduce variance in sperm traits both within species and ejaculates [ 25 , 87 , 88 , 89 , 90 , 91 , 92 , 93 ]. However, it is currently unclear if such selective pressures are also applying to interspecific diversification, for example in ungulates.…”

Fertilization Modes And The Evolution of Sperm Characteristics In Marine Fishes: Paired Comparisons of The Externally And Internally Fertilizing Species

“…Sperm competition levels of each species were estimated based on not only mating system and population density, according to previous studies (Table 1), but also relative testis mass (RTM). RTM is generally used as a proxy for sperm competition; species with high levels of sperm competition are predicted to have larger testes than species with low levels of sperm competition [6,31,[94][95][96]. We calculated the residuals (i.e.…”