Sexual selection and the evolution of sperm morphology in sharks.

Amy Rowley,Cristina Porcu,Ryan M Kempster,Ariel Kahrl,Mariana Rego,Francisco Garcia‐Gonzalez,Maria C Follesa,John L Fitzpatrick,Fabio Hazin,Toby Daly‐Engel,Eva Giacomello,Annika Boussard,Lisa Locatello,Shaun P Collin,Jonathan P Evans,Eduardo Garza‐Gisholt,Carlotta Mazzoldi

doi:10.1111/jeb.13501

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.

Highlights

Sperm exhibit extraordinary morphological diversity and are among the most variable of all known cell types (Pitnick, Hosken, & Birkhead, 2009)
Our findings demonstrate that selection influ‐ ences both the inter‐ and intraspecific variation in sperm morphology and suggest that the flagellum is an important target of sexual selection in sharks
Our results demonstrate that sperm competition acts to shape the evolution of the sperm morphology in sharks

Summary

| INTRODUCTION

Sperm exhibit extraordinary morphological diversity and are among the most variable of all known cell types (Pitnick, Hosken, & Birkhead, 2009). When females mate with multiple males, the temporal and spatial overlap of sperm from rival males within the fertilization environ‐ ment can result in sperm competition, where sperm from different males compete to fertilize the available ova (Parker, 1970), and cryp‐ tic female choice, where females bias the outcome of sperm compe‐ tition in favour of preferred males (Eberhard, 1996) These episodes of post‐copulatory sexual selection impose strong selec‐ tive pressures on males to produce more effective ejaculates. Sperm number is not the only target of post‐cop‐ ulatory sexual selection, as sperm competition and cryptic female choice can influence the evolution of sperm morphology and size (Pitnick et al, 2009; Simmons & Fitzpatrick, 2012). We examine the relationship between the size of each sperm component (head, midpiece and fla‐ gellum) and the level of sperm competition and consider how sperm competition acts on within‐male variation in sperm component size

| MATERIALS AND METHODS

| DISCUSSION

Findings

CONFLICTS OF INTEREST