Postcopulatory sexual selection is associated with reduced variation in sperm morphology.

Sara Calhim,Tim R Birkhead,Simone Immler,Tom Pizzari

doi:10.1371/journal.pone.0000413

Abstract

BackgroundThe evolutionary role of postcopulatory sexual selection in shaping male reproductive traits, including sperm morphology, is well documented in several taxa. However, previous studies have focused almost exclusively on the influence of sperm competition on variation among species. In this study we tested the hypothesis that intraspecific variation in sperm morphology is driven by the level of postcopulatory sexual selection in passerine birds.Methodology/FindingsUsing two proxy measures of sperm competition level, (i) relative testes size and (ii) extrapair paternity level, we found strong evidence that intermale variation in sperm morphology is negatively associated with the degree of postcopulatory sexual selection, independently of phylogeny.Conclusions/SignificanceOur results show that the role of postcopulatory sexual selection in the evolution of sperm morphology extends to an intraspecific level, reducing the variation towards what might be a species-specific ‘optimum’ sperm phenotype. This finding suggests that while postcopulatory selection is generally directional (e.g., favouring longer sperm) across avian species, it also acts as a stabilising evolutionary force within species under intense selection, resulting in reduced variation in sperm morphology traits. We discuss some potential evolutionary mechanisms for this pattern.

Highlights

The evolutionary role of postcopulatory sexual selection in shaping male reproductive morphology, physiology and behaviour is well documented in several taxa [1,2,3,4]
For both indices of intraspecific variation in sperm morphology ([i] index of variation in sperm size and [ii] index of variation in sperm design [see Methods]) and for both estimates of the intensity of postcopulatory sexual selection ([i] relative testes size and [ii] percent of extrapair offspring [see Methods]), the degree of variation decreased with postcopulatory sexual selection intensity (Figure 1, Table 1)
While our results are consistent with the idea that sperm competition and/or cryptic female choice account for the degree of intermale variation in sperm morphology, the relative importance of these two processes remains to be established

Summary

Introduction

The evolutionary role of postcopulatory sexual selection in shaping male reproductive morphology, physiology and behaviour is well documented in several taxa [1,2,3,4]. The evolutionary role of postcopulatory sexual selection in shaping male reproductive traits, including sperm morphology, is well documented in several taxa. Our results show that the role of postcopulatory sexual selection in the evolution of sperm morphology extends to an intraspecific level, reducing the variation towards what might be a speciesspecific ‘optimum’ sperm phenotype. This finding suggests that while postcopulatory selection is generally directional (e.g., favouring longer sperm) across avian species, it acts as a stabilising evolutionary force within species under intense selection, resulting in reduced variation in sperm morphology traits.

Methods

Results

Discussion

Conclusion