Postcopulatory sexual selection is associated with accelerated evolution of sperm morphology

Abstract

Rapid diversification of sexual traits is frequently attributed to sexual selection, though explicit tests of this hypothesis remain limited. Spermatozoa exhibit remarkable variability in size and shape, and studies report a correlation between sperm morphology (sperm length and shape) and sperm competition risk or female reproductive tract morphology. However, whether postcopulatory processes (e.g., sperm competition and cryptic female choice) influence the speed of evolutionary diversification in sperm form is unknown. Using passerine birds, we quantified evolutionary rates of sperm length divergence among lineages (i.e., species pairs) and determined whether these rates varied with the level of sperm competition (estimated as relative testes mass). We found that relative testes mass was significantly and positively associated with more rapid phenotypic divergence in sperm midpiece and flagellum lengths, as well as total sperm length. In contrast, there was no association between relative testes mass and rates of evolutionary divergence in sperm head size, and models suggested that head length is evolutionarily constrained. Our results are the first to show an association between the strength of sperm competition and the speed of sperm evolution, and suggest that postcopulatory sexual selection promotes rapid evolutionary diversification of sperm morphology.

Sperm head morphology is associated with sperm swimming speed: A comparative study of songbirds using electron microscopy

“…These data were obtained from the literature for a total of 32 species (see list of references in Table S1). Data on testes mass and body mass (n = 33) were taken from Møller (1991) and Rowe et al (2015a). Data on testes mass and body mass (n = 33) were taken from Møller (1991) and Rowe et al (2015a).…”

Sperm divergence in a passerine contact zone: Indication of reinforcement at the gametic level

“…The morphology of male gametes (spermatozoa) may be an example of such sexual traits (Rowe et al. ). Spermatozoa exhibit remarkable variability in size and shape among species (Pitnick et al.…”

“…The diversity in spermatozoa (despite the common function of sperm cells to fertilize the ova) has been mostly attributed to postcopulatory sexual selection, including sperm competition and cryptic female choice (e.g., Snook ; Simmons and Fitzpatrick ; Rowe et al. ). The contribution of sperm divergence to the origin of reproductive isolation and speciation is, however, still not broadly understood.…”

Comparative Sperm Proteomics in Mouse Species with Divergent Mating Systems

“…While it is well-established in mice that sperm fertilization capacity increases rapidly in response to increased sperm competition (Firman et al 2014) and that rates of sperm diversification are correlated with sperm competition intensity more generally (Rowe et al 2015), the ramifications of relaxed sperm competition has received far less attention. The evolution of sperm in the absence of sperm competition has been discussed in the context of a degenerative process (van der Horst and Maree 2014) and it is certainly tempting to speculate that the decreased abundance of a coherent set of proteins critical to sperm function (e.g.…”