Male evolution under relaxed selection: Evidence for degeneration in sperm produced by male snails from asexual lineages

Abstract

How drastic changes in selective regimes affect trait evolution is an important open biological question. We take advantage of naturally occurring and repeated transitions from sexual to asexual reproduction in a New Zealand freshwater snail species to address how relaxed selection on male-specific traits influences sperm morphology. The occasional production of male offspring by the otherwise all-female asexual lineages allows a unique and powerful opportunity to assess the fate of sperm traits in a context where males are unnecessary. These comparisons revealed that the sperm produced by “asexual” males are markedly distinct from sexual counterparts. In particular, the asexual male sperm harbored markedly higher phenotypic variation and was much more likely to be morphologically abnormal. Together, these data suggest that transitions to asexual reproduction might be irreversible at least in part because male function is likely to be compromised. More broadly, our results are consistent with a scenario where relaxed selection translates into rapid trait degeneration.

Spermathecal Variation in Temperate Opiliones

“…Two of the three species examined have facultatively parthenogenetic females, and past publications have focused on the antagonistic morphologies found in rare males of these species (Burns & Tsurusaki 2016). Subsequent papers have discussed the uniquely manifest nature of sexual conflict in facultatively parthenogenetic systems (Burke & Bonduriansky 2018;Gerber & Kokko 2016), where we might otherwise expect relaxed selection on males (Jalinsky et al 2019;Schwander et al 2013) given the biases towards females in populations (Burns et al 2017). While males of facultatively parthenogenetic species have larger pedipalps than expected given their body size (Burns & Tsurusaki 2016), females of these species did not have significantly larger or more complex spermatheca than a closely related sexual species.…”

“…Subsequent papers have discussed the uniquely manifest nature of sexual conflict in facultatively parthenogenetic systems ( Gerber and Kokko 2016 ; Burke and Bonduriansky 2018 ), where we might otherwise expect relaxed selection on males ( Schwander et al. 2013 ; Jalinsky et al. 2019 ) given the biases toward females in populations ( Burns et al 2017 ).…”