Male phenotypes in a female framework: evidence for degeneration in sperm produced by male snails from asexual lineages

How 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, Potamopyrgus antipodarum, to address how evolution in an asexual content— including the potential for relaxed selection on male-specific traits— influences sperm morphology. The occasional production of male offspring by the otherwise all-female asexual P. antipodarum lineages affords a unique and powerful opportunity to assess the fate of sperm traits in a context where males are exceedingly rare. These comparisons revealed that the sperm produced by \"asexual\" males are markedly distinct from sexual counterparts. We also found that 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, ...