Persistent Copulation in Asexual Female Potamopyrgus antipodarum: Evidence for Male Control with Size-Based Preferences

Abstract

Transitions from sexual to asexual reproduction provide a useful context for investigating the evolutionary loss of nonfunctional traits. It is often assumed that useless behaviors or structures will degrade, but this process is poorly understood.Potamopyrgus antipodarumis an ancestrally sexual New Zealand freshwater snail characterized by numerous independent transitions to asexual all-female lineages. The availability of multiple independently-derived asexual lineages of various time since derivation from sexual ancestors means that theP. antipodarumsystem is well-suited for the study of trait loss related to mating behavior and copulation. Here, we asked whether mating behavior in asexual femaleP. antipodarumdegrades with increasing asexual lineage age. While copulation frequency did not differ in females from old versus young asexual lineages,post hocanalyses indicated that it was instead positively associated with mean lineage female size. We observed that femaleP. antipodarumtake a passive physical role in copulatory interactions, indicating that female behavior may not be a useful variable for detection of sex-related vestigialization in this system. Instead, males seem to be in proximate control of copulation frequencies, meaning that male mating behavior may be a primary determinant of the expression of mating behavior in asexual femaleP. antipodarum.