Evidence for inter-population variation in waiting times in a self-fertilizing flatworm

ABSTRACT Self-fertilization (selfing) is thought to evolve as a way of guaranteeing reproduction when mate availability is low. Nevertheless, because it often leads to inbreeding depression (ID), many hermaphrodites postpone reproduction via selfing when mates are scarce to wait for potential outcrossing opportunities. Owing to varying demographic histories and mate availabilities causing differing optimal selfing propensities, species exhibiting such delayed selfing may show inter-population variation in waiting time (WT) to commence selfing. To test this, we quantified WT across two natural populations of the hermaphroditic flatworm Macrostomum hystrix. In one of the populations we additionally estimated ID and sex allocation (SA), which are expected to covary with selfing propensity. We found no evidence for a WT in either population, but nevertheless observed both ID in the selfed progeny and plasticity in SA in the population we measured these traits. Our results contrast starkly to the pattern previously observed in another natural M. hystrix population, which exhibited a substantial WT and a corresponding reduction in selfed offspring fitness. Our study establishes the presence of natural diversity in selfing propensity in this species and highlights the importance of characterizing such among-population variation to further understand mating system evolution.