Influences of polyploidy and mating system on life-history variation in a snail model for the maintenance of sex

Background/Question/Methods

Sexual reproduction is both extremely common and very costly, suggesting that there are profound benefits associated with sex that can counter its costs. Here, we focus on potential costs of a trait that very often co-occurs with asexual reproduction, polyploidy. Both theory and empirical data suggest that the higher nuclear DNA content of polyploids may directly or indirectly result in a relatively slow rate of growth and reproductive maturation. We are currently using a common-garden approach to test this hypothesis in diploid sexual and triploid and tetraploid asexual Potamopyrgus antipodarum. This New Zealand freshwater snail is an important model system for studying sex and ploidy-level evolution because sexual diploids and asexual triploids and tetraploids are ecologically similar and often coexist in natural populations.

Results/Conclusions

While this experiment is still in progress, there is significant across-family variation in growth rate, time to maturity, and size at reproduction within diploids, triploids, and tetraploids, revealing genetic variation for these important life-history traits in P. antipodarum. We also found that snails that grew more rapidly also reproduced at a younger age, indicating that more rapid growth translates into earlier reproduction. In contrast to our predictions, polyploid asexual P. antipodarum reproduced significantly earlier than diploid sexuals. Triploid and tetraploid asexual P. antipodarum grew and matured at similar rates, suggesting that polyploidy per se does not explain these differences in time to maturity between sexuals and asexuals. Taken together, these preliminary data suggest that the persistence of sexual diploid P. antipodarum is not obviously linked to benefits associated with more rapid maturation.