All else being equal, asexual females will produce twice as many daughters as sexual females, which make both male and female offspring. Because only females contribute directly to the rate of population growth, the production of males creates a two-fold cost that should theoretically result in the selective elimination of sex. Most research on the advantages of sex has focused on explicitly genetic mechanisms that offset male production costs. A complementary hypothesis builds from the common association between polyploidy and asexuality to posit that competitive interactions between diploid sexual and polyploid asexual animal taxa may be mediated by the availability of dietary phosphorus (P). The role of P may be important when polyploid asexual animals have higher per-mass P-rich nucleic acid content than sexual counterparts and environmental P availability limits biomass production. Here we evaluate aspects of the phosphorus hypothesis with Potamopyrgus antipodarum, a New Zealand freshwater snail. Diploid sexual and polyploid (3x and >3x) asexual individuals often coexist within lakes, and there is substantial across-lake variation in the relative frequency of sexual and asexual snails and in the relative frequency of triploid and >3x asexuals, This across-lake variation in snail population composition sets the stage for the possibility that variation in dietary P availability could influence the maintenance and distribution of mating system and ploidy-level variation.
Results/Conclusions
Consistent with the phosphorus hypothesis, we found in a lab study that individual growth rate decreased more under P scarcity for asexual tetraploids than asexual triploids. In a survey of 15 lakes across the South Island of New Zealand, we collected information on rates of biomass production in algae (the main food source of P. antipodarum), algal elemental composition, and elemental constraints on algal production. We also collected information on alkaline phosphatase expression in snails (an indicator of P limitation) and ploidy composition of local snail populations. Our results suggest that algal production in South Island lakes is strongly limited by nitrogen (N) availability. However, we also found evidence of NP co-limitation that varied in strength across lakes. We found no evidence for P limitation of algae or snail production in the absence of N additions. These results suggest that P limitation could influence competitive interactions within P. antipodarum populations to a variable degree across lakes. Our results also suggest that future work should investigate how N limitation and NP co-limitation differentially affect asexual and sexual individuals in this system.