A cost of asexuality? Evidence for higher phosphorus and nucleic acid content in asexual versus sexual freshwater snails (Potamopyrgus antipodarum)

Background/Question/Methods Given its many costs, sex should be rare or even nonexistent. The predominance of sexual reproduction thus indicates that sex provides substantive benefits. These benefits are usually thought to derive directly from the genetic consequences of sex, such as the generation of offspring which are genetically distinct or contain rare/novel combinations of alleles. While these genetic benefits are almost certainly critical to the maintenance of sex, there are also often phenotypic differences between sexuals and asexuals that could make the cost of sex dependent on ecological conditions. One way in which sexuals and asexuals can differ is that asexual taxa are nearly always polyploid. While polyploidy is often viewed as providing genetic benefits, we suggest that it could confer ecological costs related to the resource demands of the production of phosphorus-rich nucleic acids. 

Results/Conclusions We addressed this question by estimating bodily phosphorus (P) and nucleic acid concentration in sexual (diploid) vs. asexual (triploid) Potamopyrgus antipodarum, a freshwater snail native to New Zealand that has become a natural model system for the maintenance of sex. As predicted, asexual snails had significantly higher bodily P and nucleic acid concentration than sexuals. We also found that there was a strong positive correlation between nucleic acid and P content across all lineages, suggesting a causal relationship between bodily P and nucleic acid content. These data are the first to document differences in elemental composition between asexual and sexual animals, and the first to demonstrate a clear phenotypic difference between sexual and asexual P. antipodarum. Our results suggest that asexuals may be more sensitive to P availability than their diploid sexual counterparts, and, as a result, the outcome of competition between sexuals and asexuals may be directly influenced by the availability of P. The implications are that the “paradox” of sex may be mitigated in some conditions by the specific resource requirements associated with an additional set of chromosomes.