Resource limitation in sexual vs. asexual Potamopyrgus antipodarum

The question of why sexual reproduction is so prevalent in natural populations has been called the "queen of questions" in evolutionary biology (Bell, 1982). One potential answer to this question may come from studying whether sexuals and asexuals respond differently to a resource-limited environment. If asexuals require more resources to grow and reproduce at the same rate as sexuals, it could negate the two-fold cost of sexual reproduction. Here, I use Potamopyrgus antipodarum, to empirically address this possibility. First, I consider whether differences in genetic diversity between sexual and asexual components of a population could help to maintain sex in a resource-limited environment. This possible advantage associated with asexuality requires that genetically identical individuals compete more for the same resources than genetically diverse individuals. I evaluated this possibility by comparing fitness-related traits in genetically homogeneous vs. heterogeneous populations of asexual female P. antipodarum, and found that adult size was a more important determinant of individual growth and reproduction than population genetic diversity per se. Next, I used manipulations of density of experimental populations of P. antipodarum to assess whether asexual females experienced a sharper decline in fitness-related traits under high-density conditions, as expected if high sensitivity of asexual individuals to food limitation might contribute to the maintenance of sex. Counter to these predictions, I found that sexual females in fact suffered even more in high-density predictions. Finally, I considered that asexuals may respond differently to food quality, instead of quantity. Ecological stoicheometry postulates that individual growth and reproduction is associated with their bodily phosphorous (P) and the available P in food found in their environment. Because asexuals have more bodily P they may require more P rich food than sexuals to grow and reproduce at the same rate. Although asexuals reproduced less than sexuals, it was not a significant difference.