Parasite virulence, host life history, and the costs and benefits of sex.

The widespread existence of sexual reproduction is widely considered to be one of the most pressing anomalies for evolutionary theory. One possible solution is that coevolution between hosts and parasites might favor sexual over asexual reproduction (the Red Queen hypothesis), provided infection is genotype specific and highly virulent. This requirement for high virulence has been seen as a limitation of the theory. In the present study, I solve for the cost of sex per reproductive time step of the host, as well as the minimum virulence required to select for sex. The results show that the cost of sex per time step increases with increases in the host's mortality rate, reaching twofold in annual host species. The results also show that high virulence is not required to select for sexual reproduction, especially in long-lived organisms. These findings might help to explain the paucity of parthenogenesis in organisms having long generation times.