Antigenic diversity and the selective value of sex in parasites

We employ a stochastic model of helminth transmission to explore the persis tence properties of sexual versus asexual parasites in the face of a host p opulation that develops immunity. We assume that this immunity is specific to the parasite strain, such that different parasite strains express unique antigens which in turn elicit specific host immune responses. Sexual paras ites are inherently disadvantaged by a fecundity that is only half that of asexual parasites, given that males do not produce offspring. However, we d emonstrate that sexual parasites benefit from the greater production and ma intenance of antigenic genotypes than mutation alone in asexuals. The abili ty of sexual parasites to produce antigenic diversity enhances population p ersistence of the parasites, given that enhanced antigenic diversity permit s evasion of host immunity. Therefore, we argue that sexual reproduction fo r parasites under intense negative frequency dependent selection induced by host immunity is associated with advantages that may be sufficient to comp ensate for lower intrinsic reproductive potential.