The coevolution of parasites with host-acquired immunity and the evolution of sex.
Here I present a deterministic model of the coevolution of parasites with the acquired immunity of their hosts, a system in which coevolutionary oscillations can be maintained. These dynamics can confer an advantage to sexual reproduction within the parasite population, but the effect is not strong enough to outweigh the twofold cost of sex. The advantage arises primarily because sexual reproduction impedes the response to fluctuating epistasis and not because it facilitates the response to directional selection-in fact, sexual reproduction often slows the response to directional selection. Where the cost of sexual reproduction is small, a polymorphism can be maintained between the sexuals and the asexuals. A polymorphism is maintained in which the advantage gained due to recombination is balanced by the cost of sex. At much higher costs of sex, a polymorphism between the asexual and sexual populations can still be maintained if the asexuals do not have a full complement of genotypes available to them, because the asexuals only outcompete those sexuals with which they share the same selected alleles. However, over time we might expect the asexuals to amass the full array of genotypes, thus permanently eliminating sexuals from the population. The sexuals may avoid this fate if the parasite population is finite. Although the model presented here describes the coevolution of parasites with the acquired immune responses of their hosts, it can be compared with other host-parasite models that have more traditionally been used to investigate Red Queen theories of the evolution of sex.