Fitness compensation and the evolution of selfish cytoplasmic elements

Cytoplasmic incompatibility (CI) and male-killing (MK) are both examples of the action of selfish cytoplasmic elements. Both are found in a wide variety of host arthropod species and sometimes coexist within the same species. Although classical models suggest that fitness compensation is potentially important in explaining the invasion of the MK cytotype, no model of CI evolution incorporates this effect. We construct a discrete-time mathematical model which describes a host population containing both CI and MK, permitting fitness compensation for survivors of broods which suffer partial mortality as a result of the behaviour of either element. In the absence of fitness compensation, classical models of CI predict the existence of a threshold frequency at which infected individuals must be introduced in order to spread in a wild-type population. We examine whether fitness compensation will affect the case of spread of CI (reduce the lower threshold) and its equilibrium frequency. Fitness compensation is found to affect the dynamics of CI in a complex manner. This is so, regardless of whether one considers the introduction of individuals infected with the CI element alone, or the introduction of individuals infected with both CI and MK elements into a wild-type population. The model predicts that fitness compensation has only a small effect on the dynamics of CI/wild-type population. In contrast, when individuals infected with both CI and MK agents are introduced into a wild-type population, the presence of plausible levels of fitness compensation may greatly facilitate the spread of CI. We note that this apparent synergism may be expected from the coexistence of CI with other sex-ratio distorters.