FITNESS COMPENSATION AND THE EVOLUTION OF SELFISH CYTOPLASMIC ELEMENTS

Cytoplasmic incompatibility (CI) and male-killing (MK) are both exampl es of the action of selfish cytoplasmic elements. Both are found in a wide variety of host arthropod species and sometimes coexist within th e same species. Although classical models suggest that fitness compens ation is potentially important in explaining the invasion of the MK cy totype, no model of CI evolution incorporates this effect. We construc t a discrete-time mathematical model which describes a host population containing both CI and MK, permitting fitness compensation for surviv ors of broods which suffer partial mortality as a result of the behavi our of either element. In the absence of fitness compensation, classic al models of CI predict the existence of a threshold frequency at whic h infected individuals must be introduced in order to spread in a wild -type population. We examine whether fitness compensation will affect the ease of spread of CI (reduce the lower threshold) and its equilibr ium frequency. Fitness compensation is found to affect the dynamics of CI in a complex manner. This is so, regardless of whether one conside rs the introduction of individuals infected with the CI element alone, or the introduction of individuals infected with both CI and MK eleme nts into a wild-type population. The model predicts that fitness compe nsation has only a small effect on the dynamics of CI/wild-type popula tion. In contrast, when individuals infected with both CI and MK agent s are introduced into a wild-type population, the presence of plausibl e levels of fitness compensation may greatly facilitate the spread of CI. We note that this apparent synergism may be expected from the coex istence of CI with other sex-ratio distorters.