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Maternally inherited parasites are known to impose a wide variety of reproductive manipulations upon their host. These often produce strong selection on the host to suppress the parasite, resulting in a reduction in the frequency of the parasite. However, in the butterfly Hypolimnas bolina, infected with a Wolbachia bacterium, field data demonstrate that suppression of the male-killing phenotype does not depress parasite frequency. Here we test and verify one hypothesis to explain this apparent paradox— Wolbachia induces a second phenotype, Cytoplasmic Incompatibility (CI), in populations where host suppression has evolved. We further demonstrate that the capacity to induce CI has not evolved de novo, but instead is instantaneously expressed upon the survival of infected males. The significance of these results is threefold: (1) multiple phenotypes can provide Wolbachia with the means to maintain itself in a host following suppression of a single manipulative phenotype; (2) the ability to induce CI can remain hidden in systems in which male-killing is observed, just as the ability to induce male-killing may be obscured in strains exhibiting CI; (3) the evolutionary maintenance of CI in a system in which it is not expressed suggests a functional link with male-killing or other traits under selection. [ABSTRACT FROM AUTHOR]

Within the Diptera, two different selfish genetic elements are known to cause the production of female-biased sex ratios: maternally inherited bacteria that kill male zygotes (male-killers), and X chromosomes causing the degeneration of Y-bearing sperm in males (meiotic drive). We here develop a mathematical model for the dynamics of these two sex-ratio distorters where they co-occur. We show that X chromosome meiotic drive elements can be expected to substantially lower the equilibrium frequency of male-killers and can even lead to their extinction. Conversely, male-killers can also decrease the equilibrium frequency of X drivers and cause their extinction. Thus, we predict that there will be some complementarity in the incidence of X chromosome meiotic drive and male-killing in natural populations, with a lower than expected number of species bearing both elements. [ABSTRACT FROM AUTHOR]