Cytoplasmic incompatibility.

<p><b>(A)</b> When a <i>Wolbachia</i>-infected male (red) mates with an uninfected female (black), a sperm–egg incompatibility means that some or all of the embryos die. Therefore, infected females produce more offspring than uninfected females (red versus black mosquitoes). <b>(B)</b> This reproductive advantage depends on the prevalence of <i>Wolbachia</i> in the population, because when <i>Wolbachia</i> is rare, females are unlikely to mate with infected males. The <i>Wolbachia</i> strain in <i>Aedes aegypti</i> carries a physiological cost, reducing the fecundity of infected females. If this cost exceeds the advantage of cytoplasmic incompatibility, then the infection is lost from the population. This creates a threshold prevalence below which <i>Wolbachia</i> is lost and above which it invades the population. This cartoon assumes infected females transmit <i>Wolbachia</i> to all their offspring. <i>Image credit</i>: <a href="https://doi.org/10.1093/gbe/evw018" target="_blank">https://doi.org/10.1093/gbe/evw018</a>.