Filamentous fungi spontaneously undergo vegetative cell fusion events within but also between individuals. These cell fusions (anastomoses) lead to cytoplasmic mixing and to the formation of vegetative heterokaryons (i.e., cells containing different nuclear types). The viability of these heterokaryons is genetically controlled by specific loci termed het loci (for heterokaryon incompatibility)....

Many insects and other arthropods harbor maternally inherited bacteria inducing "cytoplasmic incompatibility" (CI), reduced egg hatch when infected males mate with uninfected females. CI-causing infections produce a frequency-dependent reproductive advantage for infected females. However, many such infections impose fitness costs that lead to unstable equilibrium frequencies below which the inf...