The evolutionary dynamics of male-killers and their hosts

Male-killing bacteria are cytoplasmic sex-ratio distorters that are transmi tted vertically through females of their insect hosts. The killing of male hosts by their bacteria is thought to be an adaptive bacterial trait becaus e it augments the fitness of female hosts carrying clonal relatives of thos e bacteria. Here we attempt to explain observations of multiple male-killer s in natural host populations. First we show that such male-killer polymorp hism cannot be explained by a classical model of male-killing. We then show that more complicated models incorporating the evolution of resistance in hosts can explain male-killer polymorphism. However, this is only likely if resistance genes are very costly. We also consider the long-term evolution ary dynamics of male-killers, and show that evolution towards progressively more 'efficient' male-killers can be thwarted by the appearance of host re sistance. The presence of a resistance gene can allow a less efficient male -killer to outcompete its rival and hence reverse the trend towards more ef ficient transmission and reduced metabolic load on the host.