What causes inefficient transmission of male-killing Wolbachia in Drosophila?

Selfish genetic elements that distort the sex ratio are common in arthropods. Theory predicts they will invade and spread to fixation if they are vertically transmitted with perfect fidelity, potentially leading to host extinction. For inherited microorganisms that distort the sex ratio, inefficient vertical transmission or incomplete sex ratio distorting ability is required for host persistence. However, the relative roles of genetic and environmental factors in permitting the survival of male hosts and preventing parasite transmission are poorly understood. We examined the causes of transmission infidelity and male survival for a male-killing Wolbachia strain in Drosophilabifasciata. Under standard laboratory conditions (18°C), in its standard genetic background, males are produced very rarely, and no case of reversion has been observed in 20 generations of laboratory rearing. To investigate the role of host genetic factors, Wolbachia was crossed into 27 different inbred lines of D.bifasciata, but in no case was reversion observed at preferred environmental temperatures. The role of elevated temperature in inducing inefficient transmission was examined. Whilst vertical transmission was perfect over three generations of maintenance at 23.5°, transmission infidelity was observed at 25°. We conclude that there is no evidence for the presence of either fixed or polymorphic host genes that repress transmission at standard environmental temperatures. However, severe temperature treatment does make vertical transmission imperfect. We suggest that the case of Wolbachia in D.bifasciata is one that is naturally balanced, the population being maintained polymorphic without the evolution of host resistance genes.