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

Selfish genetic elements that distort the sex ratio are common in arthropod s. Theory predicts they will invade and spread to fixation if they are vert ically transmitted with perfect fidelity, potentially leading to host extin ction. For inherited microorganisms that distort the sex ratio., inefficien t vertical transmission or incomplete sex ratio distorting ability is requi red for host persistence. However, the relative roles of genetic and enviro nmental factors in permitting the survival of male hosts and preventing par asite transmission are poorly understood. We examined the causes of transmi ssion infidelity and male survival for a male-killing Wolbachia strain in D rosophila bifasciata. Under standard laboratory conditions (18 degreesC), i n its standard genetic background. males are produced very rarely, and no c ase of reversion has been observed in 20 generations of laboratory rearing. To investigate the role of host genetic factors, Wolbachia was crossed int o 27 different inbred lines of D. bifasciata, but in no case was reversion observed at preferred environmental temperatures. The role of elevated temp erature in inducing inefficient transmission was examined. Whilst vertical transmission was perfect over three generations of maintenance at 23.5 degr ees. transmission infidelity was observed at 25 degrees. We conclude that t here is no evidence for the presence of either fixed or polymorphic host ge nes that repress transmission at standard environmental temperatures. Howev er, severe temperature treatment does make vertical transmission imperfect. We suggest that the case of Wolbachia in D. bifasciata is one that is natu rally balanced. the population being maintained polymorphic without the evo lution of host resistance genes.