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.