Why do maternally inherited microorganisms kill males?

Maternally-inherited male killing microorganisms are known in a number of insect species. We here discuss the evolutionary reasons for such behaviour through examining the ongoing dynamics of these elements. In cases where death occurs late in the life cycle, horizontal transmission associated with male death occurs but in others no such direct advantage to killing the individual appears to exist. The evidence that early male killing is analogous to primary sex ratio distortion through increasing the fitness with respect to producing females from increases in resources to surviving (female) offspring and through the prevention of inbreeding is discussed. The early life history of many of these species suggests an early resource advantage may accrue from male killing. Inbreeding avoidance appears to be an important factor in at least two cases. The potential for horizontal transmission to maintain these elements is also discussed. It is concluded that male killing appears to be an adaptive strategy for the microorganism in most cases.