The evolution of cytoplasmic incompatibility or when spite can be successful.

It is proposed that the phenomena of cytoplasmic incompatibility is explicable in terms of the selfish interests of the prokaryotic symbionts associated with the phenomena. It is hypothesized that in males the symbionts produce a product, termed wolbachin, which is carried in sperm and has the capability of inhibiting zygotic development if not neutralized. Symbionts are capable of neutralizing wolbachin. If this is the correct mechanism then the symbionts by killing eggs incapable of neutralizing wolbachin are acting spitefully. A simple model demonstrates that spiteful symbionts can invade a population of non-spiteful symbionts. The resulting population of spiteful symbionts is capable of resisting invasion by other spiteful symbionts even if the invaders have more efficient vertical transmission. Spite is successful in this system because all of the costs of being spiteful are inflicted on the host and not on the symbionts. This is in contrast to other systems of spite.