Understanding the Distribution and Effects of Wolbachia

Within isopod crustaceans, the vertically transmitted bacteria Wolbachia induces either cytoplasmic incompatibility (CI) or feminization of male hosts. One of the challenges is to understand the distribution of the different manipulations between species. The invasion conditions for feminizers are much broader than for CI inducers and so the former is expected to be the more common, all else being equal. Here we ask whether prior infection with one type predisposes or inhibits the spread of a strain causing the opposite manipulation. Were this so, historical accident might have to be evoked to explain which species is affected by which type. We consider two possibilities. First, the appearance of a new mutant bacterium capable of both manipulations. Second, the appearance, via horizontal transfer, of a new bacterium capable of only one manipulation. In a mutational model, invasion of CI into a population with a feminizer is trivial, as is the reverse. This provides the first mechanism for trivial invasion of CI, but its biological relevance is unclear. In the horizontal transfer model, replacement of one type by another can occur if infection is initially into an uninfected lineage. However, under these circumstances neither form is likely to spread. If the initial horizontal transfer event is into an infected lineage, then under the most realistic circumstances, the prior existence of one form has little effect on the conditions for spread of the other, but may marginally inhibit or promote spread. If spread does occur, stable duel infection is the most common equilibrium condition. We suggest reasons as to why this has yet to be observed.