Reviewer 1's Review (Andy Gardner)

“The role for Wolbachia-related cytoplasmic incompatibility in reproductive isolation and speciation is controversial. Although bidirectional incompatibility (where hosts carrying two different strains of Wolbachia cannot interbreed) can obviously result in reproductive isolation, it appears that the occurrence of multiple Wolbachia strains in a single population is relatively rare. More commonly, a single strain is present, in which case cytoplasmic compatibility is unidirectional (male carriers cannot produce offspring with uninfected females, but female carriers can produce offspring with uninfected males). However, because this presents little barrier to geneflow, it is unclear whether it could result in reproductive isolation. The authors address this by examining a standard mainland-island model, in which the Wolbachia-infected mainland population of hosts sends migrants to an initially-uninfected island population. They allow for the evolution of a potentially locally-adapted male trait, and female preference for this trait, in the island population, and delineate the circumstances in which cytoplasmic incompatibility will drive assortative mating and hence reproductive isolation.

The problem is very clearly stated, and is interesting. The mathematical analysis is similarly clear and easy to follow. Through the use of approximations (that are shown by simulations to be very reasonable), most of the theoretical results are expressed analytically, which allows far more insight than would be possible from a purely simulation-based study. It was a pleasure to read the ms.”

N.B. These are the general comments made by the reviewer when reviewing this paper in light of which the manuscript was revised. Specific points addressed during revision of the paper are not shown.