Arndt Telschow; Matthias Flor; Yutaka Kobayashi; Peter Hammerstein; John H. Werren

“ We assume that an ancestral host population has split into two populations, a large mainland and a small island. The scenario is analogous to a large central population with a small peripheral isolated population. The populations remain for some time in allopatry and diverge during that time at a locus which controls a male trait used in female mate choice. Further, the mainland is infected with Wolbachia but there is no infection on the island. ”

“ Discussion In this article we investigated the role of Wolbachia-induced unidirectional CI on genetic divergence and reinforcement using a mainland-island model. We demonstrated analytically that (a) the infection polymorphism between populations is stabilized by local adaptation, (b) gene flow between the populations is reduced but divergence at a selected locus enhanced, and (c) unidirectional CI selects for premating isolation in a mostly uninfected island population receiving migration from a Wolbachia-infected mainland. ”

“ After migration, viability selection takes place. We assume that selection on male trait alleles acts differently in mainland and island. On the mainland, T1 is favored and therefore fixed. On the island, however, T2-individuals have a (1+s) -times higher viability than T1-individuals. Therefore, there is differential selection for the trait on the island relative to the mainland.
Mating is nonrandom due to female mating preferences. We assume that females of genotype P1 and P2 have characteristic mating preference strengths, a1 and a2, respectively. ”

“ In all simulations shown below, females with genotype P1 show no mating preference, i.e. a1 = 1, whereas P2 genotype females favor mating with T2 males, i.e. a2>1. Mating of P1 females is random and the frequency of mating with T1 and T2 males is proportional to the relative abundance of the two male traits. P2 females, however, mate a2-times more often with T2-males than P1 females do. The frequency of mating with the different male types is proportional to the relative abundance of the respective male trait weighed by the strength of mating preference. ”

“ Second, matings with T2 males result less often in CI because T2 males are less likely to be infected with Wolbachia. The latter can be a huge advantage because P2 females are mostly not infected with Wolbachia. From the female's perspective, the advantage of being choosy is therefore not only to find a male that fits the environment but also to find a male that is compatible with one's own cytotype. Because the selected allele and cytotype are in association, female choice of the selected allele enhances chances of mating with a compatible cytotype. ”

“ A counter example is the spread of Wolbachia in uninfected populations of Drosophila simulans in uninfected North American populations [25] .
Our results also have important ramifications for classical reinforcement theory. A standard critique in reinforcement is that gene flow might overwhelm the selective advantage of being choosy [37] . Although previous work has shown that reinforcement is possible (e.g. [38] , [8] , [39] ) , most of the studies are purely simulation based (but see [40] ) . In the present study we developed a new method to analyze the reinforcement dynamics analytically. ”

“ The authors have declared that no competing interests exist.Funding: The study was funded by the Deutsche Forschungsgemeinschaft (SFB 618) , the National Science Foundation (NSF: EF-0328363) , the Grant of Biodiversity Research 21COE (A14) , and the Japanese Society for the Promotion of Science (JSPS) . ”