Wolbachia-induced incompatibility precedes other hybrid incompatibilities in Nasonia

Wolbachia are cytoplasmically inherited bacteria that cause a number of rep roductive alterations in insects, including cytoplasmic incompatibility(1,2 ), an incompatibility between sperm and egg that results in loss of sperm c hromosomes following fertilization. Wolbachia are estimated to infect 15-20 % of all insect species(3), and also are common in arachnids, isopods and n ematodes(3,4). Therefore, Wolbachia-induced cytoplasmic incompatibility cou ld be an important factor promoting rapid speciation in invertebrates(5), a lthough this contention is controversial(6,7). Here we show that high level s of bidirectional cytoplasmic incompatibility between two closely related species of insects (the parasitic wasps Nasonia giraulti and Nasonia longic ornis) preceded the evolution of other postmating reproductive barriers. Th e presence of Wolbachia severely reduces the frequency of hybrid offspring in interspecies crosses. However, antibiotic curing of the insects results in production of hybrids. Furthermore, F-1 and F-2 hybrids are completely v iable and fertile, indicating the absence of F-1 and F-2 hybrid breakdown. Partial interspecific sexual isolation occurs, yet it is asymmetric and inc omplete. Our results indicate that Wolbachia-induced reproductive isolation occurred in the early stages of speciation in this system, before the evol ution of other postmating isolating mechanisms (for example, hybrid inviabi lity and hybrid sterility).