EFFECTS OF SOCIAL-ORGANIZATION ON GENE FLOW IN THE FIRE ANT SOLENOPSIS-INVICTA

A CONTROVERSIAL model of speciation proposes that the development of a lternative social organizations within populations of group-living ani mals may drive the inception of reproductive isolation(1-3). The alter native social behaviours, which are selectively favoured in some socia l or ecological contexts, may be correlated with distinctive reproduct ive traits such that significant barriers to interbreeding emerge betw een coexisting social variants. Evidence for this mode of speciation i s almost non-existent(3-8), but it provides one of the most compelling mechanisms for sympatric sepciation(3,8) and could conceivably explai n many species origins. Here we examine variation in mitochondrial DNA and two unique nuclear genes to demonstrate that gene flow between sy mpatric social forms of the fire ant Solenopsis invicta is restricted to only one of four possible routes. The loss of the other routes resu lts from incompatibilities in the social systems of the two forms, dem onstrating the potential for social selection to generate significant barriers to gene flow and to initiate reproductive isolation.