GENETIC-STRUCTURE AND EVOLUTION OF A FIRE ANT HYBRID ZONE

Two introduced fire ants, Solenopsis invicta and S. richteri, hybridiz e over an extensive area in the United States spanning central Mississ ippi, Alabama, and western Georgia. We studied a portion of this hybri d zone in northwestern Mississippi in detail by sampling ants at many sites along two transects extending across the zone and examining gene frequency and size distributions at a large number of genetic and mor phological markers. The distributional patterns at these markers are m ost consistent with the mosaic hybrid zone model, whereby the distribu tion of various fire ant genotypes is determined initially by the hist orical patterns of colonization of newly available habitats. However, these distributional patterns probably do not reflect the equilibrium state of interactions because of the very recent secondary contact of the species (< 60 yr) and the dynamic nature of available nesting habi tats in this area. Our data suggest that, with prolonged contact and i nteraction, differential fitness of various hybrid genotypes due to in trinsic and extrinsic selective factors is important in structuring th e hybrid zone. For instance, consistent differential introgression of morphological and genetic markers, combined with previous evidence of differences in developmental stability among genotypes, suggest reduce d fitness of hybrids relative to parentals due to intrinsic selection (as may be caused by breakup of parental gene complexes). Furthermore, marked reductions in the occurrence of parental-like hybrids in areas where the similar parental species is common suggest reduced fitness of these parental-like hybrids in competition with the parentals (i.e. , extrinsic selection). Because the relative roles of such determinist ic as well as stochastic forces apparently vary both spatially and tem porally, the eventual distribution of the various fire ant genotypes a nd the fate of the hybrid zone in the United States is difficult to pr edict.