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The proposed project will employ the new method of admixture mapping to conduct detailed studies of gene and trait associations in a hybrid zone between two invasive fire ant species in the southern U.S.A., Solenopsis invicta and S. richteri. Admixture mapping takes advantage of the presence of highly admixed (introgressed) populations to survey nuclear genomes both for regions under selection in foreign genomic backgrounds and for regions influencing the development of species-specific phenotypic or ecological traits. Loci with variants that are relatively under- or over-represented in a foreign background mark genes subject to selection in hybrids; negatively selected loci may contribute to the process of speciation or to the maintenance of species differences, whereas positively selected loci may underlie important general adaptations. When such excess ancestry is shown to be associated with variation at known traits, the genetic architecture underlying such variation is revealed, thus informing the genetic basis of phenotypic novelties involved in anagenesis and speciation. Thus, admixture mapping is a useful tool for revealing gene numbers, locations, and effect sizes for a diversity of evolutionarily important traits involved in speciation and adaptation.The five specific objectives of the proposed work are to: 1) Collect and characterize samples for admixture mapping from two localities in the large S. invicta / S. richteri hybrid zone in the U.S.A.; 2) Develop several hundred microsatellite and SNP markers with large allele frequency differentials between the parental species; 3) Conduct admixture mapping on hybrids to identify loci under negative or positive selection; 4) Conduct admixture mapping on hybrids to identify loci affecting morphological traits that distinguish the two species; and 5) Construct a genetic map from hybrid males to reveal linkage relationships of the markers. The availability of relevant genetic resources for fire ants, combined with extensive background knowledge of their biology and history of hybridization, will provide unprecedented power to address central issues pertaining to the genetic bases of species differences and speciation, the molecular basis of adaptation, and the origin of morphological and ecological novelty in this evolutionarily and economically significant group. The long-term goal is to integrate results from the proposed research with previous genetic, ecological, behavioral, and phylogenetic information to produce new insights into the evolutionary diversification and ecological success of fire ants.