Gp-9 has a major effect on social organization in fire ants. Queens and workers of polygyne Solenopsis invicta homozygous for the b-like allele b suffer reduced viability, and bb queens never survive. The b allele acts as a recessive lethal. This allele differs from other b-like alleles (designated b2), and all other Gp-9 alleles, by encoding a lysine residue at position 151, leading to the hypothesis that this substitution is responsible for its deleterious effects. We compared b2b2 and bb homozygotes, in reproductive queens of S. richteri and S. invicta, and in workers of S. invicta from native polymorphic populations. 20% of S. richteri queens were b2b2 homozygotes, and 5% of S. invicta workers bore genotype b2b2, compared to the lack of bb homozygotes in the same populations. Thus, the lysine substitution at position 151 in the GP-9 protein confers the deleterious effects of the b allele in homozygous condition. Keywords: fire ants; genetics of adaptation; Gp-9; lethal allele; odorant-binding proteins; polygyny; Solenopsis invicta. MOLECULAR BASIS OF LETHALITY OF AN ALLELE IMPLICATED IN SOCIAL EVOLUTION IN FIRE ANTS by BRITTAN HALLAR B.A., Truman State University, 2002 A Thesis Submitted to the Graduate Faculty of the University of Georgia in Partial Fulfillment of the Requirements for the Degree MASTER OF SCIENCE ATHENS, GEORGIA 2005