The fire ant sex locus

Sex determination is a fundamental biological process that is regulated by a bewildering large number of molecular mechanisms. In about 20% of insects, including all ants and bees, sex is determined by the ploidy of the individual. Males are haploid and develop from unfertilized eggs with one set of chromosomes. Females are diploid and obtain two sets of chromosomes from fertilization. In several species, the mechanism to ascertain ploidy has been proposed to be complementary sex determination (csd): heterozygous individuals at the sex locus develop into females, whereas hemizygous haploid or homozygous diploid individuals develop into males. Studies in the honeybee Apis mellifera have identified a single locus, csd, as the master sex determination gene. We are using the fire ant Solenopsis invicta to study the mechanisms of sex determination in ants. Our genetic analyses have revealed that fire ants have evolved a novel master sex determination gene. We have mapped the sex locus to a 131 kb region containing at least ten genes, all without similarity to known sex genes. By comparing genomes of different individuals we also identified a hypervariable region with at least 10 alleles, consistent with balancing selection acting on the sex determination locus to maintain its heterozygosity. We are currently conducting gene expression analyses and functional tests to identify and characterize this novel sex determination system.