A supergene causes alternative colony organization in fire ants

The fire ant Solenopsis invicta is characterized by a remarkable form of social polymorphism, with the presence of one or several queens per colony being completely associated with allelic variation at a single Mendelian factor marked by the gene Gp-9. Because additional phenotypic differences in physiology, fecundity, and behavior are also completely associated with Gp-9, it has been hypothesized that the locus actually consists of multiple genes that form a supergene. To test this possibility, we mapped the monogyny/polygyne social polymorphism more precisely using restriction-site associated DNA (RAD) tag sequencing, which is a method that permits rapid SNP discovery and genotyping at low cost. We found that Gp-9 is associated with a non-recombining region covering ~13 Mb. This non-recombining region contains at least one inversion of ~9 Mb, many of the genes with demonstrated expression differences between individuals of alternate Gp-9 genotype, and an accumulation of deleterious mutations including repetitive elements. Thus, variation in colony social form in fire ants is associated with a very large supergene which we propose to be a social chromosome.