Convergent Genetic Architecture Underlies Social Organization in Ants

Complex adaptive polymorphisms are common in nature, but what mechanisms maintain the underlying favorable allelic combinations [1–4]? The convergent evolution of polymorphic social organization in two independent ant species provides a great opportunity to investigate how genomes evolved under parallel selection. Here, we demonstrate that a large, nonrecombining “social chromosome” is associated with social organization in the Alpine silver ant, Formica selysi. This social chromosome shares architectural characteristics with that of the fire ant Solenopsis invicta [2], but the two show no detectable similarity in gene content. The discovery of convergence at two levels—the phenotype and the genetic architecture associated with alternative social forms—points at general genetic mechanisms underlying transitions in social organization. More broadly, our findings are consistent with recent theoretical studies suggesting that suppression of recombination plays a key role in facilitating coordinated shifts in coadapted traits [5, 6]. [Display omitted] •Colony queen number in Alpine silver ants is controlled by a large set of linked genes•Suppression of recombination maintains this “social chromosome”•Ant species converge in the genetic architecture underlying social organization•The social chromosomes from two ant species harbor different sets of genes Behavioral strategies consist of multiple coadapted traits. How do they evolve? Purcell et al. discovered a novel “social chromosome” underlying variation in queen number in the ant Formica selysi. The phenotypes and genetic architecture associated with queen number converge in two ant species, but the social chromosomes differ in gene content.