Signatures of socially antagonistic seleciton in the fire ant social chromosome
Carlos Martinez Ruiz, Carlos Martinez-Ruiz , Richard Nichols , Yannick Wurm
Queen Mary University of London, London, UK; Queen Mary University of London, London, UK ; Queen Mary University of London, London, UK ; Queen Mary University of London, London, UK
Colonies of the red fire ant Solenopsis invicta are either monogyne or polygyne. These differences in queen number are associated with a suite of other phenotypic traits including independent colony founding. Interestingly, colony type is determined by a “social chromosome” region including >400 genes on chromosome 16. This system therefore provides a unique opportunity to understand the molecular processes underlying social evolution. The social chromosome has two variants, SB and Sb, which are inverted with respect to each other. Recombination between SB and Sb is severely repressed and, since the Sb homozygotes are lethal recessives, there is no effective recombination within Sb. This genetic architecture is very similar to that of sex chromosomes, which also have variants that are essentially non-recombining (the Y in an XY system). Here, we tested whether the evolution of the social chromosome system has been shaped by social antagonism between the two social forms, similarly to how the evolution of sex chromosomes has been shaped by sexual antagonism between males and fmeales. Specifically, we test whether expression patterns in the social chromosome match those expected by evolutionary conflict. Additionally, we model the gene flow between the two social forms to determine the potential importance of social antagonism in this system. Our results are consistent with socially antagonistic selection being responsible for the divergence of the social chromosome. These findings shed light on the phenotypic effects of genome architecture in the context of complex social behaviours.