  The evolution of ants is marked by remarkable adaptations that allowed the
development of very complex social systems. To identify how ant-specific
adaptations are associated with patterns of molecular evolution, we searched
for signs of positive selection on amino-acid changes in proteins. We
identified 24 functional categories of genes which were enriched for positively
selected genes in the ant lineage. We also reanalyzed genome-wide datasets in
bees and flies with the same methodology, to check whether positive selection
was specific to ants or also present in other insects. Notably, genes
implicated in immunity were enriched for positively selected genes in the three
lineages, ruling out the hypothesis that the evolution of hygienic behaviors in
social insects caused a major relaxation of selective pressure on immune genes.
Our scan also indicated that genes implicated in neurogenesis and olfaction
started to undergo increased positive selection before the evolution of
sociality in Hymenoptera. Finally, the comparison between these three lineages
allowed us to pinpoint molecular evolution patterns that were specific to the
ant lineage. In particular, there was ant-specific recurrent positive selection
on genes with mitochondrial functions, suggesting that mitochondrial activity
was improved during the evolution of this lineage. This might have been an
important step toward the evolution of extreme lifespan that is a hallmark of
ants.
