Phylogenomics of ants in the genus Acropyga, based on UCE sequences.

Acropyga are a world-wide clade of small subterranean formicine ants that live in obligate associations with root mealybugs. We used a data set of 944 loci of ultraconserved elements (UCEs) to reconstruct the phylogeny of 23 Acropyga species with both concatenation and species-tree approaches. The biogeographic history of the genus was investigated performing divergence dating analyses on 100 UCE loci and ancestral range reconstructions. The mutualism of Acropyga with their mealybug partners, as well as associated mandibular variation in the ants was reconstructed using methods of ancestral state reconstructions. We recovered a highly supported species phylogeny for Acropyga with both concatenation and species-tree analyses. The age for crown-group Acropyga is estimated around 30 Ma. Two main Acropyga lineages are recovered that are distributed mutually exclusive in the Old World and New World. Within the Old World clade, a Palearctic and African lineage is suggested as sister to the remaining species. Although present-day associations involve a number of mealybug genera, ancestral state reconstructions indicate that Old World species have diversified mainly in close association with xenococcines from the genus Eumyrmococcus. New World Acropyga predominantly evolved with Neochavesia until a recent (10-15 Ma) switch to rhizoecid mealybug partners (genus Rhizoecus). Overall, our results highlight the need for an extended investigation of the potential co-evolution of these ants with their mealybug partners.