Carpenter ants and their little helpers: Evolutionary history of Camponotini ants and their endosymbionts

Many insect groups have acquired obligate microbial symbionts, and the resulting associations can have important ecological and evolutionary consequences. A notable example among ants involves the tribe Camponotini, whose species harbor a bacterial endosymbiont, Blochmannia, found in no other insects and the source of significant nutritional benefits to the host ants. Numbering almost 2000 described species, camponotine ants have come to occupy a wide range of habitats across all major biogeographic regions. Here we use ultraconserved element (UCE) phylogenomic data to reconstruct a detailed evolutionary history of the Camponotini, including inference of divergence times and dispersal events. We recover a well-supported backbone phylogeny comprising three large genera (Camponotus, Colobopsis, Polyrhachis), and several smaller clades, including three novel lineages that represent new genera. The tribe Camponotini is estimated to have originated in the late Eocene (~38 Ma), probably in Australasia or Indomalaya, with subsequent dispersal to other parts of the globe. Most Neotropical species of Camponotus belong to a single clade that dispersed to the region only ~15 Ma, a surprising finding given the remarkable morphological diversity seen among Neotropical species. Phylogenetic analysis of selected Blochmannia genes from a subset of 98 camponotine taxa yields results that are largely congruent with the ant host phylogeny, at least for well supported nodes, but we find evidence that Blochmannia from some old lineages have discordant evolutionary histories, suggesting the possibility of horizontal transfer of the symbiont in the early evolution of camponotine ants.