Ants (Formicidae) in the tribe Camponotini Raw sequence reads
Many insect groups have acquired obligate microbial symbionts, and the resulting associations can have important ecological and evolutionary consequences. A notable example among ants is the tribe Camponotini, whose species harbor a vertically inherited, bacterial endosymbiont, Blochmannia, that is found in no other insect and provides significant nutritional benefits to the host ants. The tribe Camponotini comprises almost 2,000 described species, of which a little more than half belong to the gargantuan genus Camponotus. These ants are found in all major biogeographic regions, where they occupy diverse arboreal and ground-based nesting sites, across habitats ranging from deserts to rainforests. We generate ultraconserved element (UCE) phylogenomic data for a large, representative set of taxa (220 camponotine species, 5 outgroup species) to reconstruct a detailed evolutionary history of the Camponotini, including inference of divergence times and dispersal events. Under multiple modes of analysis, including both concatenation and species-tree approaches, we recover a well-supported backbone phylogeny comprising eight lineages: three large genera (Colobopsis, Polyrhachis, Camponotus) and several smaller genera or clusters of genera. Three novel lineages are uncovered that cannot be placed in any existing genera: Lathidris gen. n., from the mountains of Mesoamerica; Retalimyrma gen. n., from the Indian Himalayas; and Uwari gen. n., from eastern Asia. The species in these new genera were described and placed erroneously in Camponotus. The Polyrhachis portion of the tree is largely congruent with previous studies and supports monophyly of most of the recognized subgenera. The phylogeny of Camponotus (sensu stricto) is more complex and requires additional study, but we provisionally identify six major clades within this genus, most showing considerable fidelity to one or two biogeographic regions. There has been extensive convergence in morphology among these six clades of Camponotus, undermining the current subgeneric classification of the genus. The tribe Camponotini is estimated to have a crown origin in the Eocene (38.4 Ma), with successively younger crown ages for Colobopsis (22.5 Ma), Camponotus (18.6 Ma), and Polyrhachis (18.5 Ma). We infer an Australasian or Indomalayan origin for the tribe, with multiple dispersal events to the Afrotropics, Palearctic region, and New World. Most Neotropical species of Camponotus belong to a single clade whose most recent common ancestor apparently dispersed to that region only about 11 Ma, a surprising result given the remarkable morphological diversity seen among Neotropical species. These young divergence date estimates are consistent, however, with the scarcity of camponotine ant fossils in Baltic amber (1 species) and their absence from Dominican amber. 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 Blochmannia in the early evolution of camponotine ants.