Comprehensive phylogeny of Myrmecocystus honey ants highlights cryptic diversity and infers evolution during aridification of the American Southwest

The New World ant genus Myrmecocystus Wesmael, 1838 (Formicidae: Formicinae: Lasiini) is endemic to arid and semi-arid habitats of the southwestern United States and northern Mexico. Several intriguing life history traits have been described for the genus, the best-known of which are replete workers, that store liquified food in their largely expanded crops and are colloquially referred to as honeypots. Despite their interesting biology and ecological importance for arid ecosystems, the evolutionary history of Myrmecocystus ants is largely unknown and the current taxonomy represents an unsatisfactory systematic framework without well-defined species boundaries. We use ultraconserved elements (2,524 loci) to infer the evolutionary history of Myrmecocystus ants and provide a comprehensive, dated phylogenetic framework that resolves species boundaries, reveals cryptic diversity, and reconstructs the ancestral foraging activity. Using maximum likelihood, Bayesian and species tree approaches on a data set of 134 ingroup specimens (including samples from natural history collections and type material), we recover largely identical, well-resolved topologies that leave the position of only few clades uncertain and cover the intra- and interspecific variation of 28 of the 29 described and six undescribed species. In addition to traditional support values, such as bootstrap and posterior probability, we quantify genealogical concordance to estimate the effects of conflicting evolutionary histories on phylogenetic inference. Our analyses reveal that the current taxonomic classification of the genus is inconsistent with the molecular phylogenetic inference, and we identify cryptic diversity in seven species. Divergence dating suggests that the split between Myrmecocystus and its sister taxon Lasius occurred in the early Miocene. Crown group Myrmecocystus have evolved about 14.08 Ma ago when the gradual aridification of the southwestern United States and northern Mexico led to formation of the American deserts and to adaptive radiations of many desert taxa.