2nd Place: The evolution of ant-plant specialization in the Neotropical ant genus Myrmelachista: A phylogenomic approach

Obligatory ant-plant symbioses are generally assumed to be single evolutionary shifts within ant lineages; however, convergence can be revealed once careful natural history observations are complemented with molecular phylogenetic reconstruction. Exclusively arboreal, Myrmelachista (Hymenoptera: Formicidae) species can be generalized opportunists nesting in several plant species or obligate symbiotic (=mutualistic), nesting in live stems of a narrow set of species within a plant genus. The strong morphological and behavioral uniformity of mutualists supports the hypothesis that they form a monophyletic assemblage, diversifying after a single origin of specialization. Using phylogenomic data from ultraconserved elements, we resolve the phylogeny of the genus, recovering four strongly-supported clades: a Patagonian clade sister to the rest of Myrmelachista, two widespread Neotropical assemblages, and a clade of primarily Mesoamerican and Caribbean species. Furthermore, our analysis revealed a remarkable example of convergent evolution, with at least three instances of independently evolved mutualistic lineages within the genus. Divergence dating and biogeographic analyses indicate that Myrmelachista originated in the southern portion of South America ~30 Mya. Waves of dispersal across the Neotropics occurred at least three times: once during the Oligocene across the Neotropics, and two during the Miocene, between South America and Mesoamerica, in both directions. Furthermore, we could not distinguish the observed phylogenetic distribution of mutualists from null models in which mutualisms have no effect on diversification. We found that the evolution of obligate interactions is probably not associated with diversification, with higher diversification rates in Myrmelachista starting anteriorly to the transition to the mutualistic niche.