Molecular and morphological recognition of species boundaries in the neglected ant genus Brachymyrmex (Hymenoptera: Formicidae): toward a taxonomic revision

International audienceBrachymyrmex is a neglected genus of Formicinae because of its small body size, soft mesosoma, and superficially monotonous external morphology. These features have complicated the documentation of morphological variation, resulting in poorly defined and incompletely described species. Consequently, the taxonomy of the genus is complex and problematic, which has impeded research and conservation efforts. Here, we integrate molecular and morphological data to recognize species boundaries in Brachymyrmex and to guide its long-overdue revision. Specifically, we (1) redefine the limits of all described species, subspecies, and varieties based on intra-and interspecific morphological variation in workers; (2) document this variation quantitatively by constructing morphospace occupation and statistically analyzing measurements; (3) synthesize our findings on diagnostic traits in a dichotomous, illustrated identification key; and (4) examine the significance of our morphological identification system with molecular evidence from four gene fragments (EF1aEF1, EF1aEF2, WG, and COI). We recognize 40 species, of which four are new to science: Brachymyrmex bahamensis, Brachymyrmex bicolor, Brachymyrmex iridescens, and Brachymyrmex sosai. Furthermore, Brachymyrmex attenuatus and Brachymyrmex bonariensis are raised to species, and we propose 25 new synonyms. Morphometrics indicated that even poorly distinguishable species pairs show statistically significant differences in some traits, and that taxonomically problematic cases relate to taxa that demonstrate large intraspecific trait variance. Our molecular analysis supports the monophyly of the genus based on increased taxon sampling, and of the 19 species that were included 18 were retrieved as monophyletic. The single case of incongruence was also flagged in morphological analyses and requires extended geographic sampling before it can be resolved. In conclusion, the molecular work corroborates the morphologically recognized species boundaries. We also document the presence of worker dimorphism and putative worker-queen intercastes in several Brachymyrmex species, which indicates that the genus may present a promising study system to understand caste evolution in ants