Comparative Morphology of the Stingless Bees (Apidae, Meliponini) and Its Phylogenetic Relevance

Morphology continues to have a central role in systematics and taxonomy, especially regarding the positioning of fossil taxa in the Tree of Life and as an independent source of information to evaluate competing molecular phylogenetic hypotheses. For the stingless bees, morphology and behavior were the only sources of information about their systematics and biogeography for an extended period. Currently, Meliponini is a group with relatively well-known evolutionary relationships in comparison with other bee tribes. However, there are strong incongruences between morphological and molecular results and little progress has been made to reassess the morphological evidence and its role in our understanding of their evolutionary history. Considering this background, the present study intends to take a step further toward understanding the role of morphology in the context of evolutionary relationships among stingless bees. We built a matrix of 375 characters coded for representatives of all extant genera and subgenera of Meliponini. We also included information about the fossil tribe †Melikertini and †Proplebeia dominicana in parsimony and Bayesian analyses. Results changed according to the optimality criterion and weighting (or partitioning) scheme adopted as well as assumptions regarding outgroups. We demonstrate that the tribe †Melikertini, known only from extinct taxa, is strongly supported as the closest relative of the stingless bees. The phylogenetic position of the Miocene fossil †Proplebeia dominicana is evaluated and was consistent among all analyses. Morphological evidence shed light on incongruences found in previous phylogenomic analyses, showing that morphology can represent an essential source of evidence to increase our confidence concerning competing hypotheses. A morphological characterization of major clades is provided and synapomorphies are discussed. We argue that combining morphological, chronological, and molecular evidence will be essential for unraveling the complex biogeographic history of stingless bees.