Convergent evolution of pollen transport mode in two distantly related bee genera (Hymenoptera: Andrenidae and Melittidae)

Purposeful transport of pollen represents a key innovation in the evolution of bees from predatory wasps. Most bees transport pollen on specialized hairs on the hind legs or ventral metasoma in one of three ways: moist, dry, or glazed, which combines dry and moist transport. The evolutionary pathway among these three transport modes is unclear, though dry transport has been hypothesized to be ancestral. We address this hypothesis using museum specimens and published records of the bee genera Perdita (Andrenidae) and Hesperapis (Melittidae), two distantly related groups whose pollen transport modes appear to have converged. Most species in both genera transport moistened pollen; glazed and dry transport are limited to derived clades of specialists on floral hosts in Asteraceae and Onagraceae, with specialization on Asteraceae associated with more elaborate scopal hairs. The associations between transport mode, host plant, and hair type may be due to the sticky pollenkitt of asteraceous pollen grains and the viscin threads of Onagraceae pollen, which provide alternates to the binding properties of nectar. These findings suggest that the hypothesis that dry transport is ancestral in bees should be reexamined.