The relationship between phytophagous insects and their host plants has interested scientists since Darwinian times. Using modern phylogenetic inference, we are able to investigate these patterns using, not only the phylogenies of the insects, but the evolutionary relationships among the plants they feed on as well. The relationships between bees and the plants they pollinate were traditionally seen as mutualistic and were treated separately from the research investigating the antagonistic relationships between phytophagous insects and their host plants. However, recent phylogenetic studies have made great progress including bee-host relationships in with the larger body of work on phytophagous insects.
The genus Melissodes Latreille in the tribe Eucerini is a widespread and common group of bees. There are 129 described Melissodes species that range throughout the western hemisphere with the center of diversity in the warm deserts of southwestern North America. Here, we present a species-level phylogeny using five loci for 89 species of Melissodes. We confirm all of the subgeneric delineations constructed by LaBerge, with the exception of Heliomelissodes which renders Eumelissodes paraphyletic, and we discuss the unexpected placement of a few taxa. We combine this analysis with previous data to support the placement of Melissodes within the tribe Eucerini and add a temporal component. We find a southwestern North American origin for the genus with a model that supports widespread sympatric speciation.
This work represents the first analysis to incorporate a taxon dense phylogeny of bees, molecular barcoding of pollen to identify host plants, and a host plant phylogeny to assess the evolution of diet breadth in bees. The use of molecular barcodes to discern host identities allowed a more detailed look into specialization of bees within the major clades of the super-diverse plant family, Asteraceae. Here we assess the value of using barcoding techniques for pollen identification and the merits of various ways of inferring ancestral diet breadth. We find, not one, but three general patterns of host plant evolution within a single genus of bees. Finally, we place our findings in the context of historical biogeography and current theory on the evolution of diet breadth
