-Bees and the angiosperms they pollinate have developed intimate and often complex interactions over the past 100 million years. As in other insect-plant interactions, host-plant specificity is variable among taxa. While many solitary bee species display an obvious preference for a narrow spectrum of host-plants (oligolecty), others regularly visit a diversified array of pollen hosts (polylecty). Few studies have examined the patterns of host-plant associations in bees using well-resolved phylogenies at the species level combined with accurate and quantitative data on host-plant preferences. In this study, we examined the evolution of bee-plant relationships in several genera of specialist (oligolectic) bees. We used the Melittidae s.l. as a model taxon by mapping the preferred pollen hosts onto species-level phylogenies to investigate the frequency and pattern of host-plant switching. Our results suggest that host-plant associations are maintained over time in many lineages, but that host switches to unrelated plant families are also common. We find some evidence that host-switches occur more frequently to morphologically similar, rather than closely-related, host-plants suggesting that floral morphology plays a key role in host-plant evolution in bees. bees / Melittidae s.l. / oligolecty / polylecty / phylogeny / evolution / host-plant
Recent molecular phylogenetic data showed the Melittidae as the likely sister group to all other bees and indicated that proto-melittids could have been host-plant specialists originating in Africa. However, robust phylogenetic data at generic and species level are now needed for all melittid clades to test these hypotheses and investigate early melittid and bee evolution in general. The bee genera Haplomelitta and Samba, which comprise the former tribe Sambini (Hymenoptera : Melittidae), are revised here. The genera are endemic to the Afrotropical region, occurring in eastern and southern Africa. Previous studies hypothesised that Samba rendered Haplomelitta paraphyletic but a conclusive taxonomic decision was not proposed. By performing a comprehensive phylogenetic analysis based on five nuclear genes (28S, CAD, EF-1α (F2 copy), long-wavelength rhodopsin (opsin) and RNA polymerase II (RNAp); total 4179 bp) and morphological characters (34 characters), we here synonymise Haplomelitta with Samba. The genus is now subdivided into six subgenera containing 10 species, four of which are here described as new, namely: S. ascheri, S. gessorum, S. spinosa and S. rubigoinis. Moreover, we also considered biogeography, phenology and floral visitation data. Samba seems to have originated in southern Africa and later colonised eastern Africa. The ancestral host-plant foraging strategy was probably specialisation on one plant family (e.g. oligolectism). This result supports the hypothesis that the ancestor of bees arose in Africa and was a host-plant specialist.
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