Phylogenetic relationships and host-plant evolution within the basal clade of Halictidae (Hymenoptera, Apoidea)

Bees are among the most important pollinators of angiosperm plants. Many bee species show narrow host-plant preferences, reflected both in behavioral and morphological adaptations to particular attributes of host-plant pollen or floral morphology. Whether bee host-plant associations reflect co-cladogenesis of bees and their host plants or host-switches to unrelated host plants is not clear. Rophitinae is a basal subfamily of Halictidae in which most species show narrow host-plant preferences (oligolecty). We reconstructed the phylogenetic relationships among the rophitine genera using a combination of adult morphology (24 characters) and DNA sequence data (EF-1 alpha, LW rhodopsin, wingless; 2700 bp total). The data set was analyzed by parsimony, maximum likelihood and Bayesian methods. All methods yielded highly congruent results. Using the phylogeny, we investigated the pattern of host-plant association as well as the historical biogeography of Rophitinae. Our biogeographical analysis suggests a number of dispersal/vicariance events: (1) a basal split between North America and South America (most likely a dispersal from South America to North America), and (2) at least two subsequent interchanges between North America and Eurasia (presumably via the northern hemisphere land bridges). Our analysis of host-plant associations indicates that Rophitinae specialized on a closely related group of angiosperm orders in the Euasterid I clade (mainly Gentianales, Lamiales and Solanales). However, there is little evidence of cocladogenesis between bees and plants and strong evidence of host switches to unrelated host plants. Based on our phylogenetic results we describe two new tribes of Rophitinae: Conanthalictini new tribe (including the genus Conanthalictus) and Xeralictini new tribe (including Xeralictus and Protodufourea). (C) The Willi Hennig Society 2007.