Submitted to: Biological Journal of the Linnean Society, London
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/6/2005
Publication Date: 11/25/2005
Citation: Sipes, S.D., Tepedino, V. 2005. Pollen-host specificity and evolutionary patterns of host switching in a clade of specialist bees (Apoidea: Diadasia). Biological Journal of the Linnean Society, London. 86:487-505. Interpretive Summary: Many bee species native to the arid areas of the western United States exhibit extreme specialization in the flower species that they visit. Often, bee species may be restricted in their pollen collection activities to plant species of a single genus or family. We do not understand why bees behave in this way, or why such specialization evolved. Recently, a technique has been developed whereby host plants of bees can be associated with the way that these bee species are thought to have evolved. Thus we can answer such questions as: do more specialized bees evolve from those that are less particular in their choice of plants; and do host shifts from one kind of plant to another typically occur between plant groups that are closely related? We have asked these and other questions of the bee genus Diadasia, a moderate sized group of bees that occurs mostly in the western United States. We have found, among other things, that bee species are as likely to develop specialized pollen preferences from generalized collection behavior as the reverse, i.e., there does not appear to be a pattern of increasing specialization with time. We have also found that competition for food resources seems not to drive the evolution of bees in this group: host switches are as frequently to distantly related plant taxa as they are to closely related plant taxa. The evolution of new species in this bee genus is complicated and probably depends upon a number of factors including phenology, abundance, competitors and enemies, and nest-site availability, and how these vary unpredictably in time and space.
Technical Abstract: We examined the levels of pollen-host specificity in North American Diadasia (Hymenoptera: Apoidea), a clade of specialist bees. We analyzed the scopal pollen loads of 409 individuals representing 25 of the 30 species of Diadasia that occur in North America. Each Diadasia species showed a preference for one of five plant families. However, the 25 species varied in their level of host specificity: the average percentage by volume of the preferred host in pollen loads ranged from > 99% to < 75%. In 17 of the 25 species, all or most individuals examined contained pure loads of one host taxon, while in 8 species individuals were less specialized and carried mixtures of several unrelated host taxa. Mapping these host preferences onto a phylogenetic tree indicated that Malvaceae is the most likely ancestral host for the genus, and use of other hosts can be explained by a single switch to each of the other four host-plant families. Thus, most speciation events were not associated with a host switch; this pattern does not support host switching as a niche partitioning strategy to avoid competition. Diadasia species are more likely to use host-plant families that are used by other Diadasia and Emphorine bees; however, there was no evidence of residual adaptation to ancestral hosts. Diet breadth appears to be a labile trait: transitions from narrower to broader host use, as well as vice versa, were observed. The observed patterns of host-use evolution may be driven, in part, by host morphology and/or chemistry.