Host choice evolution in specialist bees
The approximately ~20,000 species of bees feed exclusively on floral resources, and most display some level of flower constancy when foraging. There are two important types of floral constancy. The first is the tendency of individual bees to visit only one species of plant at a time within foraging trips. This type of constancy involves learning and is especially obvious in the foraging behavior of social corbiculate bees such as honeybees and bumblebees. The second type (and the type I am interested in) is the inherent tendency of many bee species to limit their choice of floral hosts to certain taxa, or to certain morphological types of flowers. Bee species that display the first type of constancy do not necessarily display the second. In most cases, the second type of constancy pertains primarily to the choice of pollen hosts, and less so to nectar hosts. This second type of pollen constancy is a trophic specialization that parallels specialized feeding patterns found in many phytophagous insect groups.
L to R: Diadasia sp. on Sphaeralcea sp., photo credit M. Duff; Diadasia sp. on Opuntia sp. photo credit unknown; Didasia nigrifrons on Sidalcea oregana, photo credit K. Kuta
I have used a phylogenetic approach combined with pollen analysis to answer questions about the evolution of pollen host choice in a group of specialist bees (Diadasia). Diadasia is a new world genus of about 38 species, comprised of host-plant specialists on a wide range of host plants. A majority of species specialize on members of the Malvaceae (most often on Sphaeralcea), while others specialize on hosts in four distantly related plant families (Asteraceae, Cactaceae, Convolvulaceae, and Onagraceae). By analyzing pollen loads from over 400 specimens of Diadasia from 24 species, I quantitatively assessed host-plant specialization. I found that, whereas all Diadasia species are associated with preferred hosts, these associations are much more rigid for some species than for others. The fraction of pollen loads comprising the most preferred host varied from virtually 100% in some species to less than 75% in others. Moreover, individual bees of some species were found to have almost pure loads of alternative hosts, often very distantly related to the usual preferred host.
By combining my analysis of host-plant use with a phylogeny based on ~2800 bp of mitochondrial and nuclear_encoded DNA sequences, I have estimated that Malvaceae is the likely ancestral host for Diadasia, and use of other hosts can be explained by a single switch to each of the other four host plant families. This pattern suggests that factors in addition to host switching have been important in the diversification of Diadasia. This is the first study to investigate the historical patterns of host use in a monophyletic group of specialist bees, and is important for understanding the role of host-switching in bee diversification. This study resulted in 114 DNA sequence accessions deposited in Genbank. Voucher specimens for the phylogenetic study are databased and in the collection of the U.S. Pollinating Insect Collection, USDA/ARS, Logan, UT.
In the future I will be examining 1) examining host choice evolution within Emphorini in order to test hypotheses of adaptive morphology relating to host choice in Diadasia and other emphorines and 2) examining the efficacy of specialist bees such as Diadasia vs. generalist bees as pollinators of host plants.
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