Lack of Variation in Nuclear Genes Among Isolated Populations of the Sand Dune Restricted Bee Colletes stepheni (Hymenoptera: Colletidae)

Sand dunes in the Mojave and Great Basin Deserts are unique habitats that often harbor distinct and endemic taxa (Ricketts et al., 1999). Epps et al. (1998) compared sand dunes to ‘‘habitat islands’’ in the desert, which provide habitat for many sand-obligate organisms. Because many of these dunes are distant from other dunes, and often flanked by mountain ranges or alkaline hardpan, isolated populations of dune-restricted organisms are often genetically distinct (e.g., Britten and Rust, 1996; Epps et al., 1998). We attempted to discover phylogeographic patterns among populations of Colletes stepheni Timberlake, a sand dune restricted, oligolectic solitary bee. Colletes stepheni nests solely in loose sandy substrates and provisions its nest with pollen only from a limited number of plant species (Hurd and Powell, 1958; Minckley et al., 2000; Andrus, 2003). Populations of C. stepheni in the Mojave Desert use pollen only from Larrea tridentata (creosote bush; Zygophylaceae), which is a common shrub in the hot deserts of North America, while populations in the Great Basin Desert, a cold desert, use pollen from Psorothamnus (indigo bush; Fabaceae) (Andrus, 2003). Andrus (2003) attempted to find phylogeographic patterns among populations of C. stepheni from 13 dune sites across the Mojave and Great Basin Deserts using mtDNA. These attempts uncovered the presence of nuclear mitochondrial pseudogenes (NUMTS), which are mitochondrial genes that have been incorporated into the nuclear genome (Margulis, 1970). The presence of these pseudogenes in C. stepheni made the use of mtDNA for phylogeographic analyses difficult, if not impossible. While the majority of phylogeographic analyses use mtDNA, particularly the gene COI, to investigate phylogeographic patterns, several recent studies employ various nuclear genes to uncover intraspecific variation. For example, a phylogeographic study of sea urchins found that the internal transcribed spacer region 2 was more variable at the population level than COI (Iuri et al., 2007). Studies on several other taxa have also shown that the internal transcribed spacer regions 1 and 2 (ITS1 & ITS2) are variable at the population level (e.g., Mavarez et al., 2002; Gomez-Zurita and Vogler, 2003; Holderegger and Abbott, 2003). While nuclear genes are not widely used in phylogeographic studies of Hymenoptera, several recent studies have found population-level variation in various hymenopterans. For example, variation has been found among populations of stingless bees (De la Rua et al., 2007), an encyrtid wasp (Alvarez and Hoy, 2002), an oak gallwasp (Rokas et al., 2001), and several velvet ant species (Wilson and Pitts, 2008, 2009). Other studies on velvet ants (Hymenoptera: Mutillidae), while not specifically investigating intraspecific variation, showed that such variation exists in both the ITS1 and ITS2 sequences (Pilgrim and Pitts, 2006; Pitts et al., 2007; Pilgrim et al., 2008). Another nuclear gene, Long-wavelength rhodopsin (opsin), has not been widely used in population-level analyses, but has three introns (167 total bp in C. stepheni), which have been informative in differentiating between isolated populations of mutillid wasps (Wilson and Pitts, unpubl. data). Also, Almeida and Danforth (2009) in a phylogenetic analysis of colletid bees found that the introns of opsin were too variable to be used in a species-level analysis. This would suggest that the introns might be informative at the intraspecies level. Kuhlmann et al. (2007) found population-level differences in the introns of another nuclear gene Elongation Factor 1-Alpha for Colletes halophilus as well. In this study, we attempted to find phylogeographic patterns using three nuclear genes, the two rDNA internal transcribed spacer regions (ITS1 & ITS2), and the long-wavelength rhodopsin gene (Opsin).