Abstract
AbstractTo determine whether genetic variation in representative reptiles of the southwestern U.S. may have been similarly molded by the geologic history of the lower Colorado River, we examined restriction site polymorphisms in the mitochondrial DNA (mtDNA) of desert iguanas (Dipsosaurus dorsalis) and chuckwallas (Sauromalus obesus). Observed phylogeographic structure in these lizards was compared to that reported for the desert tortoise (Xerobates agassizi), whose mtDNA phylogeny demonstrates a striking genetic break at the Colorado River.Both the desert iguana and chuckwalla exhibit extensive mtDNA polymorphism, with respective genotypic diversities G = 0.963 and 0.983, close to the maximum possible value of 1.0. Individual mtDNA clones, as well as clonal assemblages defined by specific levels of genetic divergence, showed pronounced geographic localization. Nonetheless, for each species the distributions of certain clones and most major clonal groupings encompass both sides of the Colorado River valley, and hence are clearly incongruent with the phylogeographic pattern of the desert tortoise. Overall, available molecular evidence provides no indication that the intraspecific phylogenies of the southwestern U.S. herpetofauna have been concordantly shaped by a singular vicariant factor of overriding significance.
Highlights
Lamb, Jones and AviseVicariant biogeographic analyses commonly focus on congruent patterns among monophyletic assemblages of species or higher taxa, and address the role of ancient geologic events in shaping biological distributions on a broad spatial scale
The desert iguana (Dipsosaurus dorsalis) and the chuckwalla (Suuromulus obesus) were selected for this comparative survey for the following reasons: 1) they share nearly identical distributions with the desert tortoise, with the northern range bisected by the Colorado River; 2) both species are relatively common; and 3) both species exhibit pronounced habitat specificity and show reduced dispersal tendencies, two features that might accentuate mitochondrial DNA (mtDNA) differentiation
Preparation of purified mtDNA from fresh liver was conducted according to Lansman et al (1981) with the minor modifications described by Lamb et al (1989)
Summary
The efficacy of comparative surveys of mtDNA variation in testing biogeographic hypotheses, either by identifying or confirming regional tracks or barriers, will require additional examples from a variety of geographic realms. One such favorable setting is the lower Colorado River, extending through the Mojave and Sonoran deserts of Arizona, California, and Nevada. In this study we evaluate population-genetic structure in two species of iguanine lizards to determine whether their mtDNA phylogenies have been influenced by the same historical events that presumably shaped the phylogeographic subdivisions of the desert tortoise. The desert iguana (Dipsosaurus dorsalis) and the chuckwalla (Suuromulus obesus) were selected for this comparative survey for the following reasons: 1) they share nearly identical distributions with the desert tortoise, with the northern range bisected by the Colorado River; 2) both species are relatively common; and 3) both species exhibit pronounced habitat specificity and show reduced dispersal tendencies, two features that might accentuate mtDNA differentiation
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