27th Annual Meeting and Symposium of the
Desert Tortoise Council, March 22-24, 2002
Abstracts

STUDENT PAPER

Phylogeographic Patterns in Mojave and Sonoran Populations of the Desert Tortoise

Taylor Edwards¹, Cecil R. Schwalbe², and Kristin H. Berry^3
1Wildlife and Fisheries Science, University of Arizona, 125 Biological Sciences East, Tucson, AZ 85721; 
^1,2U.S. Geological Survey, Sonoran Desert Field Station, Tucson, AZ 85721
 ³U. S. Geological Survey, Box Springs Field Station, Riverside, CA 92507

Mitochondrial DNA analysis has played an important role in our understanding of the evolutionary relationships among North American gopher tortoises. In the desert tortoise (Gopherus agassizii), restriction fragment surveys of mtDNA have been used to examine intraspecific phylogeny, genetic variability, and population structure. However, the slow rate of mtDNA evolution in turtles limits the amount of variability detectable with this method. To complement our current knowledge of desert tortoise population genetics, we used DNA sequencing which provides higher resolution and allows us to examine variation among populations on a finer geographic scale. We sampled 30 individuals in 4 populations in the western Mojave Desert and 38 individuals from 7 populations in the Sonoran Desert. We obtained 987 base pairs of sequence from the ND3/ND4 region of the mtDNA molecule. Populations sampled from within each of these desert regions are within a 100-km radius of each other. Our results are consistent with previous studies in that Mojave and Sonoran populations show a fixed nucleotide difference between them indicating that gene flow has not occurred since the Bouse embayment (~5.5 Mya). However, FST values (a measure of the degree to which populations exchange migrants) show that intermediate levels of gene flow occur within each region (Sonoran = 0.404, Mojave = 0.072). Little variability is detected within each desert region (uncorrected pairwise distance p<0.001) and negative Tajima's D values calculated for each region (Sonoran = -1.134, Mojave = -1.515) indicate that there are selective pressures that reduce genetic diversity in both deserts, such as maintaining small effective populations over time with low levels of inbreeding. Despite differences in reproductive ecology and habitat selection between Mojave and Sonoran populations of the desert tortoise, the similarity of genetic variability and gene flow estimates observed within each region suggest that both populations experience similar influences on their genetic structure.