Genetic Variability in a Recruiting Population of Endangered Razorback Suckers from Lake Mead, Arizona–Nevada

The razorback sucker Xyrauchen texanus once was common and widely distributed throughout the Colorado River drainage of western North America. Water development and predation by nonnative species led to a significant decrease in the species' range and a dramatic reduction in the size of remaining populations. Here, we report analyses of mitochondrial DNA (mtDNA) and microsatellite variation in larval and adult razorback suckers from Lake Mead, one of the few locations with recent evidence of recruitment. Analysis of three temporal samples of larvae from Lake Mead yielded considerable variation, but it was significantly lower than variation in Lake Mead adults or in larval and adult samples from Lake Mohave, the largest extant population. This pattern is consistent with analyses of individual temporal samples from Lake Mohave, indicating that larvae were likely produced by a relatively small number of adults. The four adult samples spanned approximately 20 years from the early 1990s to the present. Patterns of microsatellite variation were comparable among all Lake Mead adult samples, but variation was lower than that in Lake Mohave, consistent with the smaller population size in Lake Mead. Levels of mtDNA variation in adults were significantly lower for the early 1990s and 2011 samples than for the 2002 samples. The result for the early 1990s sample was consistent with current knowledge of the Lake Mead population's dynamics; however, the reduced variation in the 2011 sample are somewhat surprising, as the Lake Mead population is thought to be expanding in size. Additional spatial and temporal sampling is essential to accurately assess levels of variation and population size of Lake Mead razorback suckers.