Studies on the Evolutionary Genetics of Pocket Gophers GenusThomomys

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Pocket gophers, subterranean rodents of western North America, are among the world's most variable mammals with respect to their genetic and morphological features. They occur in all available soil types from desert valleys to mountain tops, across an extremely wide range of ecological settings. They are, in fact, major agricultural pests in many areas of their range, particularly in areas where the native vegetation has been replaced by monocultures of alfalfa or other crops. Over the past 20 years, James Patton has studied aspects of the genetic differentiation among populations of pocket gophers in features such as chromosomal changes and shifts in gene frequencies. These have provided clues to the processes involved in the development of the enormous degree of evolutionary divergence that is readily observable among populations of pocket gophers. The current proposal will be directed primarily to aspects of morphological change among pocket gopher populations, with particular emphasis on body size variation and its influence on various aspects of life history, such as density, litter size, number of litters per year, and so forth. The focus of the study is on comparisons between populations of pocket gophers from natural desertscrub vegetation communities in the deserts of eastern California and those that have invaded recently developed monocultures of alfalfa. Observations to date show that in as few as a dozen years or so, the average body size in alfalfa populations increases over two-fold, and that there are major shifts in reproductive performances associated with these body size changes. These reproductive changes are of such magnitude that alfalfa farmers are, in fact, also farming pocket gophers. The principal investigator will investigate the causal basis of this size variation by a series of transplant experiments, shifting animals from natural vegetation to alfalfa fields, by observing growth rates of individuals in both habitats, and by rearing young, born in the laboratory to mothers caught in both habitat types, under a common nutritional regime. By so doing, it will be determined if the body size increase is merely a reflection of the radical change in diet represented by alfalfa, or whether it represents a major change in genetical constitution due to natural selection. These studies will incorporate several different paired natural and alfalfa populations, and the relationships among the pairs will be ascertained by genetic techniques, such as protein electrophoresis and DNA sequence analysis.