An Electrophoretic Comparison of Genetic Variability between Eastern and Western Populations of the Opossum (Didelphis virginiana)

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Eighty-five opossums {Didelphis virginiana) were electrophoretically analyzed for genetic variation in proteins encoded by 31 presumed structural gene loci to determine whether differences occurred between populations from the eastern and western United States. Deviations from Hardy-Weinberg equilibria, expressed as excesses of heterozygous genotypes, occurred at several polymorphic loci in most populations. Western populations exhibit higher mean percentage of loci heterozygous per individual (14.01%) than the eastern populations (9.10%). Mean heterozygosity per individual for the species is 11.56%. Mean percentage of loci polymorphic per population is also higher in the west (29.03%) than in the east (26.88%). Coefficients of genetic similarity and distance suggest that major genetic differentiation has not occurred between eastern and western populations. Introduction Genetic variation has been extensively studied in both vertebrates and invertebrates. While work on mammals has been done (Bonnell and Selander, 1974; Johnson and Selander, 1971; Nevo and Shaw, 1972; Selander et al., 1969a, 1969b, 1971; Smith et al., 1973), it has been directed mainly toward rodents. Virtually no studies of this type have been done on marsupials. The unique biological characteristics of the opossum (Didelphis virginiana Kerr) and the introduction of this animal into the Pacific coastal regions present an opportunity for the study of genetic variation in allopatric populations of this animal. The opossum, existing in wide variety of habitats, is generalist in its adaptive strategy. Omnivorous food preference and high fecundity enhance the ecological amplitude of this organism. An average of two litters per year with as many as 13 young per litter has been observed (McManus, 1970) ; the opossum may have three litters per year in warm environments (Hartman, 1928). The native range of the opossum is from the Great Plains of southern Canada and the United States to the desertlike areas of New Mexico and northwestern Mexico (Hall and Kelson, 1959). The Great Plains, desert and Rocky Mountains limited the opossum to the area of the United States E of the Continental Divide. However, the opossum's introduction by man into California in 1880 (Scheffer, 1943), Oregon in 1910 and again in 1921 (Jewett and Dobyns, 1929) and Washington in 1910 (Scheffer, 1943) has initiated thriving Pacific coastal population. The opossum, referred to as living fossil by many authors lived in the Cretaceous approximately 100 million years ago (Colbert, 1961) and is an example of an organism which has undergone bradytelic evolution (Dobzhansky, 1970). As Selander et al. (1970) state, In the early part of this century it was popularly believed that stable or slowly evolving (bradytelic) lines lacked the genetic variation necessary for more rapid evolution as result of having abnormally low mutation rates; as Mayr (1963) put it, a lack of variation in time and space was interpreted as reflection of genotypic stability. If this interpretation is correct, allelic variation in the opossum should be very low. However, recent studies of the horseshoe crab 1 Requests for reprints should be sent to S. I. Guttman. 2 Present address : Natural Resource Ecology Laboratory, Colorado State University, Fort Collins 80523.