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Journal Article

Genetic Variation in Thomomys bottae Pocket Gophers: Macrogeographic Patterns

James L. Patton and Suh Y. Yang
Evolution
Vol. 31, No. 4 (Dec., 1977), pp. 697-720
DOI: 10.2307/2407434
Stable URL: http://www.jstor.org/stable/2407434
Page Count: 24

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Topics: Alleles, Species, Evolution, Rodents, Gene flow, Genetic loci, Population geography, Kidneys, Mammals, Genetics
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Genetic Variation in Thomomys bottae Pocket Gophers: Macrogeographic Patterns
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Abstract

Genetic variation in both chromosomal and allozyme features has been summarized for the pocket gopher, Thomomys bottae. The former data supplement those presented earlier (Patton, 1972); the latter are based on the examination of 23 presumptive gene loci for 825 individuals from 50 populations. The salient points of the study are as follows: (1) Inter and intrapopulation variability in both karyotype and allozyme pattern are extreme. Populations may differ by as many as 19 cytologically detectable features, all apparently involving whole-arm constitutive heterochromatin additions/deletions. These do not impose any known meiotic load in heterozygotes. Divergence in allozyme frequencies between populations on a macrogeographic scale is quite high. Indeed, the average T. bottae population pair shares only 79 percent overall similarity (range 63 to 98 percent, based on Rogers' [1972] S-value. The geographic pattern is one of strong regionally delineated units which, while somewhat internally uniform, abut and interbreed with adjacent regional units. There is a high degree of concordance between the allozyme and chromosomal geographic patterns. (2) Most populations of T. bottae are quite variable genically, the average being polymorphic for 33 percent of the loci examined (range, 13 to 57 percent). Additionally, the average individual is heterozygous at 9.3 percent of its loci (range, 3.0 to 16.9 percent). Thus, the species is not only considerably more variable than its relative T. 'talpoides' complex (Nevo et al., 1974) but it is also more variable than the average rodent. (3) The most satisfying explanation for the generally high level of genic variability involves the lack of severe bottlenecking in a given population's history and the degree of gene flow between populations. A significant correlation was found between overall variability and both population density and degree of population geographic connectedness. The latter is reflected in an extremely strong, inverse relationship between pair-wise genic similarity (S-value) and ecogeographic distance between populations occupying relatively continuous habitat types, even over extensive geographic distances. This suggests that gene flow plays a major role in geographic structuring, despite the apparent low vagility typical of fossorial rodents. (4) The data presented here do not support the concept of homozygosity as an adaptive strategy in response to the uniform subterranean environment of fossorial rodents (e.g., Nevo et al., 1974). Instead, a model is developed which identifies historical impact of population range changes on the degree of bottlenecking and the development of reproductive isolation resulting from fixation of chromosomal rearrangements between populations as key elements to explain the level of observed variability in Thomomys species. (5) Divergence in structural gene loci as assayed by electrophoretic techniques is relatively unrelated to the attainment of reproductive isolation in pocket gophers (and probably other organisms as well, see Gottlieb, 1976; Avise, 1976). Rather, speciation events are strongly correlated with chromosomal rearrangements of the type which may cause severe meiotic imbalances in heterozygotes. This is substantiated by the fact that T. umbrinus populations, which differ from T. bottae by several translocations and are reproductively isolated (Patton, 1973), share closer genic similarity to some geographic segments of T. bottae than the latter do among themselves.

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