Abstract - Genetic Evidence of Outbreeding in the Black-Tailed Prairie Dog (Cynomys ludovicianus)

Foltz, David W. and Hoogland, John L. 1983. Genetic Evidence of Outbreeding in the Black-Tailed Prairie Dog (Cynomys ludovicianus). Evolution 37. pp. 273-281.

Abstract

The study of mating systems and the effect of social behavior on the genetic structure of animal populations is an important topic in evolutionary biology. The theoretical approaches of Wright (1978 and references therein) and Malecot (1969) have been extended by many other researchers, including Cockerham (1973), Jacquard (1974), Smith (1974) and Nei (1977). Two methods have been used to study mating systems in natural populations. The more usual approach is to assay the genetic structure by collecting a population sample and determining allele and genotype frequencies at one or more polymorphic loci. The number of such studies has increased rapidly in recent years, due to the now-routine application of gel electrophoresis in population biology. Previous research on the genetic structure of rodent populations includes that of Petras (1967), Selander (1970), Birdsall and Nash (1973), Myers (1974), Schwartz and Armitage (1980, 1981), Foltz (1981), Hanken and Sherman (1981) and Patton and Feder ((1981). A second approach involves the analysis of pedigrees obtained by long-term study of populations whose members are individually marked. Because of the obvious difficulties in obtaining this information for natural populations, such studies are relatively rare. Exceptions include the work of Howard (1949), Bulmer (1973), Missakian (1973), Greenwood et al. (1978) and Brown and Brown (1981). We here report on the genetic structure of the black-tailed prairie dog (Sciuridae: Cynomys ludovicianus), as determined by electrophoretic study of four variable blood proteins and pedigree analysis. There are several advantages to collecting electophoretci and pedigree data from the same population. First, the electrophoretic data can be used to confirm the pedigree structure through paternity anaylsis. This part of our research is described elsewhere (Folta and Hoogland, 1981). Second, analysis of several types of data from a single population may provide a better understanding of the underlying factors affecting its genetic structure than analysis of only one type of data (see, for example, Thompson and Roberts, 1980). In this paper, we calculate two measures of population structure: Wright's (1978) genotype fixation index, and Allen's (1965) coefficient of nonrandom mating. The mean fixation index is negative in each year of the study, indicating an overall excess of heterozygotes. The coefficient of nonrandom mating is also negative each year, indicating an avoidance of consanguineous matings. This result is consistent with previous behavioral research on this species (Hoogland, 1982). We conclude that the mating pattern can explain some, but not all, of the heterozygote excess.