Ronald K. Chesser

Estimation of fixation indices and gene diversities

Considering the multinomial sampling of genotypes, unbiased estimators of various gene diversity measures in subdivided populations are presented. Using these quantities, formulae for estimating Wrights fixation indices (FIS, FIT, and FST) from a finite sample are developed.

Population genetics meets behavioral ecology.

Populations are often composed of more than just randomly mating subpopulations - many organisms from social groups with distinct patterns of mating and dispersal. Such patterns have recieved much attention in behavioral ecology, yet theories of population genetics rarely take social structures into account. Consequently, population geneticists often report high levels of apparent in breeding and concomitantly low efective sizes, even for species that avoid mating between close kin. Recently, a view of gene dynamics has been introduced that takes dispersal and social structure into account. Accounting for social structure in population genetics leads to a different perspective on how genetic variation is partitoned and the rate at which genic diversity is lost in natural populations - a view that is more consistent with observed behaviors for the minimization of inbreeding.

GENETIC VARIABILITY WITHIN AND AMONG POPULATIONS OF THE BLACK-TAILED PRAIRIE DOG

Genetic heterogeneity over short geographic distances has been observed for many populations of small mammals (house mice: Selander, 1970; deer mice: Wright, 1978; pocket gophers: Patton and Yang, 1977; Smith et al., 1978; Wright, 1978; Patton and Feder, 1981) as well as for large, highly mobile species such as the elephant (Osterhoff et al., 1974), moose (Ryman et al., 1977, 1980), red deer (Gyllensten et al., 1980), and white-tailed deer (Manlove et al., 1976; Chesser et al., 1982). For most studies of the genetic structure of populations the specific mechanisms of genetic differentiation have not been identified. To understand the causes of population subdivision more fully, comparison of genetic variability should be made among the breeding and/or social units, rather than arbitrarily selected samples. Allele frequency differences among observed social groups within populations have been documented for house mice (Selander, 1970), dark-eyed juncos (Baker and Fox, 1978), marmots (Schwartz and Armitage, 1980), and humans (Neel and Ward, 1972). The organization of populations into independent breeding units may have important effects on the shortterm evolution of populations (Wright, 1980) as well as on the maintenance of genetic polymorphisms (Chesser et al., 1980; Karlin and Campbell, 1980). The black-tailed prairie dog (Cynomys ludovicianus) is perhaps the most socially complex of any rodent species (King, 1955; Koford, 1958). Prairie dog populations are comprised of several small coteries (har-

DO BLACK-TAILED PRAIRIE DOGS MINIMIZE INBREEDING ?

Considerable controversy surrounds the importance of inbreeding in natural populations. The rate of natural inbreeding and the influences of behavioral mechanisms that serve to promote or minimize inbreeding (e.g., philopatry vs. dispersal) are poorly understood. We studied inbreeding and social structuring of a population of black‐tailed prairie dogs (Cynomys ludovicianus) to assess the influence of dispersal and mating behavior on patterns of genetic variation. We examined 15 years of data on prairie dogs, including survival and reproduction, social behavior, pedigrees, and allozyme alleles. Pedigrees revealed mean inbreeding coefficients (F) of 1–2%. A breeding‐group model that incorporated details of prairie dog behavior and demography was used to estimate values of fixation indices (F‐statistics). Model predictions were consistent with the minimization of inbreeding within breeding groups (“coteries,” asymptotic FIL = –0.18) and random mating within the subpopulation (“colony,” asymptotic FIS = 0.00). Estimates from pedigrees (mean FIL = –0.23, mean FIS = 0.00) and allozyme data (mean FIL = –0.21, mean FIS = –0.01) were consistent with predictions of the model. The breeding‐group model, pedigrees, and allozyme data showed remarkably congruent results, and indicated strong genetic structuring within the colony (FLS = 0.16, 0.19, and 0.17, respectively). We concluded that although inbreeding occurred in the colony, the rate of inbreeding was strongly minimized at the level of breeding groups, but not at the subpopulation level. The behavioral mechanisms most important to the minimization of inbreeding appeared to be patterns of male‐biased dispersal of both subadults and adults, associated with strong philopatry of females. Incest avoidance also occurred, associated with recognition of close kin via direct social learning within the breeding groups.

Microsatellites indicate a high frequency of multiple paternity in Apodemus (Rodentia)

Microsatellites were employed to estimate frequency of multiple paternity litters of two species of mice (genus Apodemus): striped field mouse (A. agrarius), and wood mouse (A. sylvaticus). Ten pregnant females of A. agrarius and six of A. sylvaticus were collected from natural populations in the northern Ukraine and analysed with 11 and nine microsatellite loci, respectively. Multiple paternity was indicated in eight of 10 litters in A. agrarius and in three of six litters in A. sylvaticus. Multiple paternity was documented at several loci (ranging from two to 10). In two cases (A. agrarius), three males were estimated to have fathered the litter.

Small Mammals from the Most Radioactive Sites Near the Chornobyl Nuclear Power Plant

This study was designed to estimate the impact of pollution resulting from the meltdown of Reactor 4, Chornobyl, Ukraine, on the taxonomic diversity and abundance of small mammals in the surrounding area. Trap sites included the most radioactive areas within the 10-km exclusion zone, a site within the 30-km exclusion zone that received minimal radioactive pollution, and five sites outside of the 30-km exclusion zone. Within the exclusion zones, 355 specimens representing 11 species of small mammals were obtained, whereas 224 specimens representing 12 species were obtained from outside the exclusion zone. It is concluded that the diversity and abundance of the small-mammal fauna is not presently reduced at the most radioactive sites. Specimens from the most radioactive areas do not demonstrate aberrant gross morphological features other than enlargement of the spleen. Examination of karyotypes does not document gross chromosomal rearrangements.

Breeding Groups and Gene Dynamics in a Socially Structured Population of Prairie Dogs

Genetic substructuring of a colony of black-tailed prairie dogs ( Cynomys ludovicianus ) was examined using three different sources of information: allozyme alleles, pedigrees, and demography (a “breeding-group” model based on mating and dispersal patterns). Prairie dogs and their social breeding groups (called “coteries”) were studied under natural conditions during a 15-year period. Prairie-dog coteries exhibited substantial genetic differentiation, with 15–20% of the genetic variation occurring among coteries. Mating patterns within the colony approximated random mating, and, thus, mates tended to originate from different coteries. Social groups of black-tailed prairie dogs resulted in genetic substructuring of the colony, a conclusion that was supported by estimates from allozyme alleles and colony pedigrees. Predictions of the breeding-group model also were consistent with and supported by estimates from allozyme and pedigree data. Some methodological problems were revealed during analyses. Although individuals of all ages usually are pooled for biochemical estimates of among-group genetic differentiation, our estimates of among-coterie variation from allozyme data were somewhat higher for young than for older prairie dogs, perhaps due to sampling effects caused by mating patterns and infanticide of offspring. Pedigree estimates of among-coterie genetic differentiation were significantly positive for young prairie dogs, adult females, and adult males. Those estimates were always more accurate for the offspring generation, however, because pedigree data were always more complete for young and genetic differences among coteries were diluted by virtually complete dispersal of males away from their natal coteries.

INFLUENCES OF GENETIC VARIABILITY AND MATERNAL FACTORS ON FETAL GROWTH IN WHITE-TAILED DEER

A number of quantitative traits influence secondary productivity and are associated with genetic variability. High heterozygosity has been positively correlated with aggressiveness and exploratory behavior (Garten, 1976, 1977), social dominance (Baker and Fox, 1978), body size (Koehn et al., 1973; Boyer, 1974; Garten, 1976), growth rate (Singh and Zouros, 1978), production of offspring (Smith et al., 1975; Johns et al., 1977) and developmental stability (Mitton, 1978; Soule, 1979). In domestic animals, much of the variation in secondary productivity (i.e., somatic growth) has been explained by the effects of inbreeding depression or heterosis (Falconer, 1960). Reproductive capacity and/or physiological efficiency are apparently the characteristics most sensitive to inbreeding depression (Falconer, 1960 p. 258). Secondary productivity and the heterozygosity of individuals may be influenced by the breeding history of the population. Differences in gene frequency over relatively short distances indicate population subdivision and some degree of site tenacity for individual animals, since dispersal between subdivisions results in genetic homogeneity in the absence of large differences in selective pressures (Crow and Kimura, 1970). The combination of site tenacity and small population size increases the likelihood of inbreeding. Dispersal between the subdivisions will increase the heterozygosity of resulting offspring and eliminate the direct effects of inbreeding (i.e., apparent heterosis). In-

A Translocated Mitochondrial Cytochrome b Pseudogene in Voles (Rodentia: Microtus)

Abstract. A full-length cytochrome b pseudogene was found in rodents; it has apparently been translocated from a mitochondrion to the nuclear genome in the subfamily Arvicolinae. The pseudogene (ψcytb) differed from its mitochondrial counterpart at 201 of 1143 sites (17.6%) and by four indels. Cumulative evidence suggests that the pseudogene has been translocated to the nucleus. Phylogenetic reconstruction indicates that the pseudogene arose before the diversification of M. arvalis/M. rossiaemeridionalis from M. oeconomus, but after the divergence of the peromyscine/sigmodontine/arvicoline clades some ∼10 MYA. Published rates of divergence between mitochondrial genes and their nuclear pseudogenes suggest that the translocation of this mitochondrial gene to the nuclear genome occurred some 6 MYA, in agreement with the phylogenetic evidence.

INBREEDING AS A STRATEGY IN SUBDIVIDED POPULATIONS

A generalized expression for coefficients of consanguinity and relationship with previous inbreeding is presented to examine various breeding strategies in subdivided populations. Conditions that would favor inbreeding are developed for: 1) nonfamilial inbreeding within a deme versus outbreeding; 2) altruistic inbreeding by females versus outbreeding; 3) sib‐mating versus outbreeding; and 4) sib‐mating versus nonfamilial breeding within a deme. Inbreeding behavior is advantageous under certain conditions but depends on the types of mating, the previous breeding history of the deme, the rate of accumulation of inbreeding depression, and the cost of migration. In polygynous mating systems it is genetically more advantageous for males to migrate, because female emigration may 1) leave a related male with no mate or one fewer mate, or 2) force both male and female to risk the cost of migration. Nonfamilial breeding is always a better strategy than sib‐mating given previous inbreeding within the deme. Even when the cost of migration is zero, inbreeding is favored if the coefficient of relationship among relatives is greater than the ratio of the probabilities of offspring inviability to offspring viability. Although high inbreeding coefficients are probably not adaptive unless the costs of migration are great or inbreeding depression constants are small, low levels of inbreeding are advantageous in many situations. Therefore, increased genetic representation by way of inbreeding and inclusive fitness is a major component of the evolutionary process.