Temporal increase in mtDNA diversity in a declining population.

In small and declining populations levels of genetic variability are expected to be reduced due to effects of inbreeding and random genetic drift. As a result, both individual fitness and populations' adaptability can be compromised, and the probability of extinction increased. Therefore, maintenance of genetic variability is a crucial goal in conservation biology. Here we show that although the level of genetic variability in mtDNA of the endangered Fennoscandian lesser white-fronted goose Anser erythropus population is currently lower than in the neigbouring populations, it has increased six-fold during the past 140 years despite the precipitously declining population. The explanation for increased genetic diversity in Fennoscandia appears to be recent spontaneous increase in male immigration rate equalling 0.56 per generation. This inference is supported by data on nuclear microsatellite markers, the latter of which show that the current and the historical Fennoscandian populations are significantly differentiated (F(ST) = 0.046, P = 0) due to changes in allele frequencies. The effect of male-mediated gene flow is potentially dichotomous. On the one hand it may rescue the Fennoscandian lesser white-fronted goose from loss of genetic variability, but on the other hand, it eradicates the original genetic characteristics of this population.

Consequences of polluted environments on population structure: the bank vole (Clethrionomys glareolus) at Chornobyl.

The accident at the Chornobyl Nuclear Power Plant in April 1986, released 100-200 million Curies of radioactive material into the surrounding environment. To investigate the possible genetic and population effects resulting from chronic exposure to this environmental radiation, we have examined mitochondrial DNA (control region) sequences from bank voles, Clethrionomys glareolus, inhabiting contaminated sites. Our analysis indicates genetic diversity is elevated in the contaminated sites when compared to relatively uncontaminated reference sites. This may be attributed to either an increased mutation rate in the mtDNA control region or immigration of individuals from surrounding areas into the contaminated environment. Although our observations do indicate that the contaminated areas represent sink populations, we cannot statistically discriminate between these two alternatives at this time. In addition, we have been unable to attribute any significant detrimental effects to bank vole populations inhabiting the contaminated Chornobyl environment based on these data. This is particularly paradoxical considering bank voles in the contaminated areas harbor the highest radiocesium (137Cs) body burdens and external dose rates of any mammal ever measured. Our long-term research on the bank vole indicates that several factors, including contaminants, may affect haplotype dynamics both spatially and temporally. These multifarious influences subsequently affect population genetic estimates typically used to address the effects of environmental pollution on animal populations. Finally, we provide a general framework for designing experiments investigating the role contaminants play in altering the genetic characteristics of exposed populations.

Experimental exposure of naive bank voles (Clethrionomys glareolus) to the Chornobyl, Ukraine, environment: a test of radioresistance.

Previous studies have demonstrated no difference in micronucleus (MN) frequencies between wild rodents chronically exposed to the environmental radiation contamination of the Chornobyl (Ukraine) exclusion zone and those inhabiting reference populations. The aim of the present study was to test the hypothesis that a population of bank voles (Clethrionomys glareolus) has developed radioresistance as a result of 14 years of chronic, low-dose radiation exposure. Naive voles were placed in environmental enclosures in the Red Forest region of the exclusion zone for 30 d. Blood samples were obtained at regular intervals, and the MN assay was used to assess chromosomal damage. Additionally, radionuclide uptake was monitored throughout the study, and dose was documented for each individual as well as for their offspring. Total dose for the voles experimentally exposed in this environment averaged 1.09 Gy (36.20 mGy d(-1)) for the 30-d study period. Our results indicate that exposure to radiation levels well above regulatory statutes did not result in an increased MN frequency. Furthermore, our results do not support the hypothesis that voles chronically exposed to these radiation levels have developed a genetic basis for radioresistance that is unique from that present in naive populations. The use of C. glareolus as a sentinel species for environmental studies of radiation contamination and the question of whether the MN assay is an appropriate endpoint for studies of low-dose, chronic radiation exposure are also discussed.

Accumulation of 137Cesium and 90Strontium from abiotic and biotic sources in rodents at Chornobyl, Ukraine.

Bank voles (Clethrionomys glareolus) and laboratory strains of house mice (Mus musculus BALB and C57BL) were relocated into enclosures in a highly contaminated area of the Red Forest near the Chornobyl (Ukraine) Reactor 4 to evaluate the uptake rates of 137Cs and 90Sr from abiotic sources. Mice were provided with uncontaminated food supplies, ensuring that uptake of radionuclides was through soil ingestion, inhalation, or water. Mice were sampled before introduction and were reanalyzed every 10 d for 137Cs uptake. Levels of 90Sr were assessed in subsamples from the native populations and in experimental animals at the termination of the study. Uptake rates in house mice were greater than those in voles for both 137Cs and 90Sr. Daily uptake rates in house mice were estimated at 2.72 x 10(12) unstable atoms per gram (whole body) for 137Cs and 4.04 x 10(10) unstable atoms per gram for 90Sr. Comparable rates in voles were 2.26 x 10(11) unstable atoms per gram for 137Cs and 1.94 x 10(10) unstable atoms per gram for 90Sr. By comparing values from voles in the enclosures to those from wild voles caught within 50 m of the enclosures, it was estimated that only 8.5% of 137Cs was incorporated from abiotic sources, leaving 91.5% being incorporated by uptake from biotic materials. The fraction of 90Sr uptake from abiotic sources was at least 66.7% (and was probably much higher). Accumulated whole-body doses during the enclosure periods were estimated as 174 mGy from intramuscular 137Cs and 68 mGy by skeletal 90Sr in house mice over 40 d and 98 mGy from 137Cs and 19 mGy from 90Sr in voles over 30 d. Thus, uptake of radionuclides from abiotic materials in the Red Forest at Chornobyl is an important source of internal contamination.

Group-structured genetic models in analyses of the population and behavioral ecology of poikilothermic vertebrates.

Estimates of gene correlations among individuals within and among populations are frequently derived from statistical analyses of genetic data (e.g., F statistics). These measures can be important tools in molecular ecology and conservation, and offer important insights into population breeding structure. Using recently derived theory developed for group-structured populations, we show that fixation indices, when combined with basic population ecological and demographic data can be used to investigate population mating systems and to predict dispersal rates, trajectories and asymptotic levels of fixation indices, and effective population size. Four case studies of poikilothermic vertebrates are used to demonstrate the broad utility of evolutionary and ecological inferences afforded by group-structured models.

Sex-biased gene flow in spectacled eiders (Anatidae): inferences from molecular markers with contrasting modes of inheritance.

Genetic markers that differ in mode of inheritance and rate of evolution (a sex-linked Z-specific microsatellite locus, five biparentally inherited microsatellite loci, and maternally inherited mitochondrial [mtDNA] sequences) were used to evaluate the degree of spatial genetic structuring at macro- and microgeographic scales, among breeding regions and local nesting populations within each region, respectively, for a migratory sea duck species, the spectacled eider (Somateria fisheri). Disjunct and declining breeding populations coupled with sex-specific differences in seasonal migratory patterns and life history provide a series of hypotheses regarding rates and directionality of gene flow among breeding populations from the Indigirka River Delta, Russia, and the North Slope and Yukon-Kuskokwim Delta, Alaska. The degree of differentiation in mtDNA haplotype frequency among breeding regions and populations within regions was high (phiCT = 0.189, P < 0.01; phiSC = 0.059, P < 0.01, respectively). Eleven of 17 mtDNA haplotypes were restricted to a single breeding region. Genetic differences among regions were considerably lower for nuclear DNA loci (sex-linked: phiST = 0.001, P > 0.05; biparentally inherited microsatellites: mean theta = 0.001, P > 0.05) than was observed for mtDNA. Using models explicitly designed for uniparental and biparentally inherited genes, estimates of spatial divergence based on nuclear and mtDNA data together with elements of the species' breeding ecology were used to estimate effective population size and degree of male and female gene flow. Differences in the magnitude and spatial patterns of gene correlations for maternally inherited and nuclear genes revealed that females exhibit greater natal philopatry than do males. Estimates of generational female and male rates of gene flow among breeding regions differed markedly (3.67 x 10(-4) and 1.28 x 10(-2), respectively). Effective population size for mtDNA was estimated to be at least three times lower than that for biparental genes (30,671 and 101,528, respectively). Large disparities in population sizes among breeding areas greatly reduces the proportion of total genetic variance captured by dispersal, which may accelerate rates of inbreeding (i.e., promote higher coancestries) within populations due to nonrandom pairing of males with females from the same breeding population.

Subchronic exposure of BALB/c and C57BL/6 strains of Mus musculus to the radioactive environment of the Chornobyl, Ukraine exclusion zone.

Environmental contamination resulting from the Chornobyl, Ukraine, disaster offers a unique opportunity to examine the in vivo biological effects of chronic, low-dose exposure to radiation. Laboratory studies of acute exposure to ionizing radiation have been used to estimate risk and potential human health effects by the extrapolation of laboratory data to situations of low-dose environmental radiation exposure. Few studies, however, have explored the biological consequences of low-dose exposure via in situ environmental radiation in a sentinel species. In the present study, laboratory strains of Mus musculus (BALB/c and 57BL/6) were placed in environmental enclosures in the Red Forest region of the Chornobyl exclusion zone. Blood samples were obtained every 10 d, and the micronucleus (MN) test was employed to assess the potential for cytogenetic damage from exposure to Chornobyl radiation. Radionuclide uptake was monitored throughout the study, and dose was estimated for each individual as well as for their offspring. Total dose for the mice experimentally exposed to this environment averaged 1,162 mGy for BALB/c (30 d) and 1,629 mGy for C57BL/6 (40 d). A higher MN frequency for both strains was observed at day 10, although this change was only statistically significant in the C57BL/6 mice (chi2/3 = 13.41, p = 0.003). Subsequent samples from C57BL/6 resulted in values at or less than the initial frequencies. In BALB/c mice, an increase in MN was also evident at day 30 (chi2/3 = 10.38, p = 0.006). The experimental design employed here allows for the incorporation of traditional laboratory strains, as well as transgenic strains of Mus, as sentinels of environmental radiation contamination.

A translocated mitochondrial cytochrome b pseudogene in voles (Rodentia: Microtus)

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 (psi cytb) 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 approximately 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.

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.

Heteroplasmy and organelle gene dynamics.

This study assesses factors that influence the rates of change of organelle gene diversity and the maintenance of heteroplasmy. Losses of organelle gene diversity within individuals via vegetative segregation during ontogeny are paramount to resultant spatial and temporal patterns. Steady-state losses of organelle variation from the zygote to the gametes are determined by the effective number of organelles, which will be approximately equal to the number of intracellular organelles if random segregation prevails. Both rapid increases in organelle number after zygote formation and reductions at germ lines will reduce variation within individuals. Terminal reductions in organelles must be to very low copy numbers (<5) for substantial losses in variation to occur rapidly. Nonrandom clonal expansion and vegetative segregation during gametogenesis may be effective in reducing genetic variation in gametes. If organelles are uniparentally inherited, the asymptotic expectations for effective numbers of gametes and spatial differentiation will be identical for homoplasmic and heteroplasmic conditions. The rate of attainment of asymptote for heteroplasmic organelles, however, is governed by the rate of loss of variation during ontogeny. With sex-biased dispersal, the effective number of gametes is maximized when the proportional contributions of the sex having the higher dispersal rate are low.

Effective sizes and dynamics of uniparentally and diparentally inherited genes.

Models to determine the temporal dynamics and spatial heterogeneity for maternally and paternally inherited genes were derived for populations that may or may not exhibit spatial subdivision. Results were compared to those for diparentally inherited genes. The models permit definition of parameters for mean and variance of litter sizes, breeding group (subpopulation) sizes, and numbers of female mates per male, dispersal rates, and multiple paternity. Exact solutions for asymptotic effective size and spatial divergence (FLS) for maternal and paternal genes are derived. It is shown that solutions for effective size and FLS are transformations of the same quadratic equation. When compared to values for diparentally inherited genes, it is shown that effective sizes for maternal genes may be considerably higher when female dispersal is low as in many mammalian taxa. Likewise, effective sizes for paternal genes may be higher than for diparentally inherited traits when male dispersal is relatively low, as in many species of birds. The traditional assumption that the effective size for maternal genes is approximately equal to the number of females is seldom realized. Spatial heterogeneity and temporal dynamics of genes are inextricably linked as is shown by the interdependency of effective size and spatial heterogeneity.

High levels of genetic change in rodents of Chernobyl.

Base-pair substitution rates for the mitochondrial cytochrome beta gene of free-living, native populations of voles collected next to reactor 4 at Chernobyl, Ukraine, were estimated by two independent methods to be in excess of 10(-4) nucleotides per site per generation. These estimates are hundreds of times greater than those typically found in mitochondria of vertebrates, suggesting that the environment resulting from this nuclear power plant disaster is having a measurable genetic impact on the organisms of that region. Despite these DNA changes, vole populations thrive and reproduce in the radioactive regions around the Chernobyl reactor.

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.

Effective population sizes with multiple paternity.

While the concept of effective population size is of obvious applicability to many questions in population genetics and conservation biology, its utility has suffered due to a lack of agreement among its various formulations. Often, mathematical formulations for effective sizes apply restrictive assumptions that limit their applicability. Herein, expressions for effective sizes of populations that account for mating tactics, biases in sex ratios, and differential dispersal rates (among other parameters) are developed. Of primary interest is the influence of multiple paternity on the maintenance of genetic variation in a population. In addition to the standard inbreeding and variance effective sizes, intragroup (coancestral) and intergroup effective sizes also are developed. Expressions for effective sizes are developed for the beginning of nonrandom gene exchanges (initial effective sizes), the transition of gene correlations (instantaneous effective sizes), and the steady-state (asymptotic effective size). Results indicate that systems of mating that incorporate more than one male mate per female increase all effective sizes above those expected from polygyny and monogamy. Instantaneous and asymptotic sizes can be expressed relative to the fixation indices. The parameters presented herein can be utilized in models of effective sizes for the study of evolutionary biology and conservation genetics.

Sources of error associated with sample collection and preparation of nucleated blood cells for flow cytometric analysis.

Analysis of cellular DNA content by flow cytometry has been used to detect genetic changes associated with exposure to environmental contaminants. In lower vertebrates, nucleated red blood cells can be collected for analysis without harm to the animal. Because erythrocytes sampled from an individual should have identical amounts of DNA, the coefficient of variation (CV) around the G0/G1 peak should be small. Increases in CV can indicate genetic aberrations, but may also be caused by sample handling and preparation or problems with instrumentation. To increase confidence in associating increases in CV with external causes, artifactual changes in CV due to sample treatment and instrument parameters should be identified and minimized. We assessed the effects of various sampling and handling protocols on the CV of nucleated blood cells collected from largemouth bass (Micropterus salmoides). We also compared the distribution of cells among the G0/G1, S, and G2/M phases of the cell cycle to see whether these were affected by sampling or treatment protocols. Groups of 7 fish were bled on 7 consecutive days, and blood from each fish was analyzed by flow cytometry when freshly collected, and after freezing for 1 hour or 10 days. The same fish were bled again over a consecutive 7-day period, and the experiment was repeated. CV and cell cycle distribution were not affected by our freezing protocol. Repeat sampling from the same individual did not affect CV, but altered the distribution of cells in the cell cycle, suggesting increased hemopoiesis in response to blood sampling.(ABSTRACT TRUNCATED AT 250 WORDS)

Effective sizes for subdivided populations.

Many derivations of effective population sizes have been suggested in the literature; however, few account for the breeding structure and none can readily be expanded to subdivided populations. Breeding structures influence gene correlations through their effects on the number of breeding individuals of each sex, the mean number of progeny per female, and the variance in the number of progeny produced by males and females. Additionally, hierarchical structuring in a population is determined by the number of breeding groups and the migration rates of males and females among such groups. This study derives analytical solutions for effective sizes that can be applied to subdivided populations. Parameters that encapsulate breeding structure and subdivision are utilized to derive the traditional inbreeding and variance effective sizes. Also, it is shown that effective sizes can be determined for any hierarchical level of population structure for which gene correlations can accrue. Derivations of effective sizes for the accumulation of gene correlations within breeding groups (coancestral effective size) and among breeding groups (intergroup effective size) are given. The results converge to traditional, single population measures when similar assumptions are applied. In particular, inbreeding and intergroup effective sizes are shown to be special cases of the coancestral effective size, and intergroup and variance effective sizes will be equal if the population census remains constant. Instantaneous solutions for effective sizes, at any time after gene correlation begins to accrue, are given in terms of traditional F statistics or transition equations. All effective sizes are shown to converge upon a common asymptotic value when breeding tactics and migration rates are constant. The asymptotic effective size can be expressed in terms of the fixation indices and the number of breeding groups; however, the rate of approach to the asymptote is dependent upon dispersal rates. For accurate assessment of effective sizes, initial, instantaneous or asymptotic, the expressions must be applied at the lowest levels at which migration among breeding groups is nonrandom. Thus, the expressions may be applicable to lineages within socially structured populations, fragmented populations (if random exchange of genes prevails within each population), or combinations of intra- and interpopulation discontinuities of gene flow. Failure to recognize internal structures of populations may lead to considerable overestimates of inbreeding effective size, while usually underestimating variance effective size.