Inbreeding in reintroduced populations: the effects of early reintroduction history and contemporary processes

Maintaining genetic variation and minimizing inbreeding are central goals of conservation genetics. It is therefore crucial to understand the important population parameters that affect inbreeding, particularly in reintroduction programs. Using data from 41 reintroduced Alpine ibex (Capra ibex ibex) populations we estimated inbreeding since the beginning of reintroductions using population-specific Fst, and inbreeding over the last few generations with contemporary effective population sizes. Total levels of inbreeding since reintroduction of ibex were, on average, close to that from one generation of half-sib mating. Contemporary effective population sizes did not reflect total inbreeding since reintroduction, but 16% of variation in contemporary effective population sizes among populations was due to variation in current population sizes. Substantial variation in inbreeding levels among populations was explained by founder group sizes and the harmonic mean population sizes since founding. This study emphasizes that, in addition to founder group sizes, early population growth rates are important parameters determining inbreeding levels in reintroduced populations.     

High levels of variation despite genetic fragmentation in populations of the endangered mountain pygmy-possum, Burramys parvus, in alpine Australia

In endangered mammals, levels of genetic variation are often low and this is accompanied by genetic divergence among populations. The mountain pygmy-possum (Burramys parvus) is an endangered marsupial restricted to the alpine region of Victoria and New South Wales, Australia. By scoring variation at eight microsatellite loci, we found that B. parvus populations exhibit high levels of genetic divergence and fall into three distinct groups from the northern, central and southern areas of the distribution of this species, consistent with previous assessments of mitochondrial DNA variation. F(ST) values between populations from these regions ranged from 0.19 to 0.54. Within the central area, there was further genetic fragmentation, and a linear association between genetic and geographical distance. This pattern is likely to reflect limited dispersal across barriers despite the fact that individual B. parvus can move several kilometres. Levels of genetic variation within populations were high with the exception of a southern population where there was evidence of inbreeding. From a conservation perspective, all three areas where B. parvus are found should be considered as separate gene pools; management of populations within these areas needs to take into account the low gene flow between populations, as well as threats posed by roads, resorts and other developments in the alpine region. The low genetic variability and inbreeding in the southern population is of particular concern given the high levels of variability in other B. parvus populations.     

GENETIC VARIATION IN INBREEDING DEPRESSION IN THE RED FLOUR BEETLE TRIBOLIUM CASTANEUM

Inbreeding depression varies among species and among populations within a species. Few studies, however, have considered the extent to which inbreeding depression varies within a single population. We report on two experiments to provide evidence that inbreeding depression is genetically variable, such that within a single population some lineages suffer severe inbreeding depression, others suffer only mild inbreeding depression, and some lineages actually increase in phenotypic value at higher levels of inbreeding. We examine the effects of population structure on inbreeding depression for two traits in the first experiment (adult dry weight and female relative fitness), and for seven traits in the second experiment (female and male adult dry weight, female and male relative fitness, female and male developmental time, and egg-to-adult viability). In the first experiment, we collected data from 4 families within each of 38 lineages derived from a single ancestral stock population and maintained for four generations of full-sib mating. Both traits demonstrate significant inbreeding depression and provide evidence that even within a single lineage there is significant genetic variability in inbreeding depression. In the second experiment, we collected data from 5 replicates for each of 15 lineages derived from the same ancestral population used in the first experiment; these lineages were maintained for four generations of full-sib mating. We also collected data on outbred control beetles in each generation and incorporated these data into the analyses to account for environmental effects in an unbiased manner. All traits except female and male developmental time show significant inbreeding depression. All traits showing inbreeding depression are genetically variable in inbreeding depression, as is evident from a significant linear lineage-×-f component. For both experiments, the effect of population structure on inbreeding depression is further evident from the increasing amount of variation that can be explained by the models used to measure inbreeding depression when additional levels of population structure are included. Genetic variation in inbreeding depression has important implications for conservation biology and may be an important factor in mating-system evolution.     

Lack of nonadditive genetic effects on early fecundity in Drosophila melanogaster

Fecundity is usually considered as a trait closely connected to fitness and is expected to exhibit substantial nonadditive genetic variation and inbreeding depression. However, two independent experiments, using populations of different geographical origin, indicate that early fecundity in Drosophila melanogaster behaves as a typical additive trait of low heritability. The first experiment involved artificial selection in inbred and non-inbred lines, all of them started from a common base population previously maintained in the laboratory for about 35 generations. The realized heritability estimate was 0.151 +/- 0.075 and the inbreeding depression was very small and nonsignificant (0.09 +/- 0.09% of the non-inbred mean per 1% increase in inbreeding coefficient). With inbreeding, the observed decrease in the within-line additive genetic variance and the corresponding increase of the between-line variance were very close to their expected values for pure additive gene action. This result is at odds with previous studies showing inbreeding depression and, therefore, directional dominance for the same trait and species. All experiments, however, used laboratory populations, and it is possible that the original genetic architecture of the trait in nature was subsequently altered by the joint action of random drift and adaptation to captivity. Thus, we carried out a second experiment, involving inbreeding without artificial selection in a population recently collected from the wild. In this case we obtained, again, a maximum-likelihood heritability estimate of 0.210 +/- 0.027 and very little nonsignificant inbreeding depression (0.06 +/- 0.12%). The results suggest that, for fitness-component traits, low levels of additive genetic variance are not necessarily associated with large inbreeding depression or high levels of nonadditive genetic variance.     

Effects of inbreeding on reproductive success,performance, litter size,and survival in captive red wolves (Canis rufus)

Captive‐breeding programs have been widely used in the conservation of imperiled species, but the effects of inbreeding, frequently expressed in traits related to fitness, are nearly unavoidable in small populations with few founders. Following its planned extirpation in the wild, the endangered red wolf (Canis rufus) was preserved in captivity with just 14 founders. In this study, we evaluated the captive red wolf population for relationships between inbreeding and reproductive performance and fitness. Over 30 years of managed breeding, the level of inbreeding in the captive population has increased, and litter size has declined. Inbreeding levels were lower in sire and dam wolves that reproduced than in those that did not reproduce. However, there was no difference in the inbreeding level of actual litters and predicted litters. Litter size was negatively affected by offspring and paternal levels of inbreeding, but the effect of inbreeding on offspring survival was restricted to a positive influence. There was no apparent relationship between inbreeding and method of rearing offspring. The observable effects of inbreeding in the captive red wolf population currently do not appear to be a limiting factor in the conservation of the red wolf population. Additional studies exploring the extent of the effects of inbreeding will be required as inbreeding levels increase in the captive population. Zoo Biol 29:36–49, 2010. © 2009 Wiley‐Liss, Inc.     

Anther-stigma separation is associated with inbreeding depression in Datura stramonium,a predominantly self-fertilizing annual

Abstract.— Genetically based variation in outcrossing rate generates lineages within populations that differ in their history of inbreeding. According to some models, mating-system modifiers in such populations will demonstrate both linkage and identity disequilibrium with fitness loci, resulting in lineage-specific inbreeding depression. Other models assert that differences among families in levels of inbreeding depression are mainly attributable to random accumulation of genetic load, unrelated to variation at mating-system loci. We measured female reproductive success of selfed and outcrossed progeny from naturally occurring lineages of Datura stramonium , a predominantly self-fertilizing annual weed that has heritable variation in stigma-anther separation, a trait that influences selfing rates. Progeny from inbred lineages (as identified by high degree of anther-stigma overlap) showed equal levels of seed production, regardless of cross type. Progeny from mixed lineages (as identified by relatively high separation between anthers and stigma) showed moderate levels of inbreeding depression. We found a significant correlation between anther-stigma separation and relative fitness of selfed and outcrossed progeny, suggesting that family-level inbreeding depression may be related to differences among lineages in inbreeding history in this population. Negative inbreeding depression in putatively inbred lineages may be due in part to additive effects or to epistatic interactions among loci.     

Genetic discontinuity, breeding-system change and population history of Arabis alpina in the Italian Peninsula and adjacent Alps

Arabis alpina is a widespread plant of European arctic and alpine environments and belongs to the same family as Arabidopsis thaliana. It grows in all major mountain ranges within the Italian glacial refugia and populations were sampled over a 1300 km transect from Sicily to the Alps. Diversity was studied in nuclear and chloroplast genome markers, combining phylogeographical and population genetic approaches. Alpine populations had significantly lower levels of nuclear genetic variation compared to those in the Italian Peninsula, and this is associated with a pronounced change in within-population inbreeding. Alpine populations were significantly inbred (F(IS) = 0.553), possibly reflecting a change to the self-incompatibility system during leading edge colonization. The Italian Peninsula populations were approaching Hardy-Weinberg equilibrium (outbreeding, F(IS) = 0.076) and genetic variation was highly structured, consistent with independent local 'refugia within refugia' and the fragmentation of an established population by Quaternary climate oscillations. There is very little evidence of genetic exchange between the Alps and the Italian Peninsula main distribution ranges. The Alps functioned as a glacial sink for A. alpina, while the Italian Peninsula remains a distinct and separate long-term refugium. Comparative analysis indicated that inbreeding populations probably recolonized the Alps twice: (i) during a recent postglacial colonization of the western Alps from a Maritime Alps refugium; and (ii) separately into the central Alps from a source outside the sampling range. The pronounced geographical structure and inbreeding discontinuities are significant for the future development of A. alpina as a model species.     

Inbreeding effects on the growth in stature among Telaga boys and girls of Kharagpur, West Bengal, India

The present study is an attempt to understand the genetical effects of inbreeding on the process of growth. The inbred and non-inbred subjects were selected on the basis of extensive pedigrees of five generations in the Telaga, an endogamous population of Kharagpur, India. Preference was given to cousins belonging to the same kindreds while selecting control sample so that environmental variation was minimized. Altogether 633 boys and 614 girls of different inbreeding levels aged five to twenty years were measured for stature. Analysis has been done in different levels of inbreeding in each age and sex on mean annual increments and variances of increments. The results revealed that comparison of annual increment for each age between boys and girls with different degrees of inbreeding and application of the one-tailed t-test of significance does not provide any evidence of inbreeding effect on mean increment for stature studied in either sex. This might indicate the absence of marked dominant/recessive effects of genes determining annual increments in body size rather than the absence of genetical control of increments due to growth. Moreover, it is noteworthy that the variance of annual increment due to growth (which is estimated indirectly) consistently increases with increase of inbreeding level with only a few exceptions. The exceptions occur more often in girls than in boys, which can be explained by greater environmental stress and selection pressure and variation in X-linked inbreeding among girls. This would be worthwhile to verify in longitudinal growth data in future. Increased variances of annual increment with inbreeding, in the absence of change of mean increment on inbreeding, would indicate the influence of additive autosomal genes for the process of physical growth in children in either sex. A close scrutiny of the annual increments for the measurements in all the four levels of inbreeding in either sex fails to bring out any consistent trend of change in the age of adolescent spurt with inbreeding. This might suggest an underlying homozygosity of several genes with inbreeding in the population.     

Conservation genetics of the wood ant, Formica lugubris, in a fragmented landscape

Various intrinsic factors connected to the special features of sociality influence the persistence of social insect populations, including low effective population size, reduced amount of genetic variation easily leading to inbreeding depression, and spatially structured populations. In this work, we studied an isolated, small and fragmented population system of the red wood ant Formica lugubris, and evaluated the impact of social and genetic population structure on the persistence and conservation of the populations. The effective population size was large in our study population because all nests were polygynous. As a result, and despite the apparent isolation, the amount of nuclear genetic variability was similar to that in a nonisolated population system. Lack of inbreeding, as well as a high level of variability, indirectly suggests that this population does not suffer from inbreeding depression. The spatial distribution of genetic variation between local populations suggests intensive, but strongly male-biased, nuclear gene flow. Thus, the persistence of this population system does not seem to be threatened by any immediate social or genetic factor, but colonization of new habitat patches may be difficult because of restricted female dispersal.     

Levels of variation in stress resistance in drosophila among strains, local populations, and geographic regions: patterns for desiccation, starvation, cold resistance, and associated traits

Stress resistance traits in Drosophila often show clinal variation. Although these patterns suggest selection, there is generally no attempt to test how large differences at the geographical level are relative to levels of variation within and between local populations. Here we compare these levels in D. melanogaster from temperate Tasmania versus tropical northern Queensland by focusing on adult resistance to desiccation, cold and starvation stress, as well as associated traits (size, lipid content). For starvation and desiccation resistance, levels of variation were highest among strains from the same population. whereas there was little differentiation among local populations and a low level of differentiation at the geographic level. For adult cold resistance, there was local differentiation and strain variation but no geographic variation. For size (thorax length), geographic differentiation was higher despite some overlap among strains from the tropical and temperate locations. Finally, for lipid levels there was only evidence for variation among strains. The low level of differentiation among geographic locations for stress resistance was further verified with the characterization of isofemale strains from 18 locations along a coastal transect extending from Tasmania to northern Queensland. Crosses among some of the isofemale strains showed that results were not confounded by inbreeding effects. Strains derived from a cross between a tropical and temperate strain differed for all traits, and variation among strains for body size was higher than strain variation within the geographic regions. Unlike in previous studies, lipid content and starvation resistance were not correlated in any set of strains, but there was a correlation between cold resistance and lipid content. There was also a correlation between desiccation resistance and size but only in the geographic cross strains. These findings suggest a large amount of variation in stress resistance at the population level and inconsistent correlation patterns across experimental approaches.     

A metapopulation perspective on genetic diversity and differentiation in partially self-fertilizing plants

Abstract.— Partial self-fertilization is common in higher plants. Mating system variation is known to have important consequences for how genetic variation is distributed within and among populations. Selfing is known to reduce effective population size, and inbreeding species are therefore expected to have lower levels of genetic variation than comparable out crossing taxa. However, several recent empirical studies have shown that reductions in genetic diversity within populations of inbreeding species are far greater than the expected reductions based on the reduced effective population size. Two different processes have been argued to cause these patterns, selective sweeps (or hitchhiking) and background selection. Both are expected to be most effective in reducing genetic variation in regions of low recombination rates. Selfing is known to reduce the effective recombination rate, and inbreeding taxa are thus thought to be particularly vulnerable to the effects of hitchhiking or background selection. Here I propose a third explanation for the lower-than-expected levels of genetic diversity within populations of selfing species; recurrent extinctions and recolonizations of local populations, also known as metapopulation dynamics. I show that selfing in a metapopulation setting can result in large reductions in genetic diversity within populations, far greater than expected based the lower effective population size inbreeding species is expected to have. The reason for this depends on an interaction between selfing and pollen migration.     

The genetic characterization of an isolated remnant population of an endangered rodent (<Emphasis Type="Italic">Cricetus cricetus</Emphasis> L.) using comparative data: implications for conservation

Estimates of genetic diversity and phylogenetic affiliation represent an important resource for biodiversity assessment and a valuable guide to conservation and management. We have found a new population (Jawor—JW) of the common hamster Cricetus cricetus in western Poland that is remote from the nearest populations by 235–300 km. With the objective of genetically characterizing of this population, we compared it with other populations from Poland and Germany by taking into account sequences of four mitochondrial DNA genes and variation at 10 microsatellite loci. The JW population exhibited low levels of genetic diversity and allelic and haplotype richness, which likely reflects its extreme isolation. This factor, coupled with inbreeding and genetic drift, are major threats to JW. A neighbor-joining tree based on mtDNA haplotypes shows that JW clusters among samples representing the Central subgroup that is known from central Germany but that has not yet been identified in Poland. Findings presented here improve our understanding of the spread and diversification of the common hamster. We offer the following hypotheses to explain the observed pattern of mtDNA haplotype distribution: JW could be a byproduct of postglacial migrations or back-migrations from eastern refugia to the western part of Europe, or/and be a result of population and habitat fragmentation. We recommend translocation of individuals as an effective management strategy, both at the level of Central phylogeographic group and at the species level, to overcome the negative consequences of inbreeding and geographical isolation of the JW population.     

Consanguinity in Spain: socioeconomic,demographic, and geographic influences

Information on consanguinity in Spain was reanalyzed in order to obtain a more complete picture of consanguinity patterns by considering temporal, geographical, demographic, and economic factors. To obtain data on consanguineous marriages, we reviewed 106 published references. Only populations with homogeneous geographical characteristics, economic activities, transportation facilities, and the like, were considered. Two-way analysis of variance (ANOVA) provided highly significant differences (p < 0.001) for a both for time periods and for urban versus rural patterns, but there was no interaction between the latter two. A regression analysis shows that for rural areas the geographic characteristics, economy, communications, and index of demographic tendency were significant predictors of the inbreeding coefficient. The results obtained indicate that urban and rural patterns differ significantly and that the temporal factor needs to be taken into account before comparing inbreeding coefficients. In urban areas the census size and altitude are included in the regression equation as significant variables, but association was not found between alpha and the index of demographic tendency (population increase or decrease). Instead, the absolute number of inhabitants (census size) was significant. In rural areas geographic characteristics, communications, and index of demographic tendency were significant predictors of the inbreeding coefficient.     

Explaining Spatial Heterogeneity in Population Dynamics and Genetics from Spatial Variation in Resources for a Large Herbivore

Fine-scale spatial variation in genetic relatedness and inbreeding occur across continuous distributions of several populations of vertebrates; however, the basis of observed variation is often left untested. Here we test the hypothesis that prior observations of spatial patterns in genetics for an island population of feral horses (Sable Island, Canada) were the result of spatial variation in population dynamics, itself based in spatial heterogeneity in underlying habitat quality. In order to assess how genetic and population structuring related to habitat, we used hierarchical cluster analysis of water sources and an indicator analysis of the availability of important forage species to identify a longitudinal gradient in habitat quality along the length of Sable Island. We quantify a west-east gradient in access to fresh water and availability of two important food species to horses: sandwort, Honckenya peploides, and beach pea, Lathyrus japonicas. Accordingly, the population clusters into three groups that occupy different island segments (west, central, and east) that vary markedly in their local dynamics. Density, body condition, and survival and reproduction of adult females were highest in the west, followed by central and east areas. These results mirror a previous analysis of genetics, which showed that inbreeding levels are highest in the west (with outbreeding in the east), and that there are significant differences in fixation indices among groups of horses along the length of Sable Island. Our results suggest that inbreeding depression is not an important limiting factor to the horse population. We conclude that where habitat gradients exist, we can anticipate fine-scale heterogeneity in population dynamics and hence genetics.     

The genealogy and genetic viability of reintroduced Yellowstone grey wolves

The recovery of the grey wolf in Yellowstone National Park is an outstanding example of a successful reintroduction. A general question concerning reintroduction is the degree to which genetic variation has been preserved and the specific behavioural mechanisms that enhance the preservation of genetic diversity and reduce inbreeding. We have analysed 200 Yellowstone wolves, including all 31 founders, for variation in 26 microsatellite loci over the 10-year reintroduction period (1995-2004). The population maintained high levels of variation (1995 H(0) = 0.69; 2004 H(0) = 0.73) with low levels of inbreeding (1995 F(IS) = -0.063; 2004 F(IS) = -0.051) and throughout, the population expanded rapidly (N(1995) = 21; N(2004) = 169). Pedigree-based effective population size ratios did not vary appreciably over the duration of population expansion (1995 N(e)/N(g) = 0.29; 2000 N(e)/N(g) = 0.26; 2004 N(e)/N(g) = 0.33). We estimated kinship and found only two of 30 natural breeding pairs showed evidence of being related (average r = -0.026, SE = 0.03). We reconstructed the genealogy of 200 wolves based on genetic and field data and discovered that they avoid inbreeding through a wide variety of behavioural mechanisms including absolute avoidance of breeding with related pack members, male-biased dispersal to packs where they breed with nonrelatives, and female-biased subordinate breeding. We documented a greater diversity of such population assembly patterns in Yellowstone than previously observed in any other natural wolf population. Inbreeding avoidance is nearly absolute despite the high probability of within-pack inbreeding opportunities and extensive interpack kinship ties between adjacent packs. Simulations showed that the Yellowstone population has levels of genetic variation similar to that of a population managed for high variation and low inbreeding, and greater than that expected for random breeding within packs or across the entire breeding pool. Although short-term losses in variation seem minimal, future projections of the population at carrying capacity suggest significant inbreeding depression will occur without connectivity and migratory exchange with other populations.     

Mating patterns and demography in the tristylous daffodil Narcissus triandrus

Mating patterns in plant populations are influenced by interactions between reproductive traits and ecological conditions, both factors that are likely to vary geographically. Narcissus triandrus, a wide-ranging heterostylous herb, exhibits populations with either two (dimorphic) or three (trimorphic) style morphs and displays substantial geographical variation in demographic attributes and floral morphology. Here, we investigate this variation to determine if demography, morphology, and mating system differ between the two sexual systems. Our surveys in Portugal and NW Spain indicated that dimorphic populations were less dense, of smaller size, and had larger plants and flowers compared to trimorphic populations. Outcrossing rates estimated using allozyme markers revealed similar outcrossing rates in dimorphic and trimorphic populations (t(m) dimorphic=0.759; t(m) trimorphic=0.710). All populations experienced significant inbreeding in progeny (mean F=0.143). In contrast, parental estimates of inbreeding were not significantly different from zero (mean F=0.062), implying that few inbred offspring survive to reproductive maturity due to inbreeding depression. Although the majority of inbreeding results from selfing, significant levels of biparental inbreeding were also detected in eight of the nine populations (mean s(s)-s(m)=0.081). Density was negatively associated with levels of selfing but positively associated with biparental inbreeding. Population size was positively associated with outcrossing but not biparental inbreeding. There were no consistent differences among the style morphs in outcrossing or biparental inbreeding indicating that the maintenance of trimorphism vs dimorphism is unlikely to be associated with inbreeding of maternal parents.     

Heterozygosity, inbreeding and neonatal traits in Soay sheep on St Kilda

We investigated whether birth weight and neonatal survival, a period within which 24% of all mortalities occur, were correlated with levels of inbreeding in St Kilda Soay sheep, using pedigree inbreeding coefficients and four marker-based estimators of inbreeding. None of the inbreeding estimators, either of the offspring, or of their mothers, explained significant variation in a lamb's birth weight or probability of surviving the neonatal period, suggesting low inbreeding depression for these traits. We evaluated the correlation between the marker-based measures of inbreeding and inbreeding coefficients obtained from the Soay pedigree, where paternal links were inferred using the same panel of microsatellite markers. Even when using a relatively complete portion of the pedigree, in which all individuals had known maternal and paternal grandparents, the correlation was found to be weak (r = -0.207, where mean f = 0.0168). These results add support to the recent prediction that when the mean and variance in inbreeding are low in a population, heterozygosity-fitness correlations can be very weak or even undetectable. The pursuit of more detailed pedigrees offers the best prospect for identifying inbreeding depression within this study population.     

Interpopulation variation in inbreeding is primarily driven by tolerance of mating with relatives in a spermcasting invertebrate

The degree to which individuals inbreed is a fundamental aspect of population biology shaped by both passive and active processes. Yet, the relative influences of random and non-random mating on the overall magnitude of inbreeding are not well characterized for many taxa. We quantified variation in inbreeding among qualitatively accessible and isolated populations of a sessile marine invertebrate (the colonial ascidian Lissoclinum verrilli) in which hermaphroditic colonies cast sperm into the water column for subsequent uptake and internal fertilization. We compared estimates of inbreeding to simulations predicting random mating within sites to evaluate if levels of inbreeding were (1) less than expected because of active attempts to limit inbreeding, (2) as predicted by genetic subdivision and passive inbreeding tolerance, or (3) greater than simulations due to active attempts to promote inbreeding via self-fertilization or a preference for related mates. We found evidence of restricted gene flow and significant differences in the genetic diversity of L. verrilli colonies among sites, indicating that on average colonies were weakly related in accessible locations, but their levels of relatedness matched that of first cousins or half-siblings on isolated substrates. Irrespective of population size, progeny arrays revealed variation in the magnitude of inbreeding across sites that tracked with the mean relatedness of conspecifics. Biparental reproduction was confirmed in most offspring (86%) and estimates of total inbreeding largely overlapped with simulations of random mating, suggesting that interpopulation variation in mother–offspring resemblance was primarily due to genetic subdivision and passive tolerance of related mates. Our results highlight the influence of demographic isolation on the genetic composition of populations, and support theory predicting that tolerance of biparental inbreeding, even when mates are closely related, may be favoured under a broad set of ecological and evolutionary conditions.     

Population size and fragmentation thresholds for the maintenance of genetic diversity in the herbaceous endemic Scutellaria montana (Lamiaceae)

Abstract.-The level and distribution of genetic variation is thought to be affected primarily by the size of individual populations and by gene flow among populations. Although the effects of population size have frequently been examined, the contributions of regional gene flow to levels of genetic variation are less well known. Here I examine the effects of population size and the number of neighboring populations (metapopulation density) on the distribution and maintenance of genetic diversity in an endemic herbaceous perennial. Reductions in the proportion of polymorphic loci and the effective number of alleles per locus were apparent for many populations with a census size of less than 100 individuals, but no effects of population size on levels of inbreeding were detected. I assess the effects of regional population density on levels of diversity and inbreeding using stepwise regression analysis of metapopulation diameter (i.e., the size of a circle within which population density is estimated). This procedure provides a spatially explicit evaluation of the effects of metapopulation size on population genetic parameters and indicates the critical number of neighboring populations (fragmentation threshold) for the regional maintenance of genetic diversity. Stepwise regression analyses revealed fragmentation thresholds at two levels; at a scale of 2 km, where small metapopulations resulted in greater levels of selfing or sibling mating, and at a scale of 8 km, where metapopulation size was positively associated with higher levels of genetic diversity. I hypothesize that the smaller fragmentation threshold may reflect higher levels of selfing in isolated populations because of the absence of pollinators. The larger threshold probably indicates the maximum distance over which pollen dispersal rates are high enough to counteract genetic drift. This study demonstrates that the regional distribution of populations can be an important factor for the long-term maintenance of genetic variation.     

Inbreeding depression in perennial Lychnis viscaria (Caryophyllaceae): effects of population mating history and nutrient availability

We studied inbreeding depression in a perennial plant, Lychnis viscaria, in three populations differing in their inbreeding history and population size by measuring several traits at two nutrient levels over the plant's life cycle. The observed levels of inbreeding depression (cumulative inbreeding depression, from -0.057 to 0.629) were high for a plant with a mixed mating system. As expected, the population with a low level of isozyme variation expressed the least inbreeding depression for seed germination. Highest inbreeding depression for germination was found in the largest and genetically most variable population. No clear differences between populations in expression of inbreeding depression in the later life stages were found. The population level inbreeding depression varied with the nutrient conditions and among populations and life stages, but we found no evidence that inbreeding depression increased with lower nutrient availability. These results emphasize the importance of measuring inbreeding depression under several environmental conditions and over life stages.