Genetic isolation of fragmented populations is exacerbated by drift and selection

Reduced genetic variation at marker loci in small populations has been well documented, whereas the relationship between quantitative genetic variation and population size has attracted little empirical investigation. Here we demonstrate that both neutral and quantitative genetic variation are reduced in small populations of a fragmented plant metapopulation, and that both drift and selective change are enhanced in small populations. Measures of neutral genetic differentiation (F(ST)) and quantitative genetic differentiation (Q(ST)) in two traits were higher among small demes, and Q(ST) between small populations exceeded that expected from drift alone. This suggests that fragmented populations experience both enhanced genetic drift and divergent selection on phenotypic traits, and that drift affects variation in both neutral markers and quantitative traits. These results highlight the need to integrate natural selection into conservation genetic theory, and suggests that small populations may represent reservoirs of genetic variation adaptive within a wide range of environments.     

居群遗传学原理及其在珍稀濒危植物保护中的应用

居群遗传学在珍稀濒危植物保护研究中有着重要的应用价值。本文首先介绍了居群遗传学中的几个重要概念——有效居群大小、近交繁殖、遗传漂变和基因流,然后详细叙述了居群遗传学原理在珍稀濒危植物保护中的应用途径和前景。     

Disentangling the impact of demographic factors on population differentiation of an endangered freshwater crayfish (Austropotamobius pallipes) using population density and microsatellite data

1. Habitat fragmentation of stream ecosystems often results in decreased connectivity between populations and lower population sizes. Hence, understanding how habitat fragmentation affects genetic erosion is important for the preservation of freshwater biodiversity, in particular, as small populations suffer from loss of genetic diversity through genetic drift and loss of fitness because of inbreeding, increasing the risk of extinction. 2. Here, we assess the impact of demographic factors on population differentiation in the endangered freshwater crayfish Austropotamobius pallipes by analysing population genetic structure, estimating effective population sizes and comparing levels of polymorphism at five microsatellite loci with density estimates of 10 populations within a small French catchment that has become progressively confined to headwaters over the last six decades. 3. Levels of expected heterozygosity and allelic richness per population were relatively low (0.214–0.396 and 1.6–2.6, respectively). We found strong genetic differentiation between these geographically close populations (F_ST = 0.283), with weak statistical evidence for a pattern of isolation by distance. Estimates of effective population size were low (<150) in most populations, but potentially reached several thousands in three populations. 4. Population density and allelic richness were strongly positively correlated. A robust relationship between population density and heterozygosity values was also noted, but only after discarding two populations for which significant genetic signatures of a recent bottleneck were found; these two populations displayed high expected heterozygosity compared with a very low density. Populations with the highest densities of individuals had the highest effective population size estimates and vice versa. 5. Our results clearly show the importance of demographic factors and genetic drift on A. pallipes populations. Furthermore, analysis of genetic and population density data is a pragmatic and efficient approach to corroborate inferences from genetic data and can be particularly useful in the identification of populations experiencing a bottleneck and therefore in conservation genetics studies aiming at identifying priority populations for conservation.     

植物景观遗传学研究进展

植物景观遗传学是新兴的景观遗传学交叉学科的一个重要研究方向。目前植物景观遗传学的研究虽落后于动物,但其在生物多样性保护方面具有的巨大潜力不可忽视。从景观特征对遗传结构、环境因素对适应性遗传变异影响两个方面,系统综述了近十年来国际上植物景观遗传学的研究焦点和研究进展,比较了植物景观遗传学与动物景观遗传学研究在研究设计和研究方法上的异同,并基于将来植物景观遗传学由对空间遗传结构的描述发展为对景观遗传效应的量化分析及预测的发展框架,具体针对目前景观特征与遗传结构研究设计的系统性差、遗传结构与景观格局在时间上的误配、适应性位点与环境变量的模糊匹配、中性遗传变异与适应性遗传变异研究的分隔、景观与遗传关系分析方法的局限等五个方面提出了研究对策。     

Drift load in populations of small size and low density

According to theory, drift load in randomly mating populations is determined by past population size, because enhanced genetic drift in small populations causes accumulation and fixation of recessive deleterious mutations of small effect. In contrast, segregating load due to mutations of low frequency should decline in smaller populations, at least when mutations are highly recessive and strongly deleterious. Strong local selection generally reduces both types of load. We tested these predictions in 13 isolated, outcrossing populations of Arabidopsis lyrata that varied in population size and plant density. Long-term size was estimated by expected heterozygosity at 20 microsatellite loci. Segregating load was assessed by comparing performance of offspring from selfings versus within-population crosses. Drift load was the heterosis effect created by interpopulation outbreeding. Results showed that segregating load was unrelated to long-term size. However, drift load was significantly higher in populations of small effective size and low density. Drift load was mostly expressed late in development, but started as early as germination and accumulated thereafter. The study largely confirms predictions of theory and illustrates that mutation accumulation can be a threat to natural populations.     

湖北海棠的等位酶变异和遗传多样性研究

采用超薄平板微型聚丙烯酰胺等电聚焦电泳方法对湖北海棠（Malus hupehensis)的9个野生居群和2个人工栽培居群的等位酶变异和遗传多样性进行了初步研究。通过对12个酶系统29个酶位点的检测,结果表明湖北海棠有25个酶位点的等位基因频率分布差异,,有10个居群发现稀有等位基因,并有11个（37．9％）重复位点;湖北海棠的遗传多样性水平很高,等位基因平均数A＝2．127,多态位点百分率P＝74．927,平均预期杂合度He=0.376;居群间的基因分化系数GST＝0．224。与其他苹果属植物相比,湖北海棠具有中等丰富的遗传变异水平。居群间的基因流仅为Nm=0.866,表明遗传漂变是影响居群遗传变异和遗传结构的一个重要因素。     

Genetic Variation in Indian Populations of Scirpophaga incertulas as Revealed by RAPD-PCR Analysis

Scirpophaga incertulas, commonly referred to as yellow stem borer, is a predominant pest of rice causing serious losses in its yield. Genetic variation among populations of Scirpophaga incertulas collected from 28 hotspot locations in India was examined using the randomly amplified polymorphic DNA-polymerase chain reaction (RAPD-PCR). In all, 32 primers were used and 354 amplification products were observed. No RAPD-PCR bands diagnostic to the pest population from any specific region were identified. Cluster analysis using UPGMA showed that, with the exception of the pest population from Pattambi, all the populations cluster as one group with GD values in the range of 6–22%, suggesting that gene flow between populations is independent of geographic distance and appears to be unrestricted. The relatively high GD value of 48% exhibited by the pest population from Pattambi was the only exception.     

Genetic diversity and genetic structure of populations of Ranunculus japonicus Thunb. (Ranunculaceae)

Abstract In order to clarify the genetic diversity and population structure of Ranunculus japonicus , allozymic analysis was conducted on 60 populations in southwestern Japan. Considerable genetic variati ons were detected among the populations of R. japonicus . The genetic diversities within species ( H _es = 0.215) and within populations ( H _ep = 0.172) were slightly higher than those of other perennial herbs with widespread distribution and outcrossing plants. Significantly higher values of fixation index were detected in some populations, which might have arisen from restricted mating partners. The majority of genetic variation (approx. 80%) resided within a population and a moderate level of genetic differentiation ( G _ST = 0.203) was observed among populations. The F _ST value (0.203) suggests the existence of a substantial population structure in this species. The highly significant correlation between geographic distance and F _ST values indicates that isolation by distance has played an important role in the construction of the genetic structure of this species.     

Population structure and impact of supportive breeding inferred from mitochondrial and microsatellite DNA analyses in land-locked Atlantic salmon Salmo salar L.

Four tributaries of Lake St-Jean (Québec, Canada) are used for spawning and juvenile habitat by land-locked Atlantic salmon. Spawning runs have drastically declined since the mid-1980s, and consequently, a supportive-breeding programme was undertaken in 1990. In this study, we analysed seven microsatellite loci and mtDNA, and empirically estimated effective population sizes to test the hypotheses that (i) fish spawning in different tributaries form genetically distinct populations and (ii) the supportive breeding programme causes genetic perturbations on wild populations. Allele frequency distribution, molecular variance and genetic distance estimates all supported the hypothesis of genetic differentiation among salmon from different tributaries. Gene flow among some populations was much more restricted than previously reported for anadromous populations despite the small geographical scale (40 km) involved. Both mtDNA and microsatellites revealed a more pronounced differentiation between populations from two tributaries of a single river compared with their differentiation with a population from a neighbouring river. The comparison of wild and F₁-hatchery fish (produced from breeders originating from the same river) indicated significant changes in allele frequencies and losses of low-frequency alleles but no reduction in heterozygosity. Estimates of variance and inbreeding population size indicated that susceptibility to genetic drift and inbreeding in one population increased by twofold after only one generation of supplementation.     

A comparison of four methods for detecting weak genetic structure from marker data

Genetic structure is ubiquitous in wild populations and is the result of the processes of natural selection, genetic drift, mutation, and gene flow. Genetic drift and divergent selection promotes the generation of genetic structure, while gene flow homogenizes the subpopulations. The ability to detect genetic structure from marker data diminishes rapidly with a decreasing level of differentiation among subpopulations. Weak genetic structure may be unimportant over evolutionary time scales but could have important implications in ecology and conservation biology. In this paper we examine methods for detecting and quantifying weak genetic structures using simulated data. We simulated populations consisting of two putative subpopulations evolving for up to 50 generations with varying degrees of gene flow (migration), and varying amounts of information (allelic diversity). There are a number of techniques available to detect and quantify genetic structure but here we concentrate on four methods: F(ST), population assignment, relatedness, and sibship assignment. Under the simple mating system simulated here, the four methods produce qualitatively similar results. However, the assignment method performed relatively poorly when genetic structure was weak and we therefore caution against using this method when the analytical aim is to detect fine-scale patterns. Further work should examine situations with different mating systems, for example where a few individuals dominate reproductive output of the population. This study will help workers to design their experiments (e.g., sample sizes of markers and individuals), and to decide which methods are likely to be most appropriate for their particular data.     

Random genetic drift during cyclical ameiotic parthenogenesis

The classical models of population genetics assume sexual reproduction and do not apply to organisms in which parthenogenetic reproduction is alternated with sexual recombination. Under cyclic parthenogenesis, variation in rates or frequencies of parthenogenetic reproduction among clones produces selection that is independent of processes occurring in the sexual phases.In this paper I examine how selection during cyclic parthenogenesis influences random genetic drift and leads to a loss of variance among clones. To illustrate these effects, computer simulations are presented demonstrating the response of effective clone number and equilibrium clone diversity to selection and mutation.