Human disturbance reduces genetic diversity of an endangered tropical tree, <Emphasis Type="Italic">Prunus africana</Emphasis> (Rosaceae)

Human activities such as fragmentation and selective logging of forests can threaten population viability by modification of ecological and genetic processes. Using six microsatellite markers, we examined the effects of forest fragmentation and local disturbance on the genetic diversity and structure of adult trees (N = 110) and seedlings (N = 110) of Prunus africana in Kakamega Forest, western Kenya. Taking samples of adults and seedlings allowed for study of changes in genetic diversity and structure between generations. Thereby, adults reflect the pattern before and seedlings after intensive human impact. Overall, we found 105 different alleles in the six loci examined, 97 in adults and 88 in seedlings. Allelic richness and heterozygosity were significantly lower in seedlings than in adults. Inbreeding increased from adult tree to seedling populations. Genetic differentiation of adult trees was low (overall F _ST = 0.032), reflecting large population sizes and extensive gene flow in the past. Genetic differentiation of seedlings was slightly higher (overall F _ST = 0.044) with all of the 28 pairwise F _ST-values being significantly different from zero. These results suggest that human disturbance in Kakamega Forest has significantly reduced allelic richness and heterozygosity, increased inbreeding and slightly reduced gene flow in P. africana in the past 80–100 years.     

Restoration of genetic diversity from soil seed banks in a threatened aquatic plant, <Emphasis Type="Italic">Nymphoides peltata</Emphasis>

Populations of a threatened aquatic plant, Nymphoides peltata, have rapidly degenerated under the influence of recent artificial changes in Lake Kasumigaura of Japan. To estimate the potential of soil seed banks for genetic restoration of the species, we used 10 microsatellite markers to analyze the genetic variation in adults and in seedlings that emerged from soil seed banks. About 187 leaf samples from the cultured stocks that were collected in 17 adult subpopulations in 1995 and 2000 and from three subpopulations that were newly discovered in 2002 were analyzed. As a result, only 18 genets could be identified, suggesting that clonal diversity of the adult population had already become extremely low. Genetic tests were performed on 430 seedlings from seed banks at six locations of natural lakeshores and three of the restoration sites that were artificially constructed in an attempt to assign them to the remnant adult population; many of the seedlings showed genetic variation different from the adults. Furthermore, the seedlings preserved seven alleles that had been lost from remnant adults. However, they had lower average numbers of alleles and heterozygosity levels (NA = 1.5–3.1, H _E = 0.146–0.487) than the remnant adults (NA = 3.5, H _E = 0.539) and showed high inbreeding coefficients, suggesting that the seed banks were produced by inbreeding. Thus, although the seed banks had a certain potential to restore genetic diversity, the fitness reduction in seed banks caused by inbreeding could affect the success of restoration based on seed banks.     

Genetic diversity and seed production in Santa Lucia fir (Abies bracteata), a relict of the Miocene Broadleaved Evergreen Forest

Santa Lucia fir (Abies bracteata), is a unique fir, the sole member of the subgenus Pseudotorreya. It is a relict of the Miocene broadleaved evergreen sclerophyll forest, and is now restricted to a highly fragmented range in the Santa Lucia Mountains of central coastal California. Expected heterozygosity for 30 isozyme loci in 18 enzyme systems, averaged over six populations that spanned the species’ north–south range, was only 0.036. Despite a fragmented range and isolated populations, differentiation (F _ST) was only 0.080 for mature trees, and the number of migrants per generation (Nm) was 2.88 or 3.83, depending on the method of estimation. F _ST for embryos was lower, 0.025, and Nm correspondingly higher, 9.75. Nei’s genetic distances were small and unrelated to geographic distances between populations. The proportion of full seeds per cone was only 0.082–0.488, depending on population, which suggests a high incidence of selfing followed by embryo abortion. However, the level of accumulated inbreeding, F _IS, in mature trees was low, only 0.049. By contrast, F _IS for embryos was 0.388, which indicates a high proportion of selfed progeny, in agreement with the low seed yields. The difference in inbreeding coefficients between seed trees and their progeny suggest that most inbreds are eliminated before maturity and, therefore, seed production, already low, overestimates the true potential for regeneration of these populations. These results have implications for conservation.     

Conservation Genetics of the Endemic Mexican Heliconia uxpanapensis in the Los Tuxtlas Tropical Rain Forest

Heliconia uxpanapensis (Heliconiaceae) is an outcrossing endemic herb that grows within continuous and fragmented areas of the tropical rain forest of southeast Veracrúz (México). The genetic diversity, population differentiation, and genetic structure of seven populations of the studied species were assessed using inter‐simple sequence repeat) markers. Population differentiation was moderately high (F_ST range: 0.18–0.22) and indirect estimates of gene flow were rather low (Nm=0.65–0.83). Analysis of molecular variance indicated that the populations explained 22.2 percent of the variation, while individuals within the populations accounted for 77.8 percent. The similar and high level of genetic diversity found within populations of the continuous and fragmented forest suggests that H. uxpanapensis has not suffered yet the expected negative effect of fragmentation. Genetic structure analyses indicated the presence of fewer genetic clusters (K=4) than populations (N=7). Three of the four fragmented forest populations were assigned each to one of the clusters found within the continuous forest, suggesting the absence of a negative fragmentation effect on the amount and distribution of genetic variation. Given the significant genetic structure combined with high genetic diversity and low levels of gene flow, theoretical simulations indicated that H. uxpanapensis might be highly susceptible to changes in the mating system, which promotes inbreeding within fragmented populations. Thus, future conservation efforts in this species should be directed to ensure that levels of gene flow among populations are sufficient to prevent an increment in the magnitude of inbreeding within fragments.     

High population differentiation and genetic variation in the endangered Mexican pine Pinus Rzedowskii (Pinaceae)

Pinus rzedowskii is an endangered pine species from Michoaca´n (central Me´xico), which has been previously reported from only three localities. Classified within the subgenus Strobus, it exhibits intermediate morphological characters between subgenera Strobus and Pinus. We analyzed genetic aspects that could shed light on the evolution and conservation of this species. The genetic structure of nine populations was examined using 14 isozyme loci. Pinus rzedowskii has a relatively high level of genetic variation with 46.8% of the loci assayed being polymorphic, a total of 35 alleles, and a mean heterozygosity per population of 0.219. We calculated Wright's F_ST statistic to estimate gene flow indirectly and to evaluate whether or not there was genetic structuring among populations. We found a marked differentiation among populations (F_ST = 0.175) and significant inbreeding (F_IS = 0.247). No pattern of isolation by distance was found. We also constructed a dendrogram based on a genetic distance matrix to obtain an overview of the possible historical relationships among populations. Finally, we found a convex relationship between the genetic distance among populations and the number of ancestral lineages, suggesting that demographically this species has not been at risk recently. Although endangered, with small and fragmented populations, P. rzedowskii shows higher levels of genetic variation than other conifer species with larger populations or similar conservation status.     

Genetic variation and clonal structure of the rare,riparian shrub <Emphasis Type="Italic">Spiraea virginiana</Emphasis> (Rosaceae)

Genetic diversity is often considered important for species that inhabit highly disturbed environments to allow for adaptation. Many variables affect levels of genetic variation; however, the two most influential variables are population size and type of reproduction. When analyzed separately, both small population size and asexual reproduction can lead to reductions in genetic variation, although the exact nature of which can be contrasting. Genetic variables such as allelic richness, heterozygosity, inbreeding coefficient, and population differentiation have opposite predictions depending upon the trait (rarity or clonality) examined. The goal of this study was to quantify genetic variation and population differentiation in a species that resides in a highly stochastic environment and is both rare and highly clonal, Spiraea virginiana, and to determine if one trait is more influential genetically than the other. From populations sampled throughout the natural range of S. virginiana, we used microsatellite loci to estimate overall genetic variation. We also calculated clonal structure within populations, which included genotypic richness, evenness, and diversity. Gene flow was investigated by quantifying the relationship between genetic and geographic distances, and population differentiation (θ) among populations. Observed heterozygosity, genotypic richness, and inbreeding coefficients were found to be representative of high clonal reproduction (averaging 0.505, 0.1, and –0.356, respectively) and the number of alleles within populations was low (range = 2.0–3.6), being more indicative of rarity. Population differentiation (θ) among populations was high (average = 0.302) and there was no relationship between genetic and geographic distances. By examining a species that exhibits two traits that both can lead to reduced genetic variation, we may find an enhanced urgency for conservation. Accurate demographic counts of clonal species are rarely, if ever, possible and genetic exploration for every species is not feasible. Therefore, the conclusions in this study can be potentially extrapolated to other riparian, clonal shrubs that share similar biology as S. virginiana.     

Conservation of long-lived perennial forest herbs in an urban context: <Emphasis Type="Italic">Primula elatior</Emphasis> as study case

Urban forests are generally fragmented in small isolated remnants, embedded in an inhospitable human-used matrix, and incur strong anthropogenic pressures (recreational activities, artificialization, pollution and eutrophication). These lead to particularly high constraints even for common forest herbs, whose genetic response may depend on life-history traits and population demographic status. This study investigated genetic variation and structure for 20 allozyme loci in 14 populations of Primula elatior, a self-incompatible long-lived perennial herb, occurring in forest fragments of Brussels urban zone (Belgium), in relation to population size and young plants recruitment rate. Urban populations of P. elatior were not genetically depauperate, but the small populations showed reduced allelic richness. Small populations showing high recruitment rates—and therefore potential rejuvenation—revealed lower genetic diversity (H _o and H _e) than those with low or no recruitment. No such pattern was observed for the large populations. There was a significant genetic differentiation among populations within forest fragments (F _SC = 0.052, P < 0.001), but not between fragments (F _CT = 0.002, P > 0.10). These findings suggest restricted gene flow among populations within fragments and local processes (genetic drift, inbreeding) affecting small populations, strengthened when there is recruitment. Urban forest populations of long-lived perennial herbs can be of conservation value. However, restoration of small populations by increasing population size through regeneration by seedling recruitment may lead to negative genetic consequences. Additional management, aiming to restore gene flow among populations, may need to be applied to compensate the loss of genetic diversity and to reduce inbreeding.     

Genetic diversity of a rare species <Emphasis Type="Italic">Aristolochia contorta</Emphasis> Bunge (Aristolochiaceae) in Primorsky Krai

The herbaceous vine, twisted birthwort Aristolochia contorta Bunge, is a rare species listed in the Red Data Book of Primorsky krai (2008). On the northern boundary of its range (south of the Russian Far East), the species is represented by small isolated populations confined to the river drainages. Using allozyme analysis, genetic variation of nine natural populations of A. contorta (247 accessions), which represented the main part of the species range in Russia, was examined. The values of genetic variation indices (P = 22.7%; A = 1.28; H _o = 0.129; H _e = 0.101) were low and comparable with the data obtained for other rare plants. The proportion of unique genotypes (G/N) and Simpson’s genotypic diversity index (D) ranged from 0.32 to 0.64 and from 0.60 to 0.98, respectively. This means that A. contorta is characterized by sexual and asexual reproduction. Moreover, the ratio between these types of reproduction varied among the populations. Complete absence of inbreeding and excess of heterozygotes (F _IS = −0.282), which was low probable in case of free mating, was observed. Evidently, clonal growth and (or) apomixis enables the species to maintain certain level of heterozygosity despite of small population sizes and non-regular gene exchange.     

Past bottlenecks and current population fragmentation of endangered huemul deer (<Emphasis Type="Italic">Hippocamelus bisulcus</Emphasis>): implications for preservation of genetic diversity

Small populations in fragmented habitats can lose genetic variation through drift and inbreeding. The huemul (Hippocamelus bisulcus) is an endangered deer endemic to the southern Andes of Chile and Argentina. Huemul numbers have declined by 99% and its distribution by 50% since European settlement. The total population is estimated at less than 2,000 individuals and is highly fragmented. At one isolated population in Chilean Patagonia we sampled 56 individuals between 2005 and 2007 and genotyped them at 14 microsatellite loci. Despite low genetic variability (average 2.071 alleles/locus and average H _O of 0.341), a low inbreeding coefficient (F _IS) of 0.009 suggests nearly random mating. Population genetic bottleneck tests suggest both historical and contemporary reductions in population size. Simulations indicated that the population must be maintained at 75% of the current size of 120 individuals to maintain 90% of its current genetic diversity over the next 100 years. Potential management strategies to maintain genetic variability and limit future inbreeding include the conservation and establishment of habitat corridors to facilitate gene flow and the enlargement of protected areas to increase effective population size.     

Analysis of the relationship between allozyme heterozygosity and fitness in the rare Gentiana pneumonanthe L.

Especially for rare species occurring in small populations, which are prone to loss of genetic variation and inbreeding, detailed knowledge of the relationship between heterozygosity and fitness is generally lacking. After reporting on allozyme variation and fitness in relation to population size in the rare plant Gentiana pneumonanthe, we present a more detailed analysis of the association between heterozygosity and individual fitness. The aim of this study was to test whether increased fitness of more heterozygous individuals is explained best by the ‘inbreeding’ hypothesis or by the ‘overdominance’ hypothesis. Individual fitness was measured during 8 months of growth in the greenhouse as the performance for six life-history parameters. PCA reduced these parameters to four main Fitness Components. Individual heterozygosity was scored for seven polymorphic allozyme loci. For some of these loci (e.g. Aat3, Pgm1 and 6Pgdh2) heterozygotes showed a significantly higher relative fitness than homozygotes. To test the inbreeding model, regression analyses were performed between each Fitness Component and the number of heterozygous loci per individual. Multiple regressions with the adaptive distance of five loci as independent variables were used to test the overdominance model. Only the inbreeding model was a statistically significant explanation for the relationship between heterozygosity and fitness in G. pneumonanthe. The number of heterozygous loci was significantly negatively correlated with the coefficients of variation of three of the six initially measured fitness parameters. This suggests a lower developmental stability among more homozygous plants and may explain the higher phenotypic variation in small populations of the species observed earlier. The importance of the results for conservation biology is discussed.     

Temporal genetic changes between cohorts in a natural population of a marine fish, Diplodus sargus

Temporal changes at 17 allozyme loci in the Diplodus sargus population of Banyuls sur Mer (Mediterranean Sea, France) were monitored within a single population among ten year‐classes (cohorts) sampled over a 6‐month period. The genetic survey was combined with evaluation of the demographic structure of the population by determining variation of abundance between cohorts. The population showed variation in abundance among cohorts ranging from 16 to 214 individuals. Significant divergences in genetic structure were observed between cohorts (P < 0.0001) despite very low values of F_ST (multilocus F_ST over all cohorts = 0.0018). The heterozygosity of each cohort, as well as the F_IS values, was significantly correlated with the abundance of each cohort, with abundant cohorts showing lower heterozygosity and a significant deficit of heterozygotes (positive F_IS values). Finally, multilocus temporal genetic variance (F_k) computed between successive cohorts was higher in low abundance cohorts. Change of heterozygosity between cohorts, distribution of year‐class genetic structure, and change in the genetic structure within a cohort appear to be affected mostly by the abundance of the cohort and are therefore driven by genetic drift. We propose that the Diplodus sargus cohorts are built up from the mixing of families during the pelagic stage or later during recruitment, and that the decrease in heterozygosity leading to a deficit of heterozygotes is characteristic of a Wahlund effect. Such a Wahlund effect would derive from the mixing of the progeny of families made up of few individuals, but exhibiting high fecundity and high variability of reproductive success. Therefore, although cohorts derived from poor recruitment would only group a few families and would exhibit limited deficit of heterozygotes (higher heterozygosity values), they would lead to high genetic drift and appear more divergent (higher mean temporal genetic variance) than cohorts with high abundance. While not demonstrating directly the family structure of marine populations, our survey provides evidence of highly structured populations. © 2002 The Linnean Society of London, Biological Journal of the Linnean Society, 2002, 76 , 9–20.     

Genetic diversity and population differentiation in the endangered Siberian flying squirrel (<Emphasis Type="Italic">Pteromys volans</Emphasis>) in a fragmented landscape

Siberian flying squirrel (Pteromys volans) has declined in Finland and it is considered an endangered species. We studied microsatellite variation in four flying squirrel populations in a fragmented landscape in Finland to determine the amount of gene flow and genetic diversity in the populations. Demographic data from these areas suggest that the populations are declining. All the populations are significantly differentiated (F _ST = 0.23). The most notable result is the high degree of differentiation between two adjacent populations (F _ST = 0.11) and low genetic variability (number of alleles 3.0) in one of the populations. These findings suggest problems in dispersal and possible fragmentation effects in the landscape where only 10–20% of habitat favorable for the flying squirrel is left. Conservation ensuring dispersal should be urgently considered. Future studies should concentrate on the modeling of the population viability and on the effects of inbreeding in these small populations. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

INBREEDING INFLUENCES WITHIN‐BROOD HETEROZYGOSITY‐FITNESS CORRELATIONS (HFCS) IN AN ISOLATED PASSERINE POPULATION

Molecular estimates of inbreeding may be made using genetic markers such as microsatellites, however the interpretation of resulting heterozygosity‐fitness correlations (HFCs) with respect to inbreeding depression is not straightforward. We investigated the relationship between pedigree‐determined inbreeding coefficients (f) and HFCs in a closely monitored, reintroduced population of Stewart Island robins (Petroica australis rakiura) on Ulva Island, New Zealand. Using a full sibling design, we focused on differences in juvenile survival associated specifically with individual sibling variation in standardized multilocus heterozygosity (SH) when expected f was identical. We found that within broods, siblings with higher SH at microsatellite loci experienced a higher probability of juvenile survival. This effect, however, was detected primarily within broods that experienced inbreeding or when inbreeding had occurred in their pedigree histories (i.e., at the parents’ level). Thus we show, for the first time in a wild population, that the strength of an HFC is partially dependent on the presence of inbreeding events in the recent pedigree history. Our results illustrate the importance of realized effects of inbreeding on genetic variation and fitness and the value of full‐sibling designs for the study of HFCs in the context of small, inbred populations.     

Demographic bottlenecks and low gene flow in remnant populations of the critically endangered <Emphasis Type="Italic">Berchemiella wilsonii</Emphasis> var<Emphasis Type="Italic">. pubipetiolata</Emphasis> (Rhamnaceae) inferred from microsatellite markers

Berchemiella wilsonii var. pubipetiolata (Rhamnaceae) is an endangered plant with only four remnant populations in eastern China. Population genetic information is essential for understanding population history and formulating conservation strategies for this species. Thirteen microsatellite loci were used to investigate genetic variation and population structure of the four remnant populations. Moderate levels of expected heterozygosity (H _E = 0.466–0.543) and low allelic diversity (A = 3.1–3.6 and A _R = 2.2–2.4, respectively) were observed within populations. Bottleneck tests found three out of four populations to deviate from mutation-drift equilibrium under the two-phase model (TPM), suggesting a recent population decline, which is congruent with known demographic history. The evolutionary history of the species seems dominated by genetic drift rather than gene flow. Low historical gene flow was inferred from several different approaches and N _m ranged from 0.582 by the private allele method to 0.783 by the coalescent method. Contemporary gene flow was also found to be even lower for only one first generation migrant was detected with individual-based assignment analysis. Restricted pollen and seed dispersal as well as a recent decline in population size associated with habitat fragmentation may have contributed to low levels of historical and contemporary gene flow, and resulted in a high genetic differentiation. Under this scenario, Berchemiella wilsonii var. pubipetiolata populations are expected to display more pronounced population genetic structure in the future as a result of increased inbreeding and genetic drift.     

Effects of fragmentation phenomena on the genetic structure and gene flow in Centaurea cineraria group (Asteraceae) in the Mediterranean Basin

The complex history of the Mediterranean region illustrates how ancient and recent phenomena are closely associated with species distribution and the creation of phylogeographic divisions within Mediterranean flora. A good model to explore the genetic consequences of fragmentation can be found in Centaurea cineraria and its close relatives. We applied simple sequence repeat molecular markers to a dense population sampling throughout the distribution area of all C. cineraria taxa to study how fragmentation has altered the genetic structure and distribution of C. cineraria. The average gene diversity (H_e) was 0.286, and the average allelic richness (A_r) was 3.65 and ranged from 2.15 (C. gymnocarpa) to 5.25 (C. busambarensis). The F_IS averaged a relatively high 0.223, ranging from ? 0.724 in C. aeolica subsp. aeolica to 0.589 in C. leucadea. Our results indicate that habitat fragmentation over several generations reduced heterozygosity due to random genetic drift in populations of C. cineraria. This heterozygosity erosion becomes more severe when the inbreeding coefficient is positive and the outcrossing rates show a significant increase. The results observed for outcrossing rates and inbreeding coefficient could also indirectly support the possibility of disrupted gene flow or mating pattern changes in fragmented C. cineraria populations.     

Microsatellite variation and structure of 28 populations of the common wetland plant, Lychnis flos-cuculi L., in a fragmented landscape

Habitat fragmentation is known to cause genetic differentiation between small populations of rare species and decrease genetic variation within such populations. However, common species with recently fragmented populations have rarely been studied in this context. We investigated genetic variation and its relationship to population size and geographical isolation of populations of the common plant species, Lychnis flos-cuculi L., in fragmented fen grasslands. We analysed 467 plants from 28 L. flos-cuculi populations of different sizes (60 000-54 000 flowering individuals) in northeastern Switzerland using seven polymorphic microsatellite loci. Genetic differentiation between populations is small (F(ST) = 0.022; amova; P < 0.001), suggesting that gene flow among populations is still high or that habitat fragmentation is too recent to result in pronounced differentiation. Observed heterozygosity (H(O) = 0.44) significantly deviates from Hardy-Weinberg equilibrium, and within-population inbreeding coefficient F(IS) is high (0.30-0.59), indicating a mixed mating breeding system with substantial inbreeding in L. flos-cuculi. Gene diversity is the only measure of genetic variation which decreased with decreasing population size (R = 0.42; P < 0.05). While our results do not indicate pronounced effects of habitat fragmentation on genetic variation in the still common L. flos-cuculi, the lower gene diversity of smaller populations suggests that the species is not entirely unaffected.     

Depleted genetic variation of the European ground squirrel in Central Europe in both microsatellites and the major histocompatibility complex gene: implications for conservation

Habitat fragmentation may influence the genetic make-up and adaptability of endangered populations. To facilitate genetic monitoring of the endangered European ground squirrel (EGS), we analyzed 382 individuals from 16 populations in Central Europe, covering almost half of its natural range. We tested how fragmentation affects the genetic architecture of presumably selectively neutral (12 microsatellites) and non-neutral (the major histocompatibility class II DRB gene) loci. Spatial genetic analyses defined two groups of populations, “western” and “eastern”, with a significantly higher level of habitat fragmentation in the former group. The highly fragmented western populations had significantly lower genetic diversity in both types of markers. Only one allele of the DRB gene predominated in populations of the western group, while four alleles were evenly distributed across the eastern populations. Coefficient of inbreeding values (F _IS) calculated from microsatellites were significantly higher in the western (0.27–0.79) than in eastern populations (−0.060–0.119). Inter-population differentiation was very high, but similar in both groups (western F _ST = 0.23, eastern F _ST = 0.25). The test of isolation by distance was significant for the whole dataset, as well as for the two groups analyzed separately. Comparison of genetic variability and structure on microsatellites and the DRB gene does not provide any evidence for contemporary selection on MHC genes. We suggest that genetic drift in small bottlenecked and fragmented populations may overact the role of balancing selection. Based on the resulting risk of inbreeding depression in the western populations, we support population management by crossbreeding between the western and eastern populations.     

Genetic consequences of fragmentation in “arbor vitae,” eastern white cedar (Thuja occidentalis L.), toward the northern limit of its distribution range

We tested the hypothesis that marginal fragmented populations of eastern white cedar (EWC) are genetically isolated due to reduced pollen and gene flow. In accordance with the central-marginal model, we predicted a decrease in population genetic diversity and an increase in differentiation along the latitudinal gradient from the boreal mixed-wood to northern coniferous forest. A total of 24 eastern white cedar populations were sampled along the north-south latitudinal gradient for microsatellite genotyping analysis. Positive F_is values and heterozygote deficiency were observed in populations from the marginal (F_is = 0.244; P_HW = 0.0042) and discontinuous zones (F_is = 0.166; P_HW = 0.0042). However, populations from the continuous zone were in HW equilibrium (F_is = −0.007; P_HW = 0.3625). There were no significant latitudinal effects on gene diversity (H_s), allelic richness (AR), or population differentiation (F_st). Bayesian and NJT (neighbor-joining tree) analyses demonstrated the presence of a population structure that was partly consistent with the geographic origins of the populations. The impact of population fragmentation on the genetic structure of EWC is to create a positive inbreeding coefficient, which was two to three times higher on average than that of a population from the continuous zone. This result indicated a higher occurrence of selfing within fragmented EWC populations coupled with a higher degree of gene exchange among near-neighbor relatives, thereby leading to significant inbreeding. Increased population isolation was apparently not correlated with a detectable effect on genetic diversity. Overall, the fragmented populations of EWC appear well-buffered against effects of inbreeding on genetic erosion.     

Allozyme variation and population genetic structure of common wild rice Oryza rufipogon Griff. in China

In order to determine the genetic diversity and genetic structure of populations in common wild rice Oryza rufipogon, an endangered species, allozyme diversity was analyzed using 22 loci in 607 individuals of 21 natural populations from the Guangxi, Guangdong, Hainan, Yunnan, Hunan, Jiangxi and Fujian provinces in China. The populations studied showed a moderate allozyme variability (A=1.33, P=22.7%, Ho=0.033 and He=0.068), which was relatively high for the genus Oryza. The levels of genetic diversity for Guangxi and Guangdong were significantly higher than those for the other regions, and thus South China appeared to be the center of genetic diversity of O. rufipogon in China. A moderate genetic differentiation (F_ST=0.310, I=0.964) was found among the populations studied. Interestingly, the pattern of population differentiation does not correspond to geographic distance. An estimate of the outcrossing rate (t=0.324) suggests that the species has a typical mixed-mating system. The deficit of heterozygotes (F=0.511) indicates that some inbreeding may have taken place in outcrossing asexual populations because of intra-clone outcrossing events and ”isolation by distance” as a result of human disturbance. In order to predict the long-term genetic survival of fragmented populations, further studies on gene flow among the remaining populations and the genetic effects of fragmentation are proposed. Finally, some implications for the conservation of endangered species are suggested. Received: 22 June 1999 / Accepted: 20 December 1999     

北沙柳群体遗传多样性和遗传结构分析

以内蒙古北沙柳(Salix psammophila)国家种质资源库内9个群体(P1~P9)288个无性系为实验材料,利用TP-M13-SSR技术,选取22对具有多态性EST-SSR北沙柳引物,采用毛细管电泳对PCR产物进行检测,分析北沙柳遗传多样性、分化程度和群体遗传结构,为北沙柳种质资源库遗传管理、无性系鉴定、品种选育、遗传改良和构建指纹图谱提供理论依据。结果显示:(1)22对EST-SSR引物共检测到222个等位基因,各位点平均等位基因数(A)为10,四倍体基因型丰富度(G)和特异基因型(G1)总和分别为1 460和802个,平均特异基因型比率(P1)和种质鉴别率(P2)分别为45.86%和13.21%。(2)9个群体平均等位基因数(A)为7.475,基因型丰富度(G)为15.586,观察杂合度(Ho)和期望杂合度(He)分别为0.577和0.638。以期望杂合度He为标准,北沙柳群体遗传多样性水平最低的是P1和P9。(3)北沙柳群体遗传分化系数仅为0.02,AMOVA分子变异分析显示,北沙柳群体大部分遗传变异来自群体内(97%),群体间变异仅为3%。(4)三维主成分、聚类和Structure群体遗传结构分析显示,9个群体被划分为2个组,Mantel检验表明北沙柳遗传距离与地理距离极显著相关(r=0.684 P0.001)。研究表明,北沙柳种质资源具有丰富的遗传多样性,这是其具有耐旱、耐寒、耐高温、耐沙埋和抗风蚀等适应性较强的分子基础;北沙柳的遗传变异集中在群体内;分布区群体呈现由中心向边缘群体扩张分化的趋势。