Botryosphaeriaceae associated with Pterocarpus angolensis (kiaat) in South Africa

There have been several recent reports of Pterocarpus angolensis (kiaat) trees dying in South Africa, Zambia and Zimbabwe, where this tree is used in traditional medicine and is a valuable source of timber for woodcarving and furniture. A survey of material from diseased P. angolensis trees in South Africa yielded isolates of the Botryosphaeriaceae, an important fungal family known to cause a number of tree diseases. The aim of this study was to identify these Botryosphaeriaceae and to determine their pathogenicity to P. angolensis with branch inoculations. Seven species of the Botryosphaeriaceae were identified based on a combination of morphological characteristics and sequences from the ITS and EF-1α gene regions. Four of these represent undescribed taxa for which the names Pseudofusicoccum violaceum, P. olivaceum, Diplodia alatafructa and Fusicoccum atrovirens are provided. The remaining three species collected include Lasiodiplodia theobromae, L. pseudotheobromae and L. crassispora. Inoculation trials on tree branches showed that L. pseudotheobromae and one isolate of D. alatafructa differed significantly from control inoculations. The high levels of virulence and common occurrence of L. pseudotheobromae suggest that this species could play a role in tree dieback and death.     

Three new Lasiodiplodia spp. from the tropics, recognized based on DNA sequence comparisons and morphology

Botryosphaeria rhodina (anamorph Lasiodiplodia theobromae) is a common endophyte and opportunistic pathogen on more than 500 tree species in the tropics and subtropics. During routine disease surveys of plantations in Australia and Venezuela several isolates differing from L. theobromae were identified and subsequently characterized based upon morphology and ITS and EF1-alpha nucleotide sequences. These isolates grouped into three strongly supported clades related to but different from the known taxa, B. rhodina and L. gonubiensis, These have been described here as three new species L. venezuelensis sp. nov., L. crassispora sp. nov. and L. rubropurpurea sp. nov. The three could be distinguished easily from each other and the two described species of Lasiodiplodia, thus confirming phylogenetic separations. Furthermore all five Lasiodiplodia spp. now recognized separated from Diplodia spp. and Dothiorella spp. with 100% bootstrap support.     

中华猕猴桃和美味猕猴桃自然居群遗传结构及其种间杂交渐渗

 利用9对SSR引物对中华猕猴桃（Actinidia chinensis）和美味猕猴桃（A. deliciosa）两近缘种的5个同域分布复合体和各自1个非同域分布居群进行了遗传多样性、居群遗传结构的分析以及种间杂交渐渗的探讨。结果表明：1）两物种共有等位基因比例高达81.13%,物种特有等位基因较少（中华猕猴桃：13.27%,美味猕猴桃：5.61%）,但共享等位基因表型频率在两近缘种间存在差异,而且与各同域复合体中两物种样本的交错程度或间距存在关联;2）两种猕猴桃均具有极高遗传多样性,美味猕猴桃的遗传多样性（H_o＝0 .749, PIC＝0.818）都略高于中华猕猴桃（H_o＝0.686,PIC＝0.799）;3）两猕 猴桃物种均具有较低的Nei’s居群遗传分化度,但AMOVA分析结果揭示种内异域居群间（F_ST=0.091 5）和同域复合体种间（F_ST=0.111 5）均存在一定程度的遗传分化;中华猕猴桃居群遗传分化（G_ST=0.086; F_ST=0.212 1）高于美味猕猴桃（G_ST= 0.080;F_ST=0.142 0）;4）同域分布复合体两物种间的遗传分化（G_ST=0.020）低于物种内异域居群间的遗传分化（中华猕猴桃：G_ST=0.086; 美味猕猴桃：G_ST=0.080）,同域复合体物种间的基因流（N_m＝7.89 -29.75）远远高于 同种异域居群间（中华猕猴桃：N_m =2.663; 美味猕猴桃：N_m=2.880）; 5）居群UPGMA聚类揭示在同一地域的居群优先聚类,个体聚类结果显示多数个体聚在各自居群组内,但各地理居群并不按地理距离的远近聚类,这与Mantel相关性检测所揭示的居群间遗传距离与地理距离没有显著性相关的结果一致。进一步分析表明两种猕猴桃的遗传多样性和居群遗传结构不仅受其广域分布、远交、晚期分化等生活史特性的影响,同时还与猕猴桃的染色体基数高 (x=29)、倍性复杂和种间杂交等因素密切相关,其中两种猕猴桃的共享祖先多态性和同域分布种间杂交基因渗透对两猕猴桃的居群遗传结构产生了重要影响。     

Botryosphaeriaceae associated with <Emphasis Type="Italic">Terminalia catappa</Emphasis> in Cameroon,South Africa and Madagascar

Species in the Botryosphaeriaceae represent some of the most important fungal pathogens of woody plants. Although these fungi have been relatively well studied on economically important crops, hardly anything is known regarding their taxonomy or ecology on native or non-commercial tree species. The aim of this study was to compare the diversity and distribution of the Botryosphaeriaceae on Terminalia catappa, a tropical tree of Asian origin planted as an ornamental in Cameroon, Madagascar and South Africa. A total of 83 trees were sampled, yielding 79 Botryosphaeriaceae isolates. Isolates were initially grouped based on morphology of cultures and conidia. Representatives of the different morphological groups were then further characterised using sequence data for the ITS, tef 1-alpha, rpb2, BOTF15 and beta-tub gene regions. Five species of the Botryosphaeriaceae were identified, including Neofusicoccum parvum, N. batangarum sp. nov., Lasiodiplodia pseudotheobromae, L. theobromae and L. mahajangana sp. nov. Lasiodiplodia pseudotheobromae and L. theobromae, were the most commonly isolated species (62%), and were found at all the sites. Neofusicoccum parvum and N. batangarum were found in South Africa and Cameroon, respectively, whereas L. mahajangana was found only in Madagascar. Greenhouse inoculation trials performed on young T. catappa trees showed variation among isolates tested, with L. pseudotheobromae being the most pathogenic. The Botryosphaeriaceae infecting T. catappa appear to be dominated by generalist species that also occur on various other hosts in tropical and sub-tropical climates.     

Genetic diversity and population structure of Ascochyta rabiei from the western Iranian Ilam and Kermanshah provinces using MAT and SSR markers

Knowledge of genetic diversity in A. rabiei provides different levels of information that are important in the management of crop germplasm resources. Gene flow on a regional level indicates a significant potential risk for the regional spread of novel alleles that might contribute to fungicide resistance or the breakdown of resistance genes. Simple sequence repeat (SSR) and mating type (MAT) markers were used to determine the genetic structure, and estimate genetic diversity and the prevalence of mating types in 103 Ascochyta rabiei isolates from seven counties in the Ilam and Kermanshah provinces of western Iran (Ilam, Aseman abad, Holaylan, Chardavol, Dareh shahr, Gilangharb, and Sarpul). A set of 3 microsatellite primer pairs revealed a total of 75 alleles; the number of alleles varied from 15 to 34 for each marker. A high level of genetic variability was observed among A. rabiei isolates in the region. Genetic diversity was high (He = 0.788) within populations with corresponding high average gene flow and low genetic distances between populations. The smallest genetic distance was observed between isolates from Ilam and Chardavol. Both mating types were present in all populations, with the majority of the isolates belonging to Mat1-1 (64%), but within populations the proportions of each mating type were not significantly different from 50%. Results from this study will be useful in breeding for Ascochyta blight-resistant cultivars and developing necessary control measures.     

Allozyme diversity in natural populations of Viola palmensis Webb & Berth. (Violaceae) from La Palma (Canary Islands): implications for conservation genetics

Genetic diversity was measured by allozyme electrophoresis in eight natural populations of the threatened Canarian endemic Viola palmensis Webb & Berth. (Violaceae). Nineteen alleles corresponding to 11 gene loci were detected. High levels of genetic diversity were found, ranging from 36.3 to 45.4 % for the percentage of polymorphic loci (P), from 1.45 to 1.60 for the average number of alleles per locus (A) and from 0.128 to 0.200 for the expected heterozygosity (H(e)). Between 85.5 and 96.6 % of genetic variability was apportioned within populations. As a whole, populations were not at Hardy-Weinberg equilibrium, with a deficit of heterozygous individuals attributable to the existence of genetic structuring in the populations analysed. The levels of interpopulation genetic differentiation were low (mean F(ST) = 0.100), while genetic identity pair-wise comparisons were high (mean I = 0.973) suggesting considerable levels of gene flow among populations. No relationship was detected between genetic differentiation and geographical distances between populations. An outcrossing insect-mediated breeding system might contribute to pollen dispersion of this species. For conservation genetics we suggest in situ preservation areas are defined that are free of disturbance and that include populations with the highest genetic diversity.     

Genetic diversity in Leavenworthia populations with different inbreeding levels.

Levels of neutral genetic diversity within and between populations were compared between outcrossing (self-incompatible) and inbreeding populations in the annual plant genus Leavenworthia. Two taxonomically independent comparisons are possible, since self-incompatibility has been lost twice in the group of species studied. Within inbred populations of L.uniflora and L.crassa, no DNA sequence variants were seen among the alleles sampled, but high diversity was seen in alleles from populations of the outcrosser L. stylosa, and in self-incompatible L. crassa populations. Diversity between populations was seen in all species. Although total diversity values were lower in the sets of inbreeding populations, between-population values were as high or higher, than those in the outcrossing taxa. Possible reasons for these diversity patterns are discussed. As the effect of inbreeding appears to be a greater than twofold reduction in diversity, we argue that some process such as selection for advantageous mutations, or against deleterious mutations, or bottlenecks occurring predominantly in the inbreeders, appears necessary to account for the findings. If selection for advantageous mutations is responsible, it appears that it must be some form of local adaptive selection, rather than substitution of alleles that are advantageous throughout the species. This is consistent with the finding of high between-population diversity in the inbreeding taxa.     

Genetic Diversity and Natural Population Structure of Cacao (Theobroma cacao L.) from the Brazilian Amazon Evaluated by Microsatellite Markers

A sample of 94 accessions of Theobroma cacao L. (cacao), representing four populations from the Brazilian Amazon (Acre, Rondônia, lower Amazon and upper Amazon) were analyzed using microsatellite markers to assess the genetic diversity and the natural population structure. From the 19 microsatellite loci tested, 11 amplified scorable products, revealing a total of 49 alleles, including two monomorphic loci. The Brazilian upper Amazon population contained the largest genetic diversity, with the most polymorphic loci, the highest observed heterozygosity; and the majority of rare alleles, thereby this region might be considered part of the center of diversity of the species. The observed heterozygosity for all the Brazilian populations (H _o = 0.347) was comparable with values reported for other similar upper Amazon Forastero cacao populations, with the Acre and Rondônia displaying the lowest values. The lower Amazon population, traditionally defined as highly homozygous, presented an unexpectedly high observed heterozygosity (H _o = 0.372), disclosing rare and distinct alleles, with large identity with the upper Amazon population. It was hypothesized that part of the lower Amazon population might derive from successive natural or intentional introduction of planting material from other provenances, mainly upper Amazon. Most of the loci exhibited a lower observed heterozygosity than expected, suggesting that self-pollination might be more common than usually assumed in cacao, but excess of homozygotes might also derive from sub-grouping (Wahlund effect) or from sampling related individuals. Most of the gene diversity was found to occur within groups, with small differentiation between the four Brazilian Amazon populations, typical of species with high gene flow.     

Isolation and characterization of polymorphic nuclear microsatellite loci in <Emphasis Type="Italic">Taxus baccata</Emphasis> L.

Seven polymorphic nuclear microsatellite markers for Taxus baccata L. (English yew) were developed using an enriched-library method. An additional polymorphic SSR was obtained by testing eight primer pairs from the congeneric species Taxus sumatrana. Mendelian inheritance for the seven Taxus baccata SSRs was proved by genotyping 17 individuals and 124 megagametophytes (conifer seed haploid tissue). A total of 96 individuals from 5 different populations (10–26 samples per population) were used to estimate genetic diversity parameters. High levels of genetic diversity, with values ranging from 0.533 to 0.929 (6–28 alleles per SSR) were found. No linkage disequilibrium between pairs of loci was detected. All loci but one showed significant departures from Hardy–Weinberg equilibrium. Excess of homozygosity was probably due to high inbreeding in English yew populations, an outcome of low effective population size and/or fragmented distribution. Highly polymorphic SSRs will be used to conduct population genetic studies at different geographical scales and to monitor gene flow.     

High SSR diversity but little differentiation between accessions of Nordic timothy (Phleum pratense L.)

A large collection of genebank accessions of the hexaploid outcrossing forage grass species timothy (Phleum pratense L.) was for the first time analysed for SSR diversity on individual, population and regional level. Timothy is the most important forage grass species in the Nordic countries. Eighty-eight timothy accessions from Nordic countries and eight accessions around Europe were analysed with recently developed simple sequence repeat (SSR) markers. Timothy proved to be very polymorphic: the 13 selected SSRs amplified a total of 499 polymorphic alleles, the number of alleles per SSR locus varying from 15 to 74. Taking all SSR alleles together, the observed number in each accession ranged from 95 to 203. Levels of diversity were found to be significantly different between countries, vegetation zones and different cultivar types. However, the differentiation between accessions was low: most of the variation (94%) in the studied timothy material was due to variation within accessions and only 5% was between accessions and 1% between countries. Lack of geographical differentiation may reflect the outcrossing and hexaploid nature of timothy. Our results showed that neutral SSR markers are suitable for demonstrating levels of diversity but not alone adequate to resolve population structure in timothy. Nordic timothy material seems to be diverse enough for breeding purposes and no decline in the level of diversity was observed in varieties compared to wild timothy populations. Challenges in analysing SSR marker data in a hexaploid outcrosser were discussed.     

基于微卫星标记的中国东北地区灰飞虱遗传变异及种群遗传结构分析

【目的】本研究旨在明确我国东北地区灰飞虱Laodelphax striatellus种群遗传变异及种群遗传结构,阐明种群间遗传分化与基因流。【方法】利用9对微卫星引物对采自我国东北地区15个地理种群的375头灰飞虱样品进行测序与分析;利用GeneAlex6.51, GENEPOP4.0.9和STRUCTURE 2.3.4等软件分析灰飞虱地理种群间的遗传多样性、遗传分化、基因流及种群遗传结构。【结果】在所分析的375头灰飞虱个体中,各位点平均有效等位基因数Na=6.898;总体上,灰飞虱不同地理种群遗传多样性较高(平均观测杂合度Ho=0.548;平均期望杂合度He=0.582),各种群间基因流较低(Nm=0.660)。UPGMA聚类树、PCoA及STRCTURE分析结果表明,东北地区灰飞虱种群分为两分支:吉林(JL)和沈阳(SY2012,SY2013和SY2014)种群聚为一支;其余种群聚为一支。AMOVA分析结果表明,灰飞虱遗传变异主要来自种群内(87%),种群间变异水平较低(13%)。【结论】中国东北地区灰飞虱遗传多样性较高,不同地理种群存在一定程度的遗传分化,且基因交流较低,存在一...     

The mating system of an hydrophilous angiosperm Posidonia australis (Posidoniaceae)

The hydrophilous seagrass Posidonia australis has a wide range of multilocus outcrossing rates (t), which vary from 0 to 0.89, with "apparent' outcrossing rates varying from 0 to 0.42 among the seven populations sampled. This pattern of outcrossing rate indicates that water pollination (hydrophily) is less uniform than wind pollination and more similar to animal pollination in its variability. Variation in levels of outcrossing between populations may be due to differences in water movement; for example, open bays have greater pollen dispersal and higher outcrossing rates. Considerable pollen movement within meadows was inferred from a high frequency of nonmaternal alleles in the pollen pool. The distribution of genetic diversity among populations (GST = 0.229) suggests moderate gene flow on the local scale. These results demonstrate that successful submarine cross-pollination occurs in the hydrophile P. australis, which has a diverse mating system with populations that range from predominantly inbred to predominantly outcrossed.     

Genetic variation and population structure in Korean populations of eurya japonica (Theaceae)

We investigated levels of genetic diversity, population genetic structure, and gene flow in Eurya japonica, a widespread and broad-leaved evergreen dioecious tree native to Japan, China, Taiwan, and the southern and southwestern coast of the Korean Peninsula. Starch-gel electrophoresis was conducted on leaves collected from 1,000 plants in 20 Korean populations. All 12 loci examined were polymorphic in at least one population, and the mean number of alleles per locus was 3.79. In addition, mean observed population heterozygosity (Ho_p = 0.425), expected heterozygosity (He_p = 0.462), and total genetic diversity (H_T = 0.496) were substantially higher than average values for species with similar life history traits. Although significant differences in allele frequency were detected between populations at all loci (P < 0.001), <7% of the genetic variation was found among populations (F_ST = 0.069). There was a significant negative correlation between genetic identity and distance between populations (r = -0.341; P < 0.05), but this explained only a small amount of the diversity among populations. Indirect estimates of the number of migrants per generation (Nm) (3.37, calculated from F_ST; 3.74, calculated from the mean frequency of eight private alleles) indicate that gene flow is extensive among Korean populations of E. japonica. Factors contributing to the high levels of genetic diversity found within populations of E. japonica include large and contiguous populations, obligating outcrossing (dioecious plant), high fecundity, and long generation time. Occasional seed dispersal by humans and pollen movement by domesticated honey bees may further enhance gene flow within the species.     

Selection of high heterozygosity popcorn varieties in Brazil based on SSR markers

We analyzed genetic structure and diversity among eight populations of popcorn, using SSR loci as genetic markers. Our objectives were to select SSR loci that could be used to estimate genetic diversity within popcorn populations, and to analyze the genetic structure of promising populations with high levels of heterozygosity that could be used in breeding programs. Fifty-seven alleles (3.7 alleles per locus) were detected; the highest effective number of alleles (4.21) and the highest gene diversity (0.763) were found for the Umc2226 locus. A very high level of population differentiation was found (F(ST) = 0.3664), with F(ST) for each locus ranging from 0.1029 (Umc1664) to 0.6010 (Umc2350). This analysis allowed us to identify SSR loci with high levels of heterozygosity and heterozygous varieties, which could be selected for production of inbred lines and for developing new cultivars.     

Genetic Diversity and Structure of Neotyphodium Species and Their Host Achnatherum sibiricum in a Natural Grass–Endophyte System

Achnatherum sibiricum (Poaceae) is a perennial bunchgrass native to the Inner Mongolia Steppe of China. This grass is commonly infected by epichloë endophytes with high-infection frequencies. Previously, we identified two predominant Neotyphodium spp., N. sibiricum and N. gansuense. In the present study, genetic diversity and structure were analyzed for the two predominant Neotyphodium spp. as well as the host grass. We obtained 103 fungal isolates from five populations; 33 were identified as N. sibiricum and 61 as N. gansuense. All populations hosted both endophytic species, but genetic variation was much higher for N. gansuense than for N. sibiricum. The majority of fungal isolates were haploid, and 13% of them were heterozygous at one SSR locus, suggesting hybrid origins of those isolates. Significant linkage disequilibrium of fungal SSR loci suggested that both fungal species primarily propagate by clonal growth through plant seeds, whereas variation in genetic diversity and the presence of hybrids in both endophytic species revealed that although clonal propagation was prevalent, occasional recombination might also occur. By comparing genetic differentiation among populations, we found around 4–7-fold greater differentiation of endophyte populations than host populations, implying more restricted gene flow of endophytes than hosts. We proposed that endophyte infection of A. sibiricum might confer the host some selective advantages under certain conditions, which could help to maintain high-endophyte-infection frequencies in host populations, even when their gene flows do not match each other. Furthermore, we suggested that the same genotype of endophyte as well as host should be confirmed if the objective of the study is to know the influence of endophyte or host genotype on their symbiotic relationship, instead of just considering whether the plant is infected by an endophyte or not, since endophytes from the same host species could exhibit high levels of genetic diversity, which is likely to influence the outcome of their symbiotic relationship.     

Genetic diversity of the Bambara groundnut (Vigna subterranea (L.) Verdc.) as assessed by SSR markers

Bambara groundnut ( Vigna subterranea (L.) Verdc.) is an important African legume crop. In this study, a collection consisting of 240 accessions was analyzed using 22 simple sequence repeat (SSR) markers. In total, 166 alleles were detected, with a mean of 7.59 alleles per locus. Allelic and gene diversities were higher in the west African and Cameroon/Nigeria regions with 6.68 and 6.18 alleles per locus, and 0.601 and 0.571, respectively. The genetic distance showed high similarity between west African and Cameroon/Nigeria accessions. Principal coordinate analyses and neighbor-joining analysis consistently revealed that the majority of west African accessions were grouped with Cameroon/Nigeria accessions, but they were differentiated from east African, central African, and southeast Asian accessions. Population structure analysis showed that two subpopulations existed, and most of the east African accessions were restricted to one subpopulation with some Cameroon/Nigeria accessions, whereas most of the west African accessions were associated with most of the Cameroon/Nigeria accessions in the other subpopulation. Comparison with SSR analysis of other Vigna cultigens, i.e., cultivated azuki bean ( Vigna angularis ) and mungbean ( Vigna radiata ), reveals that the mean gene diversity of Bambara groundnut was lower than azuki bean but higher than mungbean.     

Genetic diversity and morphological differentiation in Liatris helleri (Asteraceae), a threatened plant species

Liatris helleri (Asteraceae) is an insect-pollinated herbaceous perennial endemic to several high-elevation sites in the Blue Ridge Mountains of North Carolina. Allozymes were used to describe the genetic diversity and population structure in nine populations of this rare, federally listed species. Differences in leaf morphology were also examined for greenhouse-grown plants representing several populations. The proportion of the total genetic diversity found among populations, as indicated by the allozyme data, was 16%. Higher levels of population differentiation were found for differences in leaf shape; population of origin accounted for 37% of the variation in maximum leaf width, while families within populations accounted for 7%. In contrast to many endemic species, L. helleri maintains fairly high levels of genetic diversity. For the species, the percent polymorphic loci was 87.5, the average number of alleles at variable loci was 3.00 and the gene diversity was 0.276. Mean population values were percent polymorphic loci =58.4, mean number of alleles per polymorphic locus =2.59 and gene diversity =0.219. The estimated gene flow was low (Nm=0.85–1.32) and a relatively high correlation (r=0.55; p<0.005) was found between linear geographic and genetic distance. This suggests that the populations are partially isolated by distance, despite the limited range (<60 km) of the species. We recommend that population distinetiveness be maintained in restoration efforts.     

中国落叶松-杨栅锈菌遗传多样性研究

采用SSR分子标记技术对采自全国19个地区、不同年份的落叶松-杨栅锈菌Melampsora larici-populina（简称MLP）36个单孢子堆菌系的遗传多样性和种群遗传结构进行了研究。用5对SSR多态性引物共检测到62个观察等位基因（Na）,有效等位基因数（Ne）为5.42–9.82,平均有效等位基因数为7.05。供试MLP样本在5个SSR位点上的多态性信息量（PIC）变化范围为0.64–0.89,平均值0.79。平均观察杂合度（Ho）、平均期望杂合度（He）分别为0.36和0.86,不同位点的S     

High genetic diversity in Sarracenia leucophylla(Sarraceniaceae), a carnivorous wetland herb

Eighteen allozyme loci were used to examine genetic diversity in 10 natural populations of Sarracenia leucophylla Raf., a pitcher plant restricted to the southeastern United States. One ex situ population propagated for restoration in Georgia was also analyzed. S. leucophylla is an insect-pollinated, outcrossing perennial wetland herb that is threatened over much of its geographic range. Fifteen loci (83.3%) were polymorphic, with a mean number of alleles of 3.33. Compared to species having similar life-history traits and to previously analyzed Sarracenia species, S. leucophylla displayed unexpectedly high genetic diversity. For example, genetic diversity within the species (Hes) was 0.224 and mean population genetic diversity (Hep) was 0.183. Although small S. leucophylla populations maintained less genetic diversity than larger ones, these differences were not statistically significant. Nonetheless, this suggests that small populations may have lost rare alleles. Statistically significant genetic differentiation among populations was found (theta = 0.192, P < .01), although it was not atypical considering the species' life-history characteristics. A significant correlation (P < .01) between genetic and geographic distance was found, indicating an isolation-by-distance effect. However, the correlation coefficient for this relationship was low (r = 0.46), suggesting that factors other than gene flow play a prominent role in the geographic distribution of genetic diversity within the species. The ex situ population captured most of the allozyme variation found in its source population.     

Genetic variability and distribution of mating type alleles in field populations of Leptosphaeria maculans from France

Leptosphaeria maculans is the most ubiquitous fungal pathogen of Brassica crops and causes the devastating stem canker disease of oilseed rape worldwide. We used minisatellite markers to determine the genetic structure of L. maculans in four field populations from France. Isolates were collected at three different spatial scales (leaf, 2-m2 field plot, and field) enabling the evaluation of spatial distribution of the mating type alleles and of genetic variability within and among field populations. Within each field population, no gametic disequilibrium between the minisatellite loci was detected and the mating type alleles were present at equal frequencies. Both sexual and asexual reproduction occur in the field, but the genetic structure of these populations is consistent with annual cycles of randomly mating sexual reproduction. All L. maculans field populations had a high level of gene diversity (H = 0.68 to 0.75) and genotypic diversity. Within each field population, the number of genotypes often was very close to the number of isolates. Analysis of molecular variance indicated that >99.5% of the total genetic variability was distributed at a small spatial scale, i.e., within 2-m2 field plots. Population differentiation among the four field populations was low (GST < 0.02), suggesting a high degree of gene exchange between these populations. The high gene flow evidenced here in French populations of L. maculans suggests a rapid countrywide diffusion of novel virulence alleles whenever novel resistance sources are used.