Population Genetics of Three Important Head Blight Pathogens Fusarium graminearum, F. pseudograminearum and F. culmorum

Homothallic Fusarium graminearum (teleomorph Gibberella zeae) and anamorphic F. culmorum are destructive pathogens causing Fusarium head blight (FHB) of small‐grain cereals worldwide, while heterothallic F. pseudograminearum (G. coronicola) seems to be restricted to Australia as a FHB pathogen. In a comprehensive treatise of pathogen population genetics, this review summarizes global knowledge of genetic diversity among isolates sampled at various spatial and temporal scales, examines the mechanisms that generate this diversity and explores the implications of pathogen diversity and plasticity to resistance breeding. Despite their different modes of reproduction, there is large variation among isolates of all three species originating from different countries and continents. With a few exceptions, haplotype diversity ranges from 60 to 100% even within populations from individual fields. In F. graminearum, over 90% of the variation is found within populations, even when samples are collected from areas as small as 0.25 m². Variation among populations is low (4–8%) with negligible population subdivision. This indicates a high level of gene flow (N_m = 8–71) with linkage equilibrium for the majority of selectively neutral molecular marker loci analysed. These findings for F. graminearum point to large random mating populations driven by occasional outcrossing, high gene flow across large geographical distances and a relatively low host‐mediated directional selection. Similar conclusions can be drawn for the Canadian population of F. pseudograminearum, but not for populations from Australia, where different pathogen ecology may have reduced the frequency of sexual recombination. Phylogenetic analyses indicate delineation of lineages in F. graminearum, often along geographically separated lines, while the related F. pseudograminearum is a single recombining species with limited or no lineage development. The anamorphic F. culmorum shows no obvious clonal structure in its population as might have been expected. High levels of diversity within fields may have been caused by balancing selection from frequent alternation between saprophytic and parasitical life cycle and/or a hidden or recently extinct teleomorph. Other mechanisms including parasexual cycles or active transposable elements may also be involved but these have not been investigated as yet. Crosses between and among F. graminearum lineages have shown a rather simple, additive inheritance of pathogenicity and aggressiveness with frequent transgressive segregation in crosses among isolates with moderate aggressiveness. This raises the spectre of highly aggressive and/or toxigenic isolates evolving if a limited range of quantitative trait locus for FHB resistance is deployed on a large scale. Combining more than one genetically distinct sources of resistance, possibly with different modes of action against the pathogen, will be necessary to avoid severe FHB outbreaks in the future.     

Molecular Genetic Diversity in Populations of Fusarium pseudograminearum from Tunisia

Fusarium pseudograminearum is one of the major pathogens causing crown rot of wheat in the semi‐arid and arid areas in Tunisia. In this study, the molecular diversity of 74 isolates of F. pseudograminearum representing three populations from Tunisia and a set of isolates from the world collection was investigated. The potential mycotoxin‐producing ability was tested by PCR using primer pairs specific for the Tri3, Tri7 and Tri13 genes. Results indicated that all the isolates are potentially DON and/or 3‐AcDON producers. The mating‐type idiomorphs were identified using diagnostic PCR primer for MAT1‐1 and MAT1‐2. Both mating types were recovered from the same region and in some cases from the same field. Restriction analysis of the nuclear ribosomal DNA (nrDNA) intergenic spacer region (IGS) revealed 11 haplotypes, five of which were identified in the world collection. The analysis of population structure using the combined IGS and MAT data revealed that the total gene diversity (H_T = 0.108) was mostly attributable to diversity within populations (H_S = 0.102) and that the genetic differentiation among the four populations was low (G_ST = 0.09). The analysis of molecular variance (amova ) showed that 15% of the variability was between the Tunisian populations and the world collection. These findings indicate that quarantine measures should be in place to limit the introduction of new populations of F. pseudograminearum into Tunisia.     

Competitive Aggressiveness in Binary Mixtures of Fusarium graminearum and F. culmorum Isolates Inoculated on Spring Wheat with Highly Effective Resistance QTL

Fusarium head blight (FHB) caused by Fusarium graminearum and F. culmorum is a devastating disease with high effects on grain yield and quality. We developed spring wheat lines incorporating the highly effective FHB resistance quantitative trait loci (QTL) Fhb1 and Qfhs.ifa‐5A. Whether these QTL lead to competition within Fusarium populations in the field resulting in isolates with higher aggressiveness has not been analysed. The aims of this study were to determine (i) the aggressiveness potential of F. graminearum and F. culmorum isolates, (ii) competition effects of these isolates in binary mixtures and (iii) the stability of resistant hosts. Six F. graminearum, two F. culmorum isolates and seven binary mixtures containing these isolates were tested for their aggressiveness and mycotoxin production at two locations in South Germany in 2007 and 2008. Host lines were four spring wheat lines containing the resistance QTL Fhb1 and/or Qfhs.ifa‐5A or none of them and one standard variety. Re‐isolates were sampled from plots inoculated with the binary mixtures to identify the percentage of each isolate in the mixture by simple sequence repeat markers. Resistant host lines reacted as expected and had a high stability to all isolates and mixtures. Only less important host × mixture interactions were detected. Aggressiveness among isolates and mixtures was significantly different. Type and amount of mycotoxin and high single isolate aggressiveness were not necessarily advantageous in the mixture. However, both F. culmorum isolates outcompeted F. graminearum isolates. Significant deviations from the inoculated 1 : 1 proportions occurred in 34 of 49 cases, illustrating that competition effects appeared in the mixtures. These differences depended mainly on the year and not on the level of host resistance. We conclude that resistance should not be affected by the Fusarium isolates and mixtures.     

Genetic variation in populations of the endangered fish Ladigesocypris ghigii and its implications for conservation

1. The genetic variation of the endangered freshwater fish Ladigesocypris ghigii, endemic to the island of Rhodes (Greece), was investigated for nine populations, originating from seven different stream systems and a reservoir, both at the mtDNA and nuclear level, in order to suggest conservation actions. 2. Both restriction fragment length polymorphism analysis of five segments of mitochondrial DNA (ND‐5/6, COI and 12S‐16S rRNA) amplified by polymerase chain reaction, and random amplified polymorphic DNA analysis, revealed extremely low levels of intra‐population polymorphism. It is highly likely that the low intra‐population variability is the result of successive bottleneck events evident in shrinkage and expansion of the populations year after year, which may have led to a complete loss of several genotypes and haplotypes, and an increased degree of inbreeding. 3. Inter‐population genetic structuring was high, with fixation of haplotypes within six of the nine populations and fixation of alleles within populations originating from different waterbodies. It is probable that all haplotypes and/or alleles found were initially represented in all populations. However, because of the long time of isolation coupled with successive bottleneck and subsequent genetic drift, common mtDNA haplotypes and alleles among the populations may have become rare or extinct through stochastic lineage loss. 4. Although nucleotide divergence among haplotypes was very shallow, half of the haplotypes recorded (three of six), resulted from nucleotide changes on the 12S–16S rRNA segments, which are the most conserved part of the mitochondrial genome. This fact may indicate that the observed genetic variation did not necessarily result only from the retention of ancestral polymorphism, but may have arisen through mutation and complete lineage sorting over a relatively small number of generations, once the populations had become isolated from one another. 5. Our data suggest that two of the L. ghigii populations may be on independent evolutionary trajectories. Considering that each population appears so far well adapted within each site, all populations should be managed and conserved separately.     

Genetic Structure and Aggressiveness of Rhizoctonia solani AG1‐IA,the Cause of Sheath Blight of Rice in Southern China

One hundred and eighty isolates of Rhizoctonia solani AG1‐IA, the causal agent of rice sheath blight, were obtained from six locations in southern China. The genetic structure of R. solani isolates was investigated using random amplified polymorphic DNA (RAPD) markers, and a considerable genetic variation among R. solani isolates was observed. Most of the genetic diversity was distributed within populations, rather than among them. The distribution pattern of the genetic variation of R. solani appears to be the result of high gene flow (Nm) and low‐genetic differentiation among populations. The aggressiveness of R. solani was visually assessed by rice seedlings of five different cultivars in the glasshouse. All isolates tested were found to induce significantly different levels of disease severity, reflecting considerable variation in aggressiveness. The isolates were divided into highly virulent, moderately virulent and weakly virulent groups, and the moderately virulent isolates were dominant in R. solani population. No significant correlation was observed among the genetic similarity, pathogenic aggressiveness and geographical origins of the isolates. Information obtained from this study may be useful for breeding for improved resistance to sheath blight.     

Distribution of Mating Types and Genetic Diversity of Ascochyta rabiei Populations in Tunisia Revealed by Mating-type-specific PCR and Random Amplified Polymorphic DNA Markers

The distribution of mating types of Ascochyta rabiei (teleomorph: Didymella rabiei) was determined in Tunisia using a MAT‐specific PCR assay. Among 123 isolates tested, 80% were MAT1‐1 and 20%MAT1‐2. Only MAT1‐1 isolates were present in the Beja and Bizerte regions of Tunisia, whereas both mating types were present in Nabeul, Kef and Jendouba. In the latter three regions, the hypothesis of random mating could not be rejected based on chi‐squared tests of mating‐type ratios (P > 0.05). The lower frequency of the MAT1‐2 coupled with the restricted distribution of this mating type in Tunisia may indicate a recent introduction of MAT1‐2 in Tunisia. This speculation is consistent with the recent (2001) observation of D. rabiei pseudothecia on chickpea debris in Tunisia. Forty isolates representative of the five regions were genetically analysed using 10 random amplified polymorphic DNA (RAPD) primers to provide a preliminary estimate of genetic diversity of the pathogen in Tunisia. Among 129 putative RAPD loci amplified, 81% were polymorphic and 32 unique RAPD fingerprints were detected. A high level of genetic differentiation was detected among subpopulations (G_ST = 0.33). Cluster analyses revealed that isolates from Bizerte, Beja and Jendouba were genetically similar and distinct from isolates sampled in Nabeul and Kef. MAT1‐1 isolates were clustered separately from MAT1‐2 isolates in Jendouba and Nabeul suggesting that recombination may not yet be occurring in these regions despite the occurrence of both mating types in equal frequency in these regions. This lack of recombination between MAT1‐1 and MAT1‐2 also supports the hypothesis of a recent introduction of MAT1‐2 into Tunisia.     

Analysis of Genetic Diversity of Fusarium oxysporum f. sp. phaseoli Isolates, Pathogenic and Non-pathogenic to Common Bean (Phaseolus vulgaris L.)

Twenty isolates of Fusarium oxysporum from Brazil, pathogenic and non‐pathogenic to common bean, were analysed using random amplified polymorphic DNA (RAPDs) to study the genetic diversity. RAPD analysis using 23 oligonucleotides resulted in the amplification of 229 polymorphic and 7 monomorphic DNA fragments ranging from 234 to 2590 bp. High genetic variability was observed among the isolates, with the distances varying between 8% and 76% among pathogenic, 2% and 63% among the non‐pathogenic and 45% and 76% between pathogenic and non‐pathogenic isolates. The analysis of genetic distance data showed that the pathogenic isolates tended to group in one group and the non‐pathogenic in another. The genetic distance values of 30% among the pathogenic isolates in cluster A are compatible with the genetic distance values observed within the physiological races, but the distance values among the pathogenic isolates in clusters B and G are not compatible with the distance values observed within the race. Although our results are preliminary, it was not possible to exclude the existence of more than one race of this fungus in Brazil.     

Virulence and Molecular Diversity of Venturia inaequalis in Commercial Apple Growing Regions in Kashmir

Seventy‐one isolates of Venturia inaequalis collected from commercial apple growing areas of Kashmir were characterized on international differential apple hosts and analyzed by Random Amplified Polymorphic Microsatellites (RAMS), PCR–RFLP and sequencing of rDNA for elucidation of variability. Virulence analysis on a differential set categorized them into four pathogenic races, viz. (0), (1), (2) and (1,2) in the first time comprehensive molecular analysis of this in India and especially from Jammu and Kashmir, a north‐western Himalayan state of India. Race groups (0), (1), (2) and (1,2) contained isolates from diverse areas without specificity to any geographical zone or region. Cluster analysis of the RAMS and PCR–RFLP revealed a high genotypic diversity within V. inaequalis isolates. Three major clusters were obtained and the isolates could not be categorized on the basis of either their geographical distribution or the cultivar from which they were isolated. amova analysis of pathogen populations at regional or race level revealed high diversity within the populations. Pairwise F_ST comparisons between the populations revealed less genetic differentiation, thereby indicating existence of frequent gene flow in Kashmir. The 24 rDNA sequences of V. inaequalis showed high haplotype diversity of 0.938 and 0.40 nucleotide diversity. Again clustering at regional or race level detected greater part of variability within groups than among groups, thereby indicating high diversity in V. inaequalis populations in Kashmir valley.     

Strategic Conservation of Orchard Germplasm Based on Indigenous Knowledge and Genetic Diversity: a Case Study of Sour Orange Populations in China

To effectively conserve sour orange (Citrus aurantium L.) germplasm on two Islands at the estuary of the Yangtze River In China, we estimated genetic variation and relationships of the known parental trees and their proposed descendents (young trees) using the fingerprinta of random amplified polymorphic DHA (RAPD). Results based on RAPD analyses showed considerable genetic diversity In the parental populations (H<,e> = 0.202). The overall populations including the parental and young trees showed slightly higher genetic diversity (H<,e> = 0.298) than the parents, with about 10% variation between populations. An unweighted pair group method with arithmetic mean analysis dendrogram based on cluster analysis of the Jaccard similarity among individuals demonstrated a more complicated relationship of the parental and young trees from the two islands, although the young trees showed a clear association with parental trees. This indicates a slgnificant contribution of parental trees in establishing the sour orange populations on the two islands. According to farmers' knowledge, conservation of only one or two parental trees would be sufficient because they believed that the whole populations were generated from a single mother tree. However, this study suggests that preserving most parental trees and some selected young trees with distant genetic relationships should be an effective conservation strategy for sour orange germplasm on the two islands.     

Analysis of the Genetic Structure of Sclerotinia sclerotiorum (Lib.) de Bary Populations from Different Regions and Host Plants by Random Amplified Polymorphic DNA Markers

The genetic diversity and genetic structure of a population of isolates of Sclerotinia sclerotiorum (Lib.) de Bary from different regions and host plants were investigated using the random amplified polymorphic DNA (RAPD) method with 20 random decamer primer pairs in order to provide some information on the phylogenetic taxa and breeding for resistance to sclerotinia stem rot. A minimum of three and a maximum of 15 unambiguously amplified bands were generated, furnishing a total of 170 bands ranging in size from 100to 3 200 bp, corresponding to an average of 8.5 bands per primer pair. One hundred and four of these 170bands (61.2%) were polymorphic, the percentage of polymorphic bands for each primer pair ranging from 0.0% to 86.7%. The genetic relationships among the isolates, based on the results of RAPD analysis, were examined. The genetic similarity of all selected isolates was quite high. At the species level, the genetic diversity estimated by Nei's gene diversity (h) was 0.197 and S hannon's index of diversity (I) was 0.300. The unweighted pair-group mean analysis (UPGMA) cluster analysis showed that most isolates from the same regions were grouped in the same cluster or a close cluster. The population of isolates from Hefei (Anhui Province, China) was more uniform and relatively distant to other populations. The Canadian population collected from carrot (Daucus carota var. sativa DC.) was relatively close to the Polish population collected from oilseed rape (Brassica napus L.) plants. There was no relationship between isolates from the same host plants. An analysis of molecular variance (AMOVA) revealed that the percentage of variance attributable to variation among and within populations was 50.62% and 49.38%, respectively. When accessions from China, Europe, and Canada were treated as three separate groups, the variance components among groups,among populations within groups, and within populations were -0.96%, 51.48%, and 49.47%, respectively.The genetic differentiations among and within populations were highly significant (P ＜ 0.001). Similarly, the coefficient of gene differentiation (Gst) in total populations calculated by population genetic analysis was 0.229 4, which indicated that the genetic variation among populations was 22.94%. The gene flow (Nm)was 1.68, which indicated that the gene permutation and interaction among populations was relatively high.     

Molecular Detection and Exploration of Diversity Among Fungal Consortium Involved in Phosphate Solubilization

Abstract

Phosphorous (P) that upholds life become unattainable as most of them become unavailable due to the formation of insoluble complexes with cations such as Ca²⁺, Al³⁺ and Fe³⁺ forming a complex calcium phosphate (Ca₃(PO₄)₂), aluminum phosphate (AlPO) and ferrous phosphate (FePO) that results in the decrease of soluble P to a greater extent. There are several reports stating that several rhizospheric fungal species play an important role in solubilizing these insoluble phosphates into a soluble form by the excretion of enzymes like phosphatase, phytase enzymes, and organic acids. In view of this, so we have collected twenty fungal isolates having probable phosphate solubilizing efficiency from different regions of Lucknow, India. Their morphological and biochemical characteristics were tested. Among all, six efficient fungal isolates were further checked at molecular level by using 18S rRNA universal primers and by RAPD means. A dendrogram indicated 40-90% homology i.e., highest similarity was found in between species of Aspergillus flavus and A. biplanus with 33.8% similarity while minimum similarity was observed among A. flavus and Fusarium oxysporum. These findings suggest RAPD proves as, a reliable molecular tool that helps in strain specific discrimination.     

Morphological and Genetic Analyses of Hellebore Leaf Spot Disease Isolates from Different Geographic Origins Show Low Variability and Reveal Molecular Evidence for Reclassification into Didymellaceae

Hellebore leaf spot, caused by Coniothyrium hellebori, is the most common fungal disease of Helleborus species not only in botanical and ornamental gardens but also in nurseries. To correct the current lack of knowledge regarding this widely distributed pathogen, this study investigated 25 C. hellebori isolates collected from different countries in North America and Europe, primarily Germany. The morphology, pathogenicity and molecular genetic relationships on the basis of random amplified polymorphic DNA (RAPD) of these isolates were studied. RAPD primers produced a total of 394 bands, of which 40% were polymorphic. Genetic distances were calculated, and a dendrogram with bootstrap analysis was constructed by the unweighted pair group method with arithmetic mean (UPGMA) cluster method. All isolates were identified as C. hellebori, the causal agent of the disease. Two C. hellebori subclades were found, which could not be correlated with the geographic origin of the isolate, but with the plant host species and morphological characteristics. Sequence comparisons of the large subunit and internal transcribed spacer loci between C. hellebori and sequences from GenBank revealed that C. hellebori has to be grouped into the Didymellaceae family and rather belongs to Phoma or Microsphaeropsis than to Coniothyrium. This work represents the first study of this plant pathogen causing severe damage in Helleborus stocks and provides important information for the development of future Helleborus resistance breeding strategies.     

Combined Assessment of Genetic Variability in Populations of Brown Trout (Salmo trutta L.) Based on Allozymes, Microsatellites, and RAPD Markers

Genetic variability within and among four Spanish natural populations of Salmo trutta L. was evaluated on the basis of 25 enzyme loci, 3 microsatellite loci, and 9 randomly amplified polymorphic DNAs (RAPDs). A total of 21 allelic markers were found, 12 of which were reported by microsatellites, whereas enzyme and RAPD accounted only for 6 and 3, respectively. Genetic variation within samples was significantly higher for microsatellites and RAPD than for enzyme loci. Although all methods reported a high degree of allelic heterogeneity among samples, also revealing a high degree of gene diversity, genetic relationships depicted by UPGMA dendrograms closely agreed for all kinds of data. Microsatellite loci appeared to be the most feasible technique when searching for specific alleles for a population or an area, owing to the higher number of allelic variants found. Received July 1, 1998; accepted January 14, 1999     

Race Profiling and Molecular Diversity Analysis of Fusarium oxysporum f.sp. ciceris Causing Wilt in Chickpea

Seventy isolates of Fusarium oxysporum f.sp. ciceris (Foc) causing chickpea wilt representing 13 states and four crop cultivation zones of India were analysed for their virulence and genetic diversity. The isolates of the pathogen showed high variability in causing wilt incidence on a new set of differential cultivars of chickpea, namely C104, JG74, CPS1, BG212, WR315, KWR108, GPF2, DCP92‐3, Chaffa and JG62. New differential cultivars for each race were identified, and based on differential responses, the isolates were characterized into eight races of the pathogen. The same set of isolates was used for molecular characterization with four different molecular markers, namely random amplified polymorphic DNA, universal rice primers, simple sequence repeats and intersimple sequence repeats. All the four sets of markers gave 100% polymorphism. Unweighted paired group method with arithmetic average analysis grouped the isolates into eight categories at genetic similarities ranging from 37 to 40%. The molecular groups partially corresponded to the states of origin/chickpea‐growing region of the isolates as well as races of the pathogen characterized in this study. The majority of southern, northern and central Indian populations representing specific races of the pathogen were grouped separately into distinct clusters along with some other isolates, indicating the existence of variability in population predominated by a single race of the pathogen. The present race profiling for the Indian population of the pathogen and its distribution pattern is entirely new. The knowledge generated in this study could be utilized in resistance breeding programme. The existence of more than one race, predominated by a single one, in a chickpea cultivation zone as supported by the present molecular findings is also a new information.     

Genetic Structure of Cochliobolus sativus Populations Sampled from Roots and Leaves of Barley and Wheat in North Dakota

Common root rot (CRR) and spot blotch, caused by Cochliobolus sativus (Ito and Kurib.) Drechsl. ex Dast., are important diseases of barley (Hordeum vulgare L.) and wheat (Triticum aestivum L.) worldwide. However, the population biology of C. sativus is still poorly understood. In this study, the genetic structure of three C. sativus populations, consisting of isolates sampled respectively from barley leaves (BL), barley roots (BR) and wheat roots (WR) in North Dakota, was analysed with amplified fragment length polymorphism (AFLP) markers. A total of 127 AFLP loci were generated among 208 C. sativus isolates analysed with three primer combinations. Gene diversity (H = 0.277–0.335) were high in all three populations. Genetic variation among C. sativus individuals within population accounted for 74%, whereas 26% of the genetic variation was explained among populations. Genetic differentiation was high (ØPT = 0.261, corrected = 0.39), whereas gene flow (Nm) ranged from 1.27 to 1.56 among the three populations analysed. The multilocus linkage disequilibrium (LD) (= 0.076–0.117) was moderate in C  sativus populations. Cluster analyses indicate that C. sativus populations differentiated according to the hosts (barley and wheat) and tissues (root and leaf) although generalists also exist in North Dakota. Crop breeding may benefit from combining genes for resistance against both specialists and generalists of C. sativus.     

Pathogenic Variability and Genetic Diversity of Phaeoisariopsis griseola Isolates from Two Counties in the State of Goias, Brazil

Angular leaf spot disease of common bean (Phaseolus vulgaris), caused by Phaeoisariopsis griseola, is one of the most important disease of this crop in Brazil. Control strategies for the disease include cultural practices, chemical control and genetic resistance. This pathogen is known to vary greatly in pathogenicity. For durable use of genetic resistance to control this disease, it is necessary to manage resistant cultivars by taking into account the population structure of P. griseola. Isolates of the pathogen from Goias, Brazil exhibited an important virulence polymorphism when inoculated on 12 differential cultivars. A total of 13 pathotypes was identified within a series of 96 isolates collected in Inhumas and Damolandia counties. Only pathotypes 63‐15, 63‐23, 63‐31 and 63‐63 were identified in both counties. Since all the isolates were capable of inducing disease in both Andean and Mesoamerican differential cultivars, they were considered to be of Mesoamerican origin. Random amplified polymorphic DNA (RAPD) analysis performed on the same 96 isolates revealed a great genetic diversity clustering the series into five groups at an Euclidean distance of 62.5%. Although the results did not show any clustering according to the isolate origin, it was possible to observe a tendency of the isolates to cluster in different groups according to their origin. No pathotype‐specific band was observed in the present study.     

Analysis of Cultural and Genetic Diversity in Alternaria helianthi and Determination of Pathogenic Variability Using Wild Helianthus Species

The study aims at assessment of morphological, molecular and pathogenic variability of Alternaria helianthi, incitant of leaf blight of sunflower. Morphological characteristics determined for 26 isolates of A. helianthi from India revealed variations in shape of culture, pigmentation, conidial measurements, number of septa and colony growth. The conidia of isolate Ah‐7 were long, while conidia of isolate Ah‐15 were short. Based on cultural characters, isolates were classified into four groups. Genetic variability of the isolates was assessed by random amplified polymorphic DNA analysis. Good polymorphism was observed and cluster analysis indicated presence of six genetically distinct groups among the isolates. The isolates Ah‐1, Ah‐7 and Ah‐14 were genetically distinct. Resistant sources are not available in cultivated sunflower, while wild Helianthus species possess resistance to multiple stresses. We evaluated reaction of wild Helianthus species to isolates of A. helianthi. Among wild Helianthus species, H. tuberosus followed by H. occidentalis showed moderately to highly resistant reaction to all the isolates and recorded less disease incidence. The species H. argophyllus followed by H. laevigatus showed more disease incidence. The cultivated sunflower recorded susceptible reaction to most of the isolates and recorded high disease incidence. The isolates differed significantly for pathogenic reaction and were grouped into three pathogenicity groups; low, medium and high. Six isolates induced <20% disease incidence and were included in the low pathogenicity group. Majority of isolates were in the medium pathogenicity group. Six isolates i.e. Ah‐9, Ah‐10, Ah‐18, Ah‐20, Ah‐24 and Ah‐26 induced more than 50% disease incidence and were considered high pathogenicity group. Our results demonstrate the existence of considerable variation in resistance of Helianthus species to A. helianthi and also in morphological and genetic characters of A. helianthi isolates prevalent in India.     

Genetic diversity of the endangered species Rosa rugosa Thunb. in China and implications for conservation strategies

Rosa rugosa Thunb. is one of the dominant and important shrub species in estuary dunes and shingle beaches of northern China. However, its area of distribution, the number of populations, and the size of each population have decreased rapidly in the past two decades because of habitat degradation and loss. Random amplified polymorphic DNA markers were used to determine the genetic diversity of four remaining large natural populations of R. rugosa and to discuss an effective conservation strategy for this endangered species in China. High genetic variations were detected in R. rugosa populations in China. The mean percentage of polymorphic loci （P%） within four local populations was 57.99%, with the P% of the total population being 75.30%. Mean Shannon＇s information index （H0） was 0.2826, whereas total Ho was 0.3513. The genetic differentiation among populations was 0.1878, which indicates that most genetic diversity occurs within populations. Population Tumenjiang （TMJ） showed the highest genetic diversity （P% = 66.27%; H0 = 0.3117） and contained two exclusive bands. Population Changshandao （CSD） showed higher genetic diversity （P% =59.04%; H0 = 0.3065）. Populations TMJ and CSD contained 95.33% and 99.33%, respectively, of loci with moderate to high frequency （P〉0.05） of the total population. These results indicate that populations TMJ and CSD should be given priority for in situ conservation and regarded as seed or propagule sources for ex situ conservation. The results of the present study also suggest that R. rugosa in China has become endangered as a result of human actions rather than genetic depression of populations; thus, human interference should be absolutely forbidden in R. rugosa habitats.     

巨穗小麦种质中外源遗传物质的细胞遗传学和分子生物学鉴定

利用C分带、基因组原位杂交并结合分子生物学手段,对12份巨穗小麦种质材料中的外源遗传物质进行了检测.结果表明,12份材料染色体数均为42,其中5份材料均具有一对小麦-黑麦(Triticum aestivum-Secalecereal)1BL/1RS易位染色体和一对中间偃麦草(Agropyron intermedium Garten)染色体、3份材料只具有一对中间偃麦草染色体、3份材料只具一对1BL/1RS染色体、1份材料无1BL/1RS和中间偃麦草染色体.进一步细胞学分析表明,此中间偃麦草染色体代换了普通小麦(Triticum aestivum L.)中的2D染色体,因其良好的同源补偿性,表示为2Ai.同时对2Ai在巨穗小麦种质中存在的遗传学意义及小麦遗传改良中的应用进行了讨论.