The role of selection on the genetic structure of pathogen populations: Evidence from field experiments with Mycosphaerella graminicola on wheat

Summary Coevolution refers to reciprocal genetic changes that occur in two or more ecologically interacting species. In agricultural ecosystems, we are especially concerned with the genetic response of pathogen populations to resistant cultivars produced by plant breeding programs. It would be useful to be able to predict whether disease resistance is likely to be durable or ephemeral before a cultivar is widely grown. Though it may not be possible to predict durability in advance, knowledge of the genetic structure of pathogen populations may prove useful for making predictions about the rate at which pathogens adapt to resistant varieties. Much has been learned about the genetic structure of populations of obligate fungal pathogens such as rusts and mildews, which have become paradigms for plant pathology. We have focused our effort on the population genetics of the less known, non-specialized, necrotrophic pathogens, such as the Septorias of small grains. Our approach has been to use DNA fingerprinting and RFLP analysis to conduct field experiments that elucidate how populations of fungal pathogens adapt in agroecosystems. Our results suggest that mating system may have a greater impact than natural selection on the genetic structure of populations of Mycosphaerella graminicola (anamorph Septoria tritici).     

Morphological characterization of Mediterranean populations of Biserrula pelecinus L.

A collection of 24 populations of Biserrula pelecinus L. (biserrula)from Italy (Sardinia), Spain. France (Corsica). Morocco and Greece was evaluated in nursery rows at Perth, Western Australia. Fifteen morphological traits were recorded and chromosome numbers of representative populations were counted: The quantitative characters were analysed using principal component analysis and k-means clustering. The chromosome number of all accessions was 2n= 2x= 16. In other respects there was substantial variation in biserrula collected from the Mediterranean basin, the Moroccan populations in particular being distinct, some with strongly curved pods and others with long linear pods. All Greek populations had while flowers and large seeds. Populations from Italy. France and Spain were similar, with linear pods, blue flowers and smaller seeds. Most populations from Morocco were early flowering, had larger leaves, longer pods and produced more seeds per plant than other populations. All populations were extremely hard seeded (>98% hard-seeded after a one-summer exposure on the soil surface). It wits concluded that there is sufficient variation in Mediterranean populations of biserrula lo initiate a selection programme for southern Australia, and other areas with Mediterranean-type climates, especially for areas where serradella ((Ornithopus compressus L.) is adapted. Seed production is far easier than for serradella, and this advantage alone offers potential for its use in duplex soils where the deep rooting habit of serradella gives it an ecological advantage over conventional pasture legumes.     

Inheritance of conditional dormancy in over-summering buds of Phalaris aquatica L.

Summary The relative ability of the underground buds of a Mediterranean perennial grass species to break dormancy in response to summer rain was compared between individual plants and between half-sib families in two experiments with broadly-based breeding populations under different environmental conditions. Significant genetic variation was found in summer regrowth activity in both experiments. The narrow-sense heritability of the ratio of new to old, dead tillers after summer rain under spaced plant conditions at a relatively cool site was 0.40±0.16. Under sward conditions at a warm site, the narrow-sense heritability of an index of summer regrowth potential on a half-sib family mean basis was 0.36±0.08. Summer regrowth activity was negatively but weakly correlated with flowering time (r_g=–0.22), and strongly positively correlated with early autumn herbage yield (r_g=0.79). Summer regrowth activity was not genetically correlated with seedling size or herbage yield in the second or third growing seasons. These correlations are different in sign or magnitude from those observed in collections of Mediterranean ecotypes from which the breeding populations were derived by crossing to Australian cultivars and recurrent selection for vigour and persistence. The correlations in Mediterranean populations therefore are not due to pleiotropic effects; they can be altered by recombination. Phenotypic variance relative to the population mean was considerable in both experiments, and hence the level of the responsiveness of dormant buds to summer rainfall should be manipulable readily by selection. The possible effects of altered responsiveness on long term survival and productivity in several different environments are discussed.     

Assessment of genetic variation in a collection of lentil using molecular tools

A collection of lentil landraces, mostly from Italy, but including Mediterranean and foreign germplasm as reference, was evaluated using RAPDs, microsatellite-primed PCR and ISSRs. A low level of useful polymorphic bands was detected with the former two markers, whereas ISSRs revealed a higher degree of variation. The UPGMA trees, based on Jaccard similarity index matrices, obtained with RAPD and ISSR respectively, did not produce similar clusters. However, in both cases, the accession which differentiated the most from the others was from Ethiopia, a country where also other cultivated species have shown a remarkable degree of variation. On the other hand, Italian accessions showed a trend to group together. ISSR markers proved to be useful for distinguishing closely related genotypes, and possibly for substantiating the genetic peculiarity of ecotypes applying for the obtainment of origin and quality marks. This revised version was published online in July 2006 with corrections to the Cover Date.     

Genetic relationships between cultivated and wild olives of Corsica and Sardinia using RAPD markers

In order to ensure the genetic diversity of the Olea europaea complex,it is necessary to characterize the cultivated varieties and the wildpopulations. In the present study, we focused on the olives growing on twoMediterranean islands, Corsica and Sardinia. On these two islands, there areolives with many denominations, as well as forests of oleasters. Here, it wasproposed to determine the relationships among cultivated and wild olives.Some Italian denominations were studied in addition to assess the influenceof the mainland on the two islands in this respect.The 59 RAPD markers obtained showed the existence of manysynonymous, and homonymous. A dendrogram was constructed using theUPGMA method and a FCA was carried out. The results of these twoanalyses showed the existence of a genetic divergence between the oleastersand the cultivated varieties. They suggest that some of the Corsicanvarieties were probably selected from local wild forms, contrary to theSardinian varieties. They also show that there are feral forms growing onboth islands, which result from hybridization between oleasters andvarieties.     

Genetic relationship of Mediterranean mandarins (Citrus deliciosa Tenore) using RAPD markers

Summary Random amplified polymorphic DNA (RAPD) analysis was carried out to evaluate polymorphism and genetic similarity between 39 Mediterranean mandarin genotypes. One hundred eleven amplification products were identified using 21 random primers. An average of 2.2 RAPD markers was obtained for each primer, corresponding to 42% of the amplification products. Genotype-specific RAPD markers were also found, mainly in known hybrids. UPGMA cluster analysis revealed the low level of genetic variation between accessions of Mediterranean mandarins, whereas their hybrids with other Citrus species showed greater genetic dissimilarity. Twenty accessions yielded very similar patterns, suggesting either that they could be a single clone, or that the technique was not able to detect genomic variation. However, for the other specimens genetic polymorphism can easily be detected by RAPD, although the genetic variation between accessions was quite low. The large number of hybrids and the low polymorphism between accessions support the hypothesis that Mediterranean mandarins are all true hybrid of Common mandarins (Citrus reticulata Blanco).     

Manova and discriminant analyses of phenotypic data as a guide for parent selection in kiwifruit (Actinidia deliciosa) breeding

Evaluation of seedling populations assists in identifying those plants most desirable as parents for a breeding programme. A multivariate analysis of variance (Manova) showed that the seedling populations from 6 kiwifruit (Actinidia deliciosa) crosses differed in their overall merit. Discriminant functions were then used to identify the characters most powerful in distinguishing between populations. The populations were placed in order of their function scores which represent overall merit, and the superior populations were determined. The results showed that `Bruno' was a superior female parent for the production of seedlings with high discriminant function scores for floriferousness (male offspring)and productivity and high vitamin C content (female offspring). One male parent D-1-20 was consistently superior to D-1-6 the other male parent. This revised version was published online in July 2006 with corrections to the Cover Date.     

Genetic diversity of <Emphasis Type="Italic">Patellifolia patellaris</Emphasis> from the Iberian Peninsula,a crop wild relative of cultivated beets

The genetic diversity of Patellifolia patellaris has been investigated to generate information required for the organisation of a systematic genetic resources conservation action combining the best elements of the ex situ and in situ conservation concept. To this end, ten occurrences of the species were sampled on the Iberian Peninsula in Portugal and Spain. A total of 271 individuals was analysed using 24 microsatellite markers. The factorial analysis separated the material into two occurrences from Portugal, two occurrences sampled east of Gibraltar and six occurrences from the eastern Spanish coast. The pairwise genetic distance between occurrences and the complementary compositional differentiation among occurrences were calculated. The genetic distance values were used to construct an Unweighted Pair Group Method with Arithmetic Mean tree, which can be separated into four equidistant clusters. Two clusters are located in Portugal and two further clusters in Spain. The factorial analysis as well as the genetic distance and differentiation analysis indicate that the two occurrences from Portugal are clearly genetically different from the Spanish group of occurrences. Both occurrences have low population sizes and are therefore highly endangered. In situ conservation actions are urgently required for both occurrences. Further investigations are needed to organise better informed conservation actions for P. patellaris, namely to study genetic relationships between occurrences on the Spanish mainland and occurrences distributed on the Canary Islands, Madeira and Cape Verde Islands as well as in Morocco.     

Genetic variability of Striga asiatica (L.) Kuntz based on AFLP analysis and host-parasite interaction

AFLP analysis was used to estimate genetic variability within and among 14 populations of Striga asiaticaL. Kuntze collected from different locations within the Republic of Benin. The mean within-population genetic distances ranged from 0.028 to 0.038, while the mean among-population genetic distances ranged from 0.019to 0.088, with an assumed minimum genetic distance of0.01 in each case. Intra- and inter-population variation was reflected by a highly significant R² of 0.61for the regression of geographic distance versus genetic distance. Interactions of the different Strigapopulations with susceptible host genotypes, 8338-1 (Zea mays) and CK60B (Sorghum bicolor),indicated a high degree of host-specialization, with CK60B failing to support growth of the parasite from any of the populations. The various Striga populations also exhibited different degrees of virulence on susceptible host plants. Our results support the hypothesis that different populations of this parasite may well be considered and treated as ecotypes in plant breeding programs developing resistance to S. asiatica. This revised version was published online in August 2006 with corrections to the Cover Date.     

Evaluating genetic variation and relationships among two bromegrass species and their hybrid using RAPD and AFLP markers

The two most widely grown bromegrass species in North America are smooth bromegrass (Bromus inermis Leyss.), a hay type grass, and meadow bromegrass (Bromus riparius Rehmann), a pasture type grass. Recently a hybrid bromegrass population between the two species has been produced as a dual-purpose hay-pasture grass. Molecular markers have the potential to improve selection procedures to enhance bromegrass breeding. The objective of this study was to use RAPD and AFLP markers to determine genetic relationships and variations among bromegrass populations. Forty-three RAPD markers from 21 primers and 83 AFLP markers from seven primer combinations were used. Both marker types were able to group the individuals into their respective populations. The relationships among the individuals within each of the populations were not similar between the two marker types. Analysis of molecular variance (AMOVA) detected greater within-population variation than among-population variation for both marker types. The highest variation was observed in the hybrid population followed by meadow and then smooth bromegrass. The inter-population distance from both markers indicated that the highest genetic distance was between meadow and smooth bromegrass and lowest between smooth and hybrid bromegrass, which reflect the breeding history of the hybrid population. This study showed that both markers are capable of differentiating bromegrass genotypes into their respective populations, detecting genetic variation and relationships of the populations. Results of this study suggest that these two markers can be used in the future to enhance the current breeding practices in bromegrass, however, AFLP markers would be the marker of choice due to the high number of polymorphic markers. This revised version was published online in July 2006 with corrections to the Cover Date.     

Morphological variation in Astragalus hamosus L. and Coronilla scorpioides L. populations of Tunisia

Morphological variation between Astragalus hamosus and Coronillascorpioides populations was studied using local germoplasm collected in northern and central Tunisia. Twenty-one morphological traits were recorded and data were analysed using complementary statistical analysis. Considerable variation based on morphological and agronomical traits was found between populations and for both species. Pod and peduncle lengths as well as flowers number were the most discriminant variables between populations. Differences were also found for variables like seed number per pod, seed yield and dry matter production. Significant correlations were found between plant traits and the environmental parameters of the origin sites. This study revealed enough variation among populations to initiate a selection programme for pasture improvement in arid and semi-arid areas of Tunisia, where the two species are well adapted. This revised version was published online in July 2006 with corrections to the Cover Date.     

Variation and covariation in Vicia faba L. populations of Mediterranean origins

Summary Variation and covariation in 15 Vicia faba L. ssp. major populations from Italy. Greece and Egypt were analyzed. Ten quantitative morphobiological characters of agronomical and nutritional interest were observed. Differences among countries of origin were highly significant for all characters except for lowest podded node. Phenotypic distances among populations, both within and among origins, estimated on canonical variates evidenced that the Egyptian populations from a distinct group and that a relevant degree of similarity exists among the Greek and the Italian meterial. The covariation structure within populations, which was studied by means of factor analysis, was comparatively simpler in the Egyptian material than in that form the other two Mediterranean countries. The differences evidenced can be attributed to different gene associations between characters and suggest the possibility of obtaining, through recombination, genotypes agronomically more favorable.Research work supported by C.N.R., Italy, special grant. Sub-Project 1, Paper No. 299.     

海南两个自然保护区野生荔枝遗传多样性研究

采用常规聚丙烯酰胺凝胶电泳技术,对海南吊罗山和霸王岭两个国家级自然保护区野生荔枝等位酶遗传多样性进行了研究。13个酶系统、18个酶位点的检测结果表明,两个自然保护区野生荔枝具有较低的居群间遗传变异水平和较高的居群内遗传变异。吊罗山保护区野生荔枝的遗传多样性参数为:P=61.1%,A=2.05,He=0.27。霸王岭居群野生荔枝的遗传多样性参数为:P=61.1%,A=2.05,He=0.28。两个群体的遗传分化极小,GST=0.047.居群间遗传一致度较高I=0.977。共检测到7个稀有等位基因。两个自然保护区的野生荔枝均有各自特有的稀有等位基因,都应采取措施予以保护。     

The nabicol: A horticultural crop in northwestern Spain

Nabicol (B. napus L. var. pabularia) is a traditional crop in the Northwest of Iberian Peninsula (South of Galicia and North of Portugal) where it is grown during the winter season on small farms and gardens using traditional varieties. A collection of 36 populations of nabicol from Galicia (northwestern Spain) was evaluated during 2002 and 2003 in two locations and two growing seasons (spring/summer and autumn/spring) for 28 agronomic and morphological traits. The objectives of this study were to: (i) evaluate a collection of nabicol landraces from northwestern Spain, (ii) determine the suitability of this germplasm as a summer crop and (iii) study the genetic diversity among local populations. Significant differences were observed among populations for most traits. Genotype × environment interaction was significant for most of them. Spring/summer growing season could be recommended for growing nabicol but resistance to Lepidoptera pests attacking Brassica crops should be improved. Most populations had an agronomic value similar to the commercial variety. The most promising variety for horticultural use was MBG-BRS0063, which showed the highest yield. Morphological and agronomic data were subjected to cluster analysis and four groups were defined with a group clustering most populations. The low genetic diversity could be explained because populations were collected in close geographical areas and the selection made by farmers was always for a horticultural use. These results give information about the diversity and breeding value of the nabicol Spanish germplasm, which could be useful in breeding programs.     

Correlation of isoenzyme polymorphism and Bayoud-disease resistance in date palm cultivars and progeny

Summary Seedlings of date palm (Phoenix dactylifera L.), obtained from seven cultivars crossed with two males, were analyzed by polyacrylamide gel electrophoresis for esterase (EST), glutamate oxaloacetate transaminase (GOT), endopeptidase (ENP) and alcohol dehydrogenase (ADH) polymorphisms. Eleven, eight, five and two phenotypes were revealed for the enzymes tested, respectively. Seedlings of F₁ populations derived from Bayoud (Fusarium)-resistant and low fruit quality cultivars were characterized by a high electrophoretic polymorphism, when compared with progenies of Bayoud-susceptible and high fruit quality cultivars. In almost all cases, the most frequent electrophoretic phenotypes scored for each enzyme in different F₁ populations, were similar to those of the corresponding parent cultivars. Heterozygous phenotypes have been found for, at least, 3 loci; Got-2, Est-1 and Enp, indicating that such loci could be used to screen for hybrid seedlings. The expected Mendelian segregation of allozymes has been observed for these 3 loci, in many F₁ populations. It seems that progenies of Bayoud-resistant cultivars are characterized by a high level of electrophoretic polymorphism. The estimation of this index and the search for genetic linkage with segregating allozymes, may be biochemical criteria useful as an aid in distinguishing date palm seedling populations derived from Bayoud-resistant cultivars and suitable for breeding programs.     

耳石形态在斑尾复鰕虎鱼群体鉴别中的研究

为验证耳石形态指标在斑尾复鰕虎鱼群体鉴别中的有效性。本研究运用4种统计方法对黄、渤海斑尾腹鰕虎鱼4个群体矢耳石进行分析。结果显示：烟台群体与青岛群体、烟台群体与天津群体、丹东群体与天津群体的9个耳石形态指标均存在显著差异;平均判别正确率为86.5%;构建的3个主成分贡献率分别为50.927%、24.179%、12.321%,累积贡献率为87.427%;聚类分析显示青岛和天津群体聚为一组,烟台与丹东群体聚为一组。4种分析方法均显示不同群体间耳石的差异,证实了耳石形态指标可作为黄、渤海斑尾复鰕虎鱼群体判别的参考依据。研究结果可为渔业资源的研究提供基础资料。     

Genetic characterization of Malawian cowpea (Vigna unguiculata (L.) Walp) landraces: diversity and gene flow among accessions

Cowpea has been cultured from ancient timesin Africa and Asia and is now widespreadthroughout the tropics and subtropics.Cowpea accessions can be distinguishedphenotypically from one another by theirgrowth habit, time to maturity, yield, andseed size and colour. Data on geneticdiversity of farmer-developed accessionsare lacking. The main objective of thisstudy was to determine the pattern andextend of RAPD marker variation within andamong cowpea populations from differentagroecological zones and to determine thedegree of genetic relationships and geneflow among different landraces used bylocal farmers. Twenty of the 30 RAPDprimers tested allowed amplifications ofrandom polymorphic (RAPD) loci. Overall,80% of the scored loci were polymorphic. The degree of band sharing was used toevaluate genetic distance betweenaccessions and to construct a phylogenetictree. The genetic distance values amongaccessions varied between 0.09 to 0.57.Although some small clusters groupedaccessions of the same growth habits, ageneral lack of agreement betweenclustering and morphological features wasobserved. The analysis of molecularvariance revealed that within-region ortypes (among accessions) variationaccounted for 96% of the total molecularvariance. This high within-accessionvariability is being sustained by anuncontrolled gene flow among populations.     

Penetrance of creeping-rootedness in clonal progenies of lucerne and observations on underground morphology of plants differing for this character

Elaeis oleifera or ‘caiaué’, a close relative of oil palm (E. guineensis), has some agronomic traits of great interest for the oil palm genetic breeding such as slow growth, oil quality (mostly unsaturated) and disease resistance. An analysis of a Brazilian oil palm germplasm collection was carried out using RAPD (Random Amplified Polymorphic DNA) markers with the objective of understanding the genetic variation of ‘caiaué’ accessions collected in the Amazon Forest in the last two decades. A sample of 175 accessions obtained along the Amazon River Basin was analyzed and compared to 17 accessions of oil palm from Africa. Ninety-six RAPD markers were used in the analysis, of which fourteen were shown to be specific to oil palm, while twelve were specific to ‘caiaué’. Results showed that the Brazilian ‘caiaué’ accessions studied have moderate levels of genetic diversity as compared to oil palm accessions. The data allowed the establishment of similarity groups for ‘caiaué’ accessions, which is useful for selecting parental plants for population breeding. Cluster analysis showed that, in general, genetic similarities are not correlated to geographical distances, but consistent with geographical dispersal along the Amazon River network. AMOVA showed that most of the genetic variation is found within populations, as expected for anallogamous and long-lived perennial species. The study provides important information to define strategies for future collection expeditions, for germplasm conservation and for the use of E. oleifera in breeding programs. This revised version was published online in August 2006 with corrections to the Cover Date.     

Construction of Genetic Linkage Map of Bread Wheat （Triticum aestivum L.） Using an Intervarietal Cross and QTL Map for Spike Related Traits

Most often a genetic linkage map is prepared using populations obtained from two highly diverse genotypes. However, the markers from such a map may not be useful in a breeding program as these markers may not be polymorphic among the varieties used in breeding. For the past nine years, intraspecific maps have been gaining importance and such maps based on Swiss （PaiUard et al., 2003）, Japanese （Suenaga et al., 2005）, Australian （Chaimcrs et al., 2001） wheat varieties arc available. A map based on Indian wheat varieties however has not been reported. We constructed a genetic linkage map based on a cross between two Indian bread wheat （Triticum aestivum L.） varieties, Sonalika and Kalyansona. One hundred and fifty F2 individuals were analyzed for arbitrarilyprimed polymerase Chain reaction （AP-PCR）, random amplified polymorphic DNA （RAPD）, inter simple sequence repeats （ISSR）, Sequence Tagged Microsatelhte Sites （STMS）, Amplified Fragment Length Polymorphism （AFLP） markers, seed storage proteins and known genes. A linkage map was constructed consisting of 236 markers and spanning a distance of 3 639 cM with 1 211.2 cM for A genome, 1 669.2 cM for B genome, 192.4 cM for D genome and 566.2 cM for unassigned groups,