Showing papers by "Celeste C. Linde published in 2006"

Differential Selection on Rhynchosporium secalis During Parasitic and Saprophytic Phases in the Barley Scald Disease Cycle

Abstract: Competition among eight Rhynchosporium secalis isolates was assessed during parasitic and saprophytic phases of the disease cycle in field experiments conducted at two locations and over two growing seasons. The eight isolates were inoculated onto six barley populations exhibiting varying degrees of resistance. Microsatellite analysis of 2,866 isolates recovered from the field experiments showed significant, and sometimes opposite, changes in the frequencies of R. secalis genotypes during the growing season (parasitic phase) and between growing seasons (saprophytic phase). Isolates that showed the most complex virulence in greenhouse seedling assays had the lowest fitness in the field experiment. Significant differences in isolate fitness were found on different host populations and in different environments. Selection coefficients were large, indicating that evolution can occur rapidly in field populations. Although inoculated isolates had the lowest overall fitness on the moderately resistant landrace cv. Arabi Aswad, some isolates were more virulent and consistently increased in frequency on this landrace, suggesting a risk of directional selection and possible erosion of the resistance following its widespread deployment in monoculture. These results provide the first direct evidence that R. secalis pathogen genotypes differ in their saprophytic ability and parasitic fitness under field conditions.

Global Hierarchical Gene Diversity Analysis Suggests the Fertile Crescent Is Not the Center of Origin of the Barley Scald Pathogen Rhynchosporium secalis

Abstract: A total of 1,366 Rhynchosporium secalis isolates causing scald on barley, rye, and wild barley (Hordeum spontaneum) were assayed for restriction fragment length polymorphism loci, DNA fingerprints, and mating type, to characterize global genetic structure. The isolates originated from 31 field populations on five continents. Hierarchical analysis revealed that more than 70% of the total genetic variation within regions was distributed within a barley field. At the global level, only 58% of the total genetic variation was distributed within fields, while 11% was distributed among fields within regions, and 31% was distributed among regions. A significant correlation was found between genetic and geographic distance. These findings suggest that gene flow is common at the local level while it is low between regions on the same continent, and rare between continents. Analyses of multilocus associations, genotype diversity, and mating type frequencies indicate that sexual recombination is occurring in most of the populations. We found the highest allele richness in Scandinavia followed by Switzerland. This suggests that R. secalis may not have originated at the center of origin of barley, the Fertile Crescent, nor in a secondary center of diversity of barley, Ethiopia.

Population genetic structure of Plasmopara viticola after 125 years of colonization in European vineyards.

Abstract: SUMMARY To examine the within- and among-population genetic structure of Plamopara viticola oosporic populations in Europe, 8991 lesions from 32 vineyard plots were collected and analysed. Four multi-allelic microsatellite markers were used to genotype the pathogen. All populations had high levels of gene and genotypic diversity. Most populations were in Hardy–Weinberg equilibrium and thus randomly mating. Among P. viticola populations, significant low to moderate genetic differentiation was observed, even between geographically close populations. This genetic differentiation was also evident in the neighbour-joining phylogenetic genetic distance tree, showing clear substructure and distinguishing mainly five clusters based on geographical origin. Significant isolation by distance was found in central European P. viticola populations, suggesting a step-wise migration model. No significant isolation by distance was found within Greek populations, most probably owing to natural geographical barriers such as the sea and mountains, as well as the frequent population bottlenecks occurring in these populations, preventing natural migration among populations. The high variability of P. viticola provides explanation for its successful infestation of the heterogeneous European vineyards in the last 125 years after its introduction.

Genetic Structure of Mycosphaerella graminicola Populations from Iran, Argentina and Australia

Abstract: Restriction fragment length polymorphism (RFLP) markers were used to assess the genetic structure of populations of Mycosphaerella graminicola collected from wheat fields. A total of 585 isolates representing 10 field populations were sampled from Iran, Argentina and Australia. The genetic structure of M. graminicola populations from Iran and Argentina is described for the first time. Results were compared to previously investigated populations from Israel, Uruguay and Australia. Populations from Iran exhibited high clonality and low gene diversity, suggesting an inoculation event. Populations from uninoculated fields in Argentina had gene and genotype diversities similar to previously described European and North American populations. Genotype diversity was high for populations from Australia and tests for multilocus associations were consistent with sexual recombination in these populations. Gene diversity was low and fixed alleles were found for several loci. These findings are consistent with a relatively small founding population for Australia. These 10 new populations were integrated into a genetic distance comparison with 13 global populations that were characterized earlier.

Genotypic Diversity of the Wheat leaf blotch pathogen ( Septoria tritici ) in Buenos Aires Province

Abstract: The population structure and genotypic diversity of Septoria tritici Rob ex Desm. from two crop field populations in Buenos Aires Province were studied with DNA restriction fragment length polymorphism. Hierarchical samples were taken from different cultivars. A total of 120 single spore isolates was subject to molecular analysis to compare the genetic structure of Los Hornos and Balcarce populations separated by 500km. Eight RFLP loci and 1DNA fingerprinting were used. Among the total of the isolates, 24 RFLP alleles were detected from the first population and 22 from the second. Gene and genotype diversity were high in both populations. Different haplotypes for each region were identified. Identical haplotypes were clustered in the same location in the field. The mean gene diversity and the mean genotypic diversity were high for the 8 loci of RFLP. This means that Balcarce and Los Hornos were significantly different. The X2 was significant (P>0.005) only for two RFLP locus, then both populations were not independant. They had to be compared with an European and an American populations. Balcarce population was very different to all of them. Both Argentine populations were very similar at regional level but strongly different at a continental level. The greatest genetic variability between continents belonged to Argentine population. The amount of gene flow was high when all the populations were compared.