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The population structure of Armillaria gallica, an important pathogen of Quercus spp., was investigated from mixed oak forests in central Massachusetts, encompassing a sampling area over 500 km². From 16 plots at four sites a total of 153 isolates (34-40 isolates per site) was analyzed with amplified fragment length polymorphisms (AFLPs). Analyses of 204 polymorphic loci detected 38 AFLP genotypes from a sample area of 4.51 hectares (ha). Genets ranged in distribution from five to 33 genets per hectare (GPH), with a mean of eight GPH and the average A. gallica genet occupying 0.13 ha. Allele frequencies produced an unbiased expected heterozygosity (HE) value of 0.112 (SE = 0.006) and a Nei's expected heterozygosity (Hj) value of 0.190 (SE = 0.009), indicating low genetic diversity within the population. Analysis of molecular variation (ΦPT = 0.301; P < 0.001) indicates high genetic differentiation, with 70% of the molecular variation explained at the site-level within A. gallica subpopulations. However, results of the Mantel test, used to assess the isolation-by-distance hypothesis, were inconclusive in determining whether the subpopulations were truly isolated by distance. A neighbor-joining tree constructed from a genetic distance matrix grouped genotypes from the same site (subpopulation) together, but from three of four sites genotypes were randomly clustered at the plot level. The results suggest that basidiospore dispersal is an important means of new genet formation at linear distances up to 2000 m. [ABSTRACT FROM AUTHOR]

Neonectria ditissima is a fungal pathogen native to eastern North America that causes disfiguring cankers on numerous tree species, particularly birches ( Betula spp.). In order to develop control strategies, fundamental knowledge of the pathogen's reproductive and dispersal dynamics is necessary. To undertake these studies, we developed genomic libraries enriched for clones containing microsatellites, and then designed primers flanking each locus. Of 34 loci that were screened, 11 were polymorphic among 38 isolates obtained from a single population. Gene diversities ranged from 0.05 to 0.862 with a mean of 0.409. These markers will be invaluable in population studies of this fungus. [ABSTRACT FROM AUTHOR]

The study of quantitative traits provides a window on the interactions between multiple unlinked genetic loci. The interaction between hosts and pathogenic microbes, such as fungi, involves aspects of quantitative genetics for both partners in this dynamic equilibrium. One important pathogenic fungus is Cryptococcus neoformans, a basidiomycete yeast that can infect the human brain and whose mating system has two mating type alleles, a and α. The α mating-type allele has previously been linked to increased virulence potential. Here congenic C. neoformans strains were generated in the two well-characterized genetic backgrounds B3501α and NIH433a to examine the potential influence of genes outside of the mating-type locus on the virulence potential of mating type. The congenic nature of these new strain pairs was established by karyotyping, amplified fragment length polymorphism genotyping, and whole-genome molecular allele mapping (congenicity mapping). Virulence studies revealed that virulence was equivalent between the B3501 a and α congenic strains but the a strain was more virulent than its a counterpart in the NIH433 genetic background. These results demonstrate that genomic regions outside the mating type locus contribute to differences in virulence between a and α cells. The congenic strains described here provide a foundation upon which to elucidate at genetic and molecular levels how mating-type and other unlinked loci interact to enable microbial pathogenesis. [ABSTRACT FROM AUTHOR]

To construct a genetic linkage map of the heterothallic yeast, Cryptococcus neoformans (Filobasidiella neoformans), we crossed two mating-compatible strains and analyzed 94 progeny for the segregation of 301 polymorphic markers, consisting of 228 restriction site polymorphisms, 63 microsatellites, two indels, and eight mating-type (MAT)-associated markers. All but six markers showed no significant (P< 0.05) segregation distortion. At a minimum LOD score of 6.0 and a maximum recombination frequency of 0.30, 20 linkage groups were resolved, resulting in a map length of ∼1500 cM. Average marker density is 5.4 cM (range 1-28.7 cM). Hybridization of selected markers to blots of electrophoretic karyotypes unambiguously assigned all linkage groups to chromosomes and led us to conclude that the C. neoformans genome is ∼20.2 Mb, comprising 14 chromosomes ranging in size from 0.8 to 2.3 Mb, with a ratio of ∼13.2 kb/cM averaged across the genome. However, only 2 of 12 ungrouped markers hybridized to chromosome 10. The hybridizations revealed at least one possible reciprocal translocation involving chromosomes 8, 9, and 12. This map has been critical to genome sequence assembly and will be essential for future studies of quantitative trait inheritance. [ABSTRACT FROM AUTHOR]