Clonal population structure of the chestnut blight fungus in expanding ranges in southeastern Europe

Abstract

Expanding populations are often less genetically diverse at their margins than at the centre of a species' range. Established, older populations of the chestnut blight fungus, Cryphonectria parasitica, are more variable for vegetative compatibility (vc) types than in expanding populations in southeastern Europe where C. parasitica has colonized relatively recently. To test whether vc types represent clones, we genotyped 373 isolates of C. parasitica from southern Italy, Romania, Bulgaria, Macedonia, Greece and Turkey using 11 sequence-characterized amplified region (SCAR) markers. Ten SCAR loci and six vegetative incompatibility (vic) loci were polymorphic in these samples. These populations are clonal by all criteria tested: (i) among 373 isolates, we found only eight multilocus haplotypes, and the same haplotypes were found in multiple countries, sometimes separated in time by as much as 12 years; (ii) the number of haplotypes observed was significantly less than expected under random mating; (iii) populations are in linkage disequilibrium; (iv) the two sets of independent markers, SCARs and vc types, are highly correlated; and (v) sexual structures of C. parasitica were found only in Bulgaria and Romania. One mating type (MAT-1) was found in 98% of the isolates sampled. In contrast, a population in northern Italy, in the central part of the range in Europe, had 12 multilocus haplotypes among 19 isolates. The spread of a few clones could be the result either of founder effect and restricted migration, or these clones have greater fitness than others and spread because they are better adapted to conditions in southeastern Europe.

Multiple introductions of divergent genetic lineages in an invasive fungal pathogen, Cryphonectria parasitica, in France

“…In both continents, the genetic diversity of C. parasitica populations seemed lower than in Japan and China, the native area of C. parasitica (for example Liu and Milgroom, 2007). Although the recent analysis of South-Eastern European populations showed a clonal expansion of C. parasitica likely from Italy (Milgroom et al, 2008), genetic analyses of populations from the Atlantic part of Europe suggested that other populations genetically divergent from Italian populations could have been introduced and would spread clonally (Breuillin et al, 2006;Braganca et al, 2007;Montenegro et al, 2008;Robin et al, 2009). However, in contrast to Eastern European populations in which only one mating was generally detected (Milgroom et al, 2008), in these Western populations both mating types were often found allowing sexual reproduction among isolates in this heterothallic haploid species (Braganca et al, 2007;Dutech et al, 2008;Robin et al, 2009).…”

“…Although the recent analysis of South-Eastern European populations showed a clonal expansion of C. parasitica likely from Italy (Milgroom et al, 2008), genetic analyses of populations from the Atlantic part of Europe suggested that other populations genetically divergent from Italian populations could have been introduced and would spread clonally (Breuillin et al, 2006;Braganca et al, 2007;Montenegro et al, 2008;Robin et al, 2009). However, in contrast to Eastern European populations in which only one mating was generally detected (Milgroom et al, 2008), in these Western populations both mating types were often found allowing sexual reproduction among isolates in this heterothallic haploid species (Braganca et al, 2007;Dutech et al, 2008;Robin et al, 2009). The studies in the Atlantic part of Europe were performed with phenotypic markers for which the genetic determinants have not been completely elucidated (that is vegetative compatible (vc) types; Robin et al, 2000;Braganca et al, 2007;Montenegro et al, 2008;Robin et al, 2009) or with molecular markers, but for a low number of sampled populations (Breuillin et al, 2006).…”

“…As fungal pathogens can generally reproduce asexually, clonal expansion can be the main mode of reproduction during colonization of the areas of introduction limiting recombination among genotypes (see for example Milgroom et al, 2008). On one hand, asexual reproduction provides the possibility of reproducing with a single founding genotype, in the absence of a compatible mating type, or climatic conditions conducive for sexual reproduction (that is sexual failure; Silvertown, 2008).…”

Multigene phylogenetic and population differentiation data confirm the existence of a cryptic species within Chrysoporthe cubensis

“…This is because continuous admixture between disparately distributed populations can be detected from discordant genealogies for multiple genetic loci and/or low levels of population differentiation and high numbers of migrants (Geiser et al 1998;Fisher et al 2002;Zhou et al 2007;Milgroom et al 2008). Conversely, concordance among genealogies for multiple loci and diminished gene flow due to ecological, geographical or historical processes are generally regarded as useful indicators of species divergence (Avise and Wollenberg 1997;Barraclough and Nee 2001).…”

Population structure of Cylindrocladium parasiticum infecting peanuts (Arachis hypogaea) in Georgia, USA

“…d Nei's genetic diversity (Nei 1973) Table 1 Microsatellite analysis to determine the genotypic and genetic diversities, and the occurrence of multilocus haplotypes (MLH) among three sub-populations, the GAMULT group of isolates and the entire sampled population of Cylindrocladium parasiticum isolated from peanuts in Georgia other haplotypes that were also collected, or by simple recombinations with similar haplotypes. The simple loops likewise suggest convergent mutations or recombinations (Milgroom et al 2008;Posada and Crandall 2001). The only exception to this pattern is MLH no.…”