Fusion of two divergent fungal individuals led to the recent emergence of a unique widespread pathogen species

Abstract

In a genome alignment of five individuals of the ascomycete fungus Zymoseptoria pseudotritici, a close relative of the wheat pathogen Z. tritici (synonym Mycosphaerella graminicola), we observed peculiar diversity patterns. Long regions up to 100 kb without variation alternate with similarly long regions of high variability. The variable segments in the genome alignment are organized into two main haplotype groups that have diverged ∼3% from each other. The genome patterns in Z. pseudotritici are consistent with a hybrid speciation event resulting from a cross between two divergent haploid individuals. The resulting hybrids formed the new species without backcrossing to the parents. We observe no variation in 54% of the genome in the five individuals and estimate a complete loss of variation for at least 30% of the genome in the entire species. A strong population bottleneck following the hybridization event caused this loss of variation. Variable segments in the Z. pseudotritici genome exhibit the two haplotypes contributed by the parental individuals. From our previously estimated recombination map of Z. tritici and the size distribution of variable chromosome blocks untouched by recombination we estimate that the hybridization occurred ∼380 sexual generations ago. We show that the amount of lost variation is explained by genetic drift during the bottleneck and by natural selection, as evidenced by the correlation of presence/ absence of variation with gene density and recombination rate. The successful spread of this unique reproductively isolated pathogen highlights the strong potential of hybridization in the emergence of pathogen species with sexual reproduction. species dynamics | incipient species | population genomics | genome mosaic | genome scans

Hybridization of powdery mildew strains gives rise to pathogens on novel agricultural crop species

“…39 and 40, and Supplementary Note). These findings are in contrast to those on the plant pathogen Zymoseptoria pseudotritici, whose origin was traced back to a single hybridization event 8 . Unlike Z. pseudotritici, the new forma specialis B.g.…”

“…We estimated the number of sexual generations after hybridization for each B.g. triticale isolate using the following formula modified from Stukenbrock et al 8 sexual generations (SG) ((NR BGSS)/(2 BGSG)) 0.22…”

Mosaic genome structure of the barley powdery mildew pathogen and conservation of transcriptional programs in divergent hosts

“…Even though divergence time estimates are error-prone due to indirect estimates of the mutation rate in the powdery mildews (Materials and Methods), at least the regions with high SNP density very likely existed before barley domestication ∼10,000 y ago. Similar peculiar genome diversity patterns alternating between fully conserved and variable regions were observed in a genome alignment of five individuals of Zymoseptoria pseudotritici, a close relative of the wheat pathogen Z. tritici (40). However, in contrast to our findings, the blocks are 10-fold smaller, 54% of the Z. tritici genome is monomorphic between all isolates, and the variable regions showed only two haplotypes.…”

Modelling the Effects of Mass Drug Administration on the Molecular Epidemiology of Schistosomes

“…An exemplary study on the plant pathogenic fungus Zymoseptoria pseudotritici (Stukenbrock et al, 2012) used the nuclear genomes of five individuals to investigate a recent hybridization event.…”