Abstract
Invasive microbial species constitute a major threat to biodiversity, agricultural production and human health. Invasions are often dominated by one or a small number of genotypes, yet the underlying factors driving invasions are poorly understood. The chestnut blight fungus Cryphonectria parasitica first decimated the North American chestnut, and a more recent outbreak threatens European chestnut stands. To unravel the chestnut blight invasion of southeastern Europe, we sequenced 230 genomes of predominantly European strains. Genotypes outside of the invasion zone showed high levels of diversity with evidence for frequent and ongoing recombination. The invasive lineage emerged from the highly diverse European genotype pool rather than a secondary introduction from Asia or North America. The expansion across southeastern Europe was mostly clonal and is dominated by a single mating type, suggesting a fitness advantage of asexual reproduction. Our findings show how an intermediary, highly diverse bridgehead population gave rise to an invasive, largely clonally expanding pathogen.
Highlights
Over the past century, a multitude of invasive species have emerged as threats to forest and agricultural ecosystems worldwide (Santini et al, 2013; Wingfield et al, 2010)
We analyzed the invasion of the chestnut blight pathogen C. parasitica across southeastern Europe after a first establishment on the continent
Deep sampling of recent invasive genotypes showed that the secondary invasion in southeastern Europe was caused by a single, highly homogeneous lineage consisting nearly exclusively of a single mating type
Summary
A multitude of invasive species have emerged as threats to forest and agricultural ecosystems worldwide (Santini et al, 2013; Wingfield et al, 2010). To successfully colonize a new environment, fungal pathogens have to overcome several invasion barriers including effective dispersal abilities, changes in available hosts, competition with other fungi, and niche availability (Gladieux et al, 2015; Hayes and Barry, 2008). This may be achieved with plastic phenotypic changes followed by rapid genetic adaptation (Garbelotto et al, 2015). Many fungal plant pathogen invasions were successful despite low genetic diversity within founding populations (Fontaine et al, 2013; Raboin et al, 2007; Wuest et al, 2017)
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