Successful asexual lineages of the Irish potato Famine pathogen are triploid

The oomycete Phytophthora infestans was the causal agent of the Irish Great Famine and is a recurring threat to global food security1. The pathogen can reproduce both sexually and asexually and has a potential to adapt both abiotic and biotic environment2. Although in many regions the A1 and A2 mating types coexist, the far majority of isolates belong to few clonal, asexual lineages3. As other oomycetes, P. infestans is thought to be diploid during the vegetative phase of its life cycle3, but it was observed that trisomy correlated with virulence and mating type locus4 and that polyploidy can occur in some isolates5,6. It remains unknown about the frequency of polyploidy occurrence in nature and the relationship between ploidy level and sexuality. Here we discovered that the sexuality of P. infestans isolates correlates with ploidy by comparison of microsatellite fingerprinting, genome-wide polymorphism, DNA quantity, and chromosome numbers. The sexual progeny of P. infestans in nature are diploid, whereas the asexual lineages are mostly triploids, including successful clonal lineages US-1 and 13_A2. This study reveals polyploidization as an extra evolutionary risk to this notorious plant destroyer.