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Successful asexual lineages of the Irish potato Famine pathogen are triploid

By Ying Li, Qian Zhou, Kun Qian, Theo van der Lee, Sanwen Huang

Posted 13 Aug 2015
bioRxiv DOI: 10.1101/024596

The oomycete Phytophthora infestans was the causal agent of the Irish Great Famine and is a recurring threat to global food security. The pathogen can reproduce both sexually and asexually and has a potential to adapt both abiotic and biotic environment. Although in many regions the A1 and A2 mating types coexist, the far majority of isolates belong to few clonal, asexual lineages. As other oomycetes, P. infestans is thought to be diploid during the vegetative phase of its life cycle, but it was observed that trisomy correlated with virulence and mating type locus and that polyploidy can occur in some isolates. 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.

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