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Abstract:
Crop disease pandemics are often driven by clonal lineages of plant pathogens that reproduce asexually. How these clonal pathogens adapt to their hosts despite harboring limited genetic variation is poorly understood. Here, we show multiple instances of horizontal chromosome transfer involving clonal lineages of the rice blast fungus Magnaporthe (Syn. Pyricularia) oryzae. We identified the horizontally transferred chromosome as a 1.2Mb supernumerary mini-chromosome, mChrA, which is remarkably conserved across blast fungus isolates from lineages infecting the wild grass Eleusine indica (Indian goosegrass) and rice. Further analyses revealed mChrA was acquired by clonal rice blast fungi through parasexual-mediated horizontal transfer, with evidence of at least eight distinct transfer events over the past four centuries. These findings establish horizontal mini-chromosome transfer as a mechanism facilitating genetic exchange among blast fungi infecting different hosts. We propose that blast fungus populations infecting wild grasses act as genetic reservoirs that contribute to the evolvability of pandemic clonal lineages that afflict crops.
