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Abstract

The blast fungus Magnaporthe (Syn. Pyricularia) oryzae is a destructive pathogen that targets cereal crops and wild grasses. This fungus consists of distinct lineages specialized for specific hosts, with limited genetic exchange between them. Most of the blast fungus strains responsible for widespread crop pandemics are clones that reproduce asexually. Despite their limited genetic diversity due to the absence of sexual recombination, clonal M. oryzae strains can rapidly adapt to new host plants and overcome disease resistance, posing a significant challenge for developing resistant crops. This leads to the question of how clonal blast fungus populations can continually adapt to different host plants despite having minimal genetic variation. One potential source of genetic variation is the presence of supernumerary mini- chromosomes (mChrs), additional chromosomes found in many plant pathogenic fungi alongside the essential core chromosomes. We propose that the exchange of mChrs between different blast fungus lineages contributes to their adaptive evolution. To investigate this hypothesis, we traced the origin of a specific mChr shared between blast fungus lineages infecting rice and goosegrass. Our findings suggest that clonal rice blast fungus lineages acquired this mChr through horizontal transfer from the lineage that infects goosegrass. Furthermore, this mChr carries multiple virulence genes, implying its potential role in influencing the pathogen's disease-causing ability. In conclusion, the horizontal acquisition of mChrs may play a significant role in the adaptation of the blast fungus, particularly for clonal lineages. This mechanism provides a means for introducing genetic variation and potentially impacting the pathogen's virulence. Understanding the importance of horizontal mChr transfer can shed light on the adaptive processes of the blast fungus and has implications for managing and developing resistant crop varieties.