Comparative analysis of "chromosome for pathogenicity" in phytopathogenic fungi

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We have found, in Alternaria alternata, productivity of host-specific toxin(HST) controls pathogenicity on plants, genes listed so far were specifically involved in metabolic pathway of synthesis for each of or a group of HST, and genes involved in HST production located only on a small chromosome that was conditionally dispensable. In this research, we focused on comparative analysis of structure and function of conditionally dispensable chromosome(CD chromosome) among pathotypes of A.alternata to understand molecular basis for evolution and differentiation of plant pathogen.1.We constructed three sets of CD chromosome-specific BAC library and cDNA library from three pathotypes of A.alternata (apple, strawberry and tomato pathotype). Clones of interests were subjected to DNA sequencing and genes involved in HST biosynthesis were identified.2.Sixteen genes located around 300-kb region were identified for HST biosynthesis of strawberry pathotype. Two genes were identified in apple pathotype and thirteen genes located around 150-kb region were also identified in tomato pathotype, respectively for each HST.3.One gene for aldo-keto reductase was conserved between CD chromosomes of apple and strawberry pathotype while apple pathotype produces a HST containing cyclic peptide and strawberry pathotype produces HSTs containing epoxy-decatrienoic acid (EDT). The introduced homolog of strawberry pathotype could functionally complement knocked out function of the other homolog in apple pathotype.4.Any genes for HST biosynthesis were not conserved between CD chromosomes of apple and/or strawberry pathotype and tomato pathotype that produces a HST containing polyketide moiety.These results revealed CD chromosomes of apple and strawberry may be derived from an ancient chromosome but that of tomato may be not although three pathotypes are in the same species A.alternata and suggest a novel example of differentiation of phytopathogenic fungi.