Comparative analysis of "chromosome for pathogenicity" in phytopathogenic fungi

我们已经发现，在链格孢中，寄主特异性毒素（HST）的产生控制着植物的致病性，迄今为止所列出的基因都是特异性参与每一种或一组HST合成代谢途径的基因，而参与HST产生的基因只位于一条条件性染色体上。本研究旨在比较分析互隔孢菌不同致病型间的条件染色体（CD染色体）结构和功能，以了解植物病原菌进化和分化的分子基础。对感兴趣的克隆进行了DNA测序，并鉴定了HST生物合成相关基因。2.在300-kb区域内鉴定了16个与草莓致病型HST生物合成相关的基因。在苹果和草莓的CD染色体上分别检测到一个醛酮还原酶基因，苹果致病型产生一个含环肽的HST，草莓致病型产生一个含环氧癸三烯酸（EDT）的HST。导入的草莓致病型同源基因可以在功能上补充苹果致病型同源基因的功能。4.苹果和/或其它苹果的CD染色体上没有HST合成基因的保守性。这些结果表明苹果和草莓的CD染色体可能来源于一个古老的染色体，而番茄的CD染色体可能不是，三种致病型在互隔链格孢菌中均为同一种，为植物病原真菌的分化提供了新的例证。

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.