Evolution and differentiation of plant pathogenic fungi mediated by pathogenicity chromosomes

互隔交链孢霉(A.ternata f.sp.)番茄)产生寄主特有的AAL毒素，并引起链格孢菌茎溃烂。测定了番茄致病型AAL毒素生物合成(ALT)基因簇的120kb基因组序列，并与串珠赤霉菌的伏马菌素生物合成(FUM)基因簇的相应序列进行了比较。基因组区域包括19个可能的ORF，其中12个与FUM基因簇中的基因具有相似性。ALT基因簇位于一条1.0Mb的条件可支配染色体(CDC)上，仅在致病菌株和产AAL毒素菌株中发现，而在非致病菌株中未检测到同源基因。将ALT1和位于ALT基因簇外的另一个PKS基因的基因组序列与世界各地收集的番茄致病菌株进行了比较。这表明，在不同地理来源的菌株中，CDC上的基因序列是相同的。另一方面，CDC以外的其他基因在每个品系中的序列并不完全相同，这表明CDC的来源可能与番茄致病类型中的其他染色体不同。我们提出了一个假说，即通过CDC的水平转移，产生决定病原菌特定致病性的AAL毒素的能力可能分布在互隔交链孢霉菌株之间，从而提供了一种可能的机制，从而在自然界中可能出现新的病原体。

The tomato pathotype of A. alternata (A. alternata f. sp. lycopersici) produces host-specific AAL-toxins and causes Alternaria stem canker. A 120 kb genomic region that contains the AAL-toxin biosynthetic (ALT) gene cluster in the tomato pathotype was sequenced and compared with corresponding sequences of the fumonisin biosynthetic (FUM) gene cluster in Gibberella moniliformis. The genomic region includes 19 putative ORFs and 12 of those showed similarity to the genes in the FUM gene cluster. The ALT gene cluster resides on a 1.0 Mb conditionally dispensable chromosome (CDC) found only in the pathogenic and AAL-toxin-producing strains of A. alternata, and homologues of the genes were not detected in non-pathogenic strains of A. alternata. Genomic sequences of ALT1 and another PKS gene located outside of the ALT gene cluster, both of which reside on CDCs in the tomato pathotype strains were compared to tomato pathotype strains collected worldwide. This revealed that the sequences of the genes located on the CDCs, in the strains with different geographical origins, are identical. On the other hand, sequences of other genes located on the chromosomes other than CDCs are not identical in each strain, indicating that the origin of the CDCs might be different from other chromosomes in the tomato pathotype. We propose a hypothesis whereby the ability to produce AAL-toxins determining specific pathogenicity of the pathogen could be potentially distributed among A. alternata strains by horizontal transfer of the CDCs, and thus provide a possible mechanism whereby new pathogens can arise in nature.