Wang Mingshuang, Fu Huilan, Shen Xing-Xing, Ruan Ruoxin, Rokas Antonis, Li Hongye
Key Lab of Molecular Biology of Crop Pathogens and Insects, Institute of Biotechnology, Zhejiang University, Hangzhou, 310058, China.
Department of Biological Sciences, Vanderbilt University, Nashville, TN, 37235, USA.
Mol Plant Pathol. 2019 Oct;20(10):1425-1438. doi: 10.1111/mpp.12848. Epub 2019 Jul 11.
The tangerine pathotype of the ascomycete fungus Alternaria alternata is the causal agent of citrus brown spot, which can result in significant losses of both yield and marketability for tangerines worldwide. A conditionally dispensable chromosome (CDC), which harbours the host-selective ACT toxin gene cluster, is required for tangerine pathogenicity of A. alternata. To understand the genetic makeup and evolution of the tangerine pathotype CDC, we isolated and sequenced the CDCs of the A. alternata Z7 strain and analysed the function and evolution of their genes. The A. alternata Z7 strain has two CDCs (~1.1 and ~0.8 Mb, respectively), and the longer Z7 CDC contains all but one contig of the shorter one. Z7 CDCs contain 254 predicted protein-coding genes, which are enriched in functional categories associated with 'metabolic process' (55 genes, P = 0.037). Relatively few of the CDC genes can be classified as carbohydrate-active enzymes (CAZymes) (4) and transporters (19) and none as kinases. Evolutionary analysis of the 254 CDC proteins showed that their evolutionary conservation tends to be restricted within the genus Alternaria and that the CDC genes evolve faster than genes in the essential chromosomes, likely due to fewer selective constraints. Interestingly, phylogenetic analysis suggested that four of the 25 genes responsible for the ACT toxin production were likely transferred from Colletotrichum (Sordariomycetes). Functional experiments showed that two of them are essential for the virulence of the tangerine pathotype of A. alternata. These results provide new insights into the function and evolution of CDC genes in Alternaria.
子囊菌交链孢霉的橘生致病型是柑橘褐斑病的病原体，可导致全球范围内橘子的产量和市场价值大幅损失。交链孢霉的橘生致病性需要一条含有宿主选择性ACT毒素基因簇的条件性可缺失染色体（CDC）。为了解橘生致病型CDC的基因组成和进化，我们分离并测序了交链孢霉Z7菌株的CDC，并分析了其基因的功能和进化。交链孢霉Z7菌株有两条CDC（分别约为1.1和0.8 Mb），较长的Z7 CDC包含了较短CDC除一个重叠群外的所有重叠群。Z7 CDC包含254个预测的蛋白质编码基因，这些基因在与“代谢过程”相关的功能类别中富集（55个基因，P = 0.037）。CDC基因中相对较少的基因可归类为碳水化合物活性酶（CAZymes）（4个）和转运蛋白（19个），没有一个归类为激酶。对254个CDC蛋白的进化分析表明，它们的进化保守性往往局限于交链孢霉属内，并且CDC基因的进化速度比必需染色体上的基因快，这可能是由于选择性限制较少。有趣的是，系统发育分析表明，负责ACT毒素产生的25个基因中有4个可能是从炭疽菌属（粪壳菌纲）转移而来的。功能实验表明，其中两个基因对于交链孢霉橘生致病型的毒力至关重要。这些结果为交链孢霉中CDC基因的功能和进化提供了新的见解。
