Lifestyle, mating type and mitochondrial genome features of the plant pathogen <i>Calonectria hawksworthii</i> (Hypocreales, Nectriaceae) as revealed by genome analyses

In 2019, fungal necrotic spots were observed on the cotyledons of grafted avocado (<i>Persea americana</i>) seedlings from a nursery in Ventura County, California. Morphology and comparison of DNA sequences from eight loci identified the isolate as <i>Calonectria hawksworthii</i>, a species in the Nectriaceae not yet recorded in the United States. <i>Calonectria hawksworthii</i> is a necrotrophic fungal pathogen in the <i>C. cylindrospora</i> species complex. Most species in this group are associated with <i>Eucalyptus</i>, but the type host for <i>C. hawksworthii</i> is waterlilies (<i>Nelumbo nucifera</i>). In this study, the <i>C. hawksworthii</i> genome was sequenced using Illumina technologies. The draft assembly of 64.8 Mb contained 18 703 predicted gene models, of which 70% could be assigned to a GO functional category. The mating type loci indicated this species is heterothallic. Approximately, 3.79% of the draft genome consists of transposable elements (TE), and close to 36% of the predicted proteins were homologous to those known to be involved in pathogenicity in other fungal species. The <i>C. hawksworthii</i> genome displays elements typical of a necrotrophic lifestyle based on the composition of predicted genes encoding for carbohydrate-active enzymes, number of predicted secretory proteins including effectors, secondary metabolite biosynthesis clusters and cytochrome oxidase P450 (CYP) genes. The annotated mitochondrial genome was found to be 35.7 kb with very few intra or intergenic introns. This report constitutes the first draft genome of a <i>Calonectria</i> species belonging to the <i>C. cylindrospora</i> species complex, as well as the first report of <i>C. hawksworthii</i> species occurring in the United States on avocado.

2019年，美国加利福尼亚州文图拉县某苗圃的嫁接鳄梨（*Persea americana*）幼苗子叶上观测到真菌性坏死斑。通过形态学分析及8个位点的DNA序列比对，该分离菌株被鉴定为霍克斯沃斯镰座壳孢菌（*Calonectria hawksworthii*）——一种隶属于赤壳科（Nectriaceae）且此前尚未在美国有记录的物种。霍克斯沃斯镰座壳孢菌属于*C. cylindrospora*物种复合群中的死体营养型真菌病原物。该类群的多数物种与桉树（*Eucalyptus*）相关，但该种的模式寄主为莲（*Nelumbo nucifera*）。本研究采用Illumina测序技术对该菌株的基因组进行了测序，其草图组装基因组大小为64.8 Mb，包含18703个预测基因模型，其中70%可被注释至基因本体（GO）功能类别。交配型位点分析显示该物种为异宗配合型。草图基因组中约3.79%的序列为转座因子（TE），近36%的预测蛋白与其他真菌物种中已知的致病性相关蛋白同源。根据预测的碳水化合物活性酶编码基因组成、包含效应因子在内的预测分泌蛋白数量、次级代谢产物生物合成基因簇以及细胞色素氧化酶P450（CYP）基因的特征，霍克斯沃斯镰座壳孢菌的基因组展现出死体营养型生活方式的典型元素。注释后的线粒体基因组大小为35.7 kb，基因内及基因间区的内含子极少。本报告是首个隶属于*C. cylindrospora*物种复合群的镰座壳孢菌（*Calonectria*）物种的草图基因组，同时也是霍克斯沃斯镰座壳孢菌在美国鳄梨上发生的首次报道。