Exserohilum turcicum and Exserohilum rostratum Genome sequencing and assembly

Exserohilum turcicum and E. rostratum, two closely related fungal species, are both economically important pathogens but have quite different target hosts (specific to plants and cross-kingdom infection, respectively). In the present study, complete mitochondrial genomes of the two Exserohilum species were sequenced, assembled, annotated and compared. Both the mitogenomes consist of circular DNA molecules, comprising a set of core protein-coding genes (PCGs), rRNAs, tRNAs and unidentified ORFs, but with vastly different sizes, 264,948 bp and 64,620 bp, respectively. Comparative mitogenomic analysis showed that these two fungi had rather consistent mitochondrial gene arrangement, though three tRNAs (trnN-3, trnC-2 and trnV-2) were absent in E. rostratum. However, by contrast with the 17 introns containing 17 intronic ORFs in E. rostratum mitogenome, E. turcicum involves far more introns (70) and intronic ORFs (126), which was considered as the main contributing factor of their mitogenome expansion/contraction. Within the generally intron-rich gene cox1, a total of 18 and 10 intron position classes (Pcls) were identified separately in the two mitogenomes. Moreover, 16.16% and 10.85% ratios of intra-genomic repetitive regions were detected in E. turcicum and E. rostratum, respectively. Based on the combined mitochondrial gene dataset, we obtained a well-supported topology of phylogenetic tree for 98 ascomycetes, implying that mitogenome is possibly an effective molecular marker for phylogeny reconstruction. This study served as the first report on mitogenomes in the genus Exserohilum, which could have implications in understanding the genetics and evolution of Exserohilum species.

突脐蠕孢属（Exserohilum）的两个近缘真菌物种——大斑突脐蠕孢菌（Exserohilum turcicum）与喙突脐蠕孢菌（E. rostratum），均为具有重要经济价值的病原菌，但二者的寄主范围差异显著：前者专性侵染植物，后者则可发生跨界侵染。本研究对这两个突脐蠕孢菌的完整线粒体基因组（mitogenome）进行了测序、组装、注释，并开展比较分析。二者的线粒体基因组均为环状DNA分子，包含一套核心蛋白质编码基因（protein-coding genes, PCGs）、核糖体RNA（ribosomal RNA, rRNA）、转运RNA（transfer RNA, tRNA）以及未鉴定的开放阅读框（open reading frame, ORF），但基因组大小悬殊，分别为264948 bp与64620 bp。比较线粒体基因组学分析显示，尽管喙突脐蠕孢菌的线粒体基因组中缺失了3个转运RNA基因（trnN-3、trnC-2与trnV-2），二者的线粒体基因排列顺序却高度保守。然而，相较于喙突脐蠕孢菌线粒体基因组仅含17个内含子并携带17个内含子开放阅读框，大斑突脐蠕孢菌的内含子（70个）与内含子开放阅读框（126个）数量远多于此，这被认为是导致二者线粒体基因组大小扩张与收缩的主要因素。在通常富含内含子的cox1基因中，两个线粒体基因组分别鉴定出18个与10个内含子位置类别（intron position classes, Pcls）。此外，大斑突脐蠕孢菌与喙突脐蠕孢菌的基因组内重复区域占比分别为16.16%与10.85%。基于整合的线粒体基因数据集，我们对98个子囊菌构建了具有高支持率拓扑结构的系统发育树，表明线粒体基因组或许可作为系统发育重建的有效分子标记。本研究首次报道了突脐蠕孢属的线粒体基因组，相关结果可为解析该属真菌的遗传特性与演化历程提供重要参考。