Coronocyclus labiatus and Cylicodontophorus bicoronatus are two significant horse parasitic nematodes which are classified into subfamily Cyathostominae, family Strongylidae, however, the classification of these nematodes has been controversial for more than a century. Mitochondrial (mt) genomes are considered valuable sources for parasite taxonomy, population genetics, and systematics studies. In the present study, the mt genomes of Co. labiatus and Cd. bicoronatus (type species) were determined and subsequently compared with those from closely related species by phylogenetic analysis based on concatenated datasets of amino acid sequences predicted from mt protein-coding genes. The complete mt genomes of Co. labiatus and Cd. bicoronatus were circular with 13,827 bp and 13,753 bp in size, respectively. Both mt genomes consisted of a total of 12 protein-coding genes, 22 transfer RNA genes, two ribosomal RNA genes and two non-coding regions. All protein coding genes were transcribed in the same direction, and the gene order in both mt genomes belonged to the gene arrangement type 3 (GA3). There were 19 intergenic spacers with 1 bp to 35 bp and one overlap with 4 bp in mt genome of Co. labiatus, and 22 intergenic spacers with 1–29 bp in size but no overlap in the mt genome of Cd. bicoronatus. The A + T content of Co. labiatus and Cd. bicoronatus mt genomes were 75.87 % and 75.16 %, respectively. Similar to mt genones of other Strongylidae species published in GenBank, they also exhibited a strong A + T bias not only in the nucleotide composition but also in codon usage. Comparative analyses of mt genomes nucleotide sequence showed that mt genomes of Co. labiatus and Cd. bicoronatus had higher identities to that of Cylicostephanus goldi (90.3 % and 86.9 %, respectively), followed by those of two Cyathostomum species (89.9∼90.0 %; 86.4 %), respectively. Phylogenetic analyses using mt genomes of 26 Strongyloidea nematodes revealed that Co. labiatus was closely related to Cyathostomum species, and Cd. bicoronatus formed a distinct branch with Cyathostominae species, which was closer to Triodontophorus than Poteriostomum imparidentatum. We concluded Coronocyclus might be closely related with Cyathostomum but represent a distinct genus based on comparative mt genome sequences and phylogenetic analyses. The availability of complete mt genome sequences of Co. labiatus and Cd. bicoronatus provides new and useful genetic markers for further studies on Strongylidae nematodes.

Coronocyclus labiatus和Cylicodontophorus bicoronatus是两种重要的马寄生线虫，它们被分为Cyathostominae亚科和Strongylidae亚科，然而，这些线虫的分类已经有一个多世纪的争议。线粒体 (mt) 基因组被认为是寄生虫分类学、种群遗传学和系统学研究的宝贵来源。在本研究中，Co. labiatus和Cd的 mt 基因组。bicoronatus（类型物种）被确定并随后通过系统发育分析与来自密切相关物种的那些进行比较，该分析基于从 mt 蛋白质编码基因预测的氨基酸序列的串联数据集。Co. labiatus的完整 mt 基因组和镉。bicoronatus是圆形的，大小分别为 13,827 bp 和 13,753 bp。两个 mt 基因组都由总共 12 个蛋白质编码基因、22 个转移 RNA 基因、两个核糖体 RNA 基因和两个非编码区组成。所有蛋白质编码基因的转录方向相同，两个 mt 基因组中的基因顺序都属于基因排列类型 3（GA3）。Co. labiatus的mt基因组中有19个1 bp至35 bp的基因间隔区，1个与4 bp重叠，而Cd的mt基因组中有22个大小为1-29 bp但没有重叠的基因间隔区。双冠。Co. labiatus和Cd的 A+T 含量。双冠龙mt 基因组分别为 75.87% 和 75.16%。与 GenBank 中发表的其他圆线虫科物种的 mt 基因类似，它们不仅在核苷酸组成上而且在密码子使用上也表现出强烈的 A+T 偏向。mt基因组核苷酸序列的比较分析表明，Co. labiatus和Cd的mt基因组。bicoronatus具有更高的身份到的Cylicostephanus goldi（分别为90.3％和86.9％，），其次是两个Cyathostomum分别;物种（86.4％89.9~90.0％）。使用 26 条圆线虫的 mt 基因组进行的系统发育分析表明Co. labiatus与Cyathostomum物种和Cd密切相关。双冠龙形成有Cyathostominae物种，这是更接近不同分支Triodontophorus比Poteriostomum imparidentatum。我们得出的结论Coronocyclus可能与密切相关Cyathostomum但表示基于比较MT基因组序列和系统发育分析明显属。Co. labiatus和Cd的完整 mt 基因组序列的可用性。bicoronatus为进一步研究圆线虫科线虫提供了新的有用的遗传标记。