Mitochondrial genome of Strongylus edentatus confirms monophyly of the genus Strongylus within Strongylinae

Abstract

Strongylus vulgaris, Strongylus equinus, Strongylus edentatus and Strongylus asini are important equine nematodes, classified under the genus Strongylus (subfamily Strongylinae). However, this taxonomy is controversial. Thus, the mitochondrial (mt) genome of S. edentatus collected from horses in Heilongjiang Province, China was sequenced and compared with those of closely related species to clarify the classification and phylogenetic relationships of this genus. The circular mt genome of S. edentatus was 14,765 bp long, comprising 12 protein-coding genes, 22 transfer RNA genes, two ribosomal RNA genes, and three noncoding regions. All protein-coding genes were transcribed in the same direction and had a type 3 gene arrangement. The genome contained 22 intergenic spacers (1–146 bp) and four overlapping regions (1–14 bp). The A+T content was 78.15 %, reflecting a strong nucleotide and codon usage bias. The mt genome of S. edentatus showed higher similarity with that of S. equinus (96.3 %) than with that of S. vulgaris (76.9 %). Phylogenetic analyses showed that S. edentatus and S. equinus were classified on the same branch, forming a sister branch with S. vulgaris within Strongylinae, with S. edentatus and S. equinus having a closer phylogenetic relationship. Although traditionally assigned to Strongylinae, Strongylus and Triodontophorus did not cluster together. This study provides the first complete mt genome of S. edentatus and confirmed that S. equinus, S. vulgaris, and S. edentatus belong to the genus Strongylus, which is monophyletic, with the subfamilies Strongylinae and Cyathostominae being paraphyletic. Thus, overall, this study provides valuable data for addressing taxonomic ambiguities in the Strongylidae.