Mitochondrial genome evidence suggests Cooperia sp. from China may represent a distinct species from Cooperia oncophora from Australia
Introduction
Cooperia spp. are important nematodes parasitizing in the small intestine of cattle and sheep. It is widely distributed in temperate regions and can lead to severe parasitic gastrointestinal diseases, ultimately causing major socio-economic losses worldwide [[1], [2], [3], [4], [5]]. However, the majority of studies over Cooperia species are around the high prevalence and harms in ruminants, but it is yet to know whether there is genetic diversity in mitochondrial (mt) genome sequences of Cooperia spp. from different geographic locations.
The traditional identification of parasites mainly relies on morphological characteristics [6,7]. However, some closely related species are very similar in morphology and they are hard to distinguished merely based on morphological and morphometric features. To overcome the limitation of morphological taxonomy, molecular approaches based on sequences of the internal transcribed spacer (ITS-1 and ITS-2) regions of nuclear ribosomal DNA (rDNA) [8,9], as well as the mt cytochrome c oxidase subunit 1 (cox1) have been widely used for specific identification and differentiation of nematodes [6,[10], [11], [12], [13], [14], [15]]. Our preliminary comparative study revealed that the mt cox1 sequence of Cooperia sp. from China (GenBank accession No. MK295856) and C. oncophora from Australia (GQ888713) differed by 10.16%, which is much higher than the intra-specific variation in cox1 among related nematodes (among 18 Toxocara canis samples: 0.2–3.7% [16], and among 17 Oesophagostomum dentatum samples: 0.3–5.2% [17]), but is slightly lower than the inter-specific differences between Chabertia ovina and Ch. erschowi (11.98%) [15] and between Nematodirus spathiger and N. oiratianus (11.55%) [18]. However, we have noticed that sequences of Cooperia sp. from China have 99% similarity with those from C. oncophora in both ITS-1 (AJ544290) [19] and ITS-2 (AB245040 [20,21] and AJ000032 [22]). This raised the question whether Cooperia sp. from China and C. oncophora from Australia represent different nematode species, despite the high similarities in their ITS sequences and high difference in cox1 sequences.
The sequences of complete mt genome are frequently used as effective genetic markers in differentiating cryptic species, species complex and synonymy species in parasites [23,24], in particular for the nematodes within Trichostrongylina [15,25]. In the present study, we determined the complete sequences of mt genome of Cooperia sp. from China and compared the sequences with those of C. oncophora from Australia, to test the hypothesis that Cooperia sp. from China is a genetically distinct species.
Section snippets
Parasites, DNA extraction and ITS acquirement
Eleven adult nematode specimens were collected from the small intestine of a Tianzhu White yak in Tianzhu County, Gansu Province, northwestern China. The white yak was handled strictly in accordance with the Animal Ethics Procedures and Guidelines of the People's Republic of China. The study protocol was reviewed and approved by the Animal Administration and Ethics Committee of Lanzhou Veterinary Study Institute, Chinese Academy of Agricultural Sciences. These nematodes were putatively
Identity of the specimens
The ITS sequences of the 11 Cooperia samples were all 779 bp in length, and were identical in both ITS-1 and ITS-2 rDNA, indicating that they represent the same species. The ITS sequences were assembled and compared with the available sequences by BLAST through NCBI, resulting in 99% similarity with that of C. oncophora from Brazil (AJ544290) [19] in ITS-1, and 99% similarity with those of C. oncophora from Denmark (AB245040) [20,21] in ITS-2. The detailed information of comparison in ITS
Conclusions
The present study determined the complete mt genome sequences of one Cooperia sp. from China. The results of comparative sequence analyses, together with the phylogenetic analysis, demonstrated that, despite the high similarity in ITS sequences with those of C. oncophora from Australia, Cooperia sp. from China may represent a distinct taxon.
Declaration of Competing Interest
The authors declare that they have no competing interests.
Acknowledgements
Project support was provided by National Natural Science Foundation of China (Grant No. 31702225), the International Science and Technology Cooperation Project of Gansu Provincial Key Research and Development Program (Grant No. 17JR7WA031), the Elite Program of Chinese Academy of Agricultural Sciences, and the Agricultural Science and Technology Innovation Program (ASTIP) (Grant No. CAAS-ASTIP-2016-LVRI-03).
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These authors have contributed equally to this work.