Sequencing And Analysis Of The Complete Mitochondrial Genome Of Toxascaris Leonina From The Cheetah

Toxascaris leonina is a most common intestinal parasite of canid and felid with global distribution,causing great harm to animal health and leading to public health problems.With the continuous development of sequencing technology,molecular/DNA markers have been extensively used for specific identification and differentiation of various nematodes.Very recently,some previous study raised the possibility that T.leonina from canid and felid hosts may represent two different nematode species.However,these results have only obtained from small dataset.Therefore,this hypothesis should be re-estimated by greater number of genes.In the present study,sequence variation in sections of the mt cox1 gene was assessed among 23 individuals of T.leonina from canid and felid.Nucleotide variation among the 6 T.leonina from canid individuals was 0-3.0%.Nucleotide variation among the 17 T.leonina from felid individuals was 5.4%-7.4%.In addition,phylogenetic analyses of the mt cox1 sequence data revealed also strong support for the separation of T.leonina from canid and felid individuals into two distinct cladesWe sequenced and characterised the complete mt genome of T.leonina.The entire mt genome of T.leonina-cheetah was 14,685 bp in size.This genome contains 36 genes.The nucleotide content of the mt genome sequence of T.leonina-cheetah is biased toward A+T(71.1%).All mt genes of T.leonina-cheetah are transcribed in the same direction.The mt gene arrangement of T.leonina-cheetah are the same as those of other Ascaris spp.(Ascarididae).The T.leonina-cheetah mt genome encodes 12 protein-coding genes.The most common start codon for T.leonina-cheetah is TTG,followed by ATT.In this mt genome,most of the protein-coding genes are predicted to use complete stop codon(TAG).The other four genes are predicted to use incomplete stop codons(T or TA).The entire mt genome sequence of T.leonina from the cheetah was significantly longer(375 bp)than that from the dog.The overall nucleotide sequence(except for the non-coding region)identity was 92.8% between the two mt genomes of T.leonina from the cheetah and the dog.For the 12 protein-coding genes,sequence difference was 5.0%-9.7% at the nucleotide level,and was 1.0%-7.2% at the amino acid level.The phylogenetic analyses based on amino acid sequences of 12 protein-coding genes showed the clear genetic distinctiveness between T.leonina from cheetah and dog,and the genetic distance between the two T.leonina isolates is approximately the same(looking at tree topologies and branch lengths)as that among the that of other complex species or related nematodesTaken together,these molecular datasets indicated that T.leonina from canid and felid hosts presents a species complex.Our results have implications for further studies of the molecular epidemiology and population genetics of this nematode.