Abstract
Ascaridomorph nematodes threaten the health of humans and other animals worldwide. Despite their medical, veterinary and economic importance, the identification of species lineages and establishing their phylogenetic relationships have proved difficult in some cases. Many working hypotheses regarding the phylogeny of ascaridomorphs have been based on single-locus data, most typically nuclear ribosomal RNA. Such single-locus hypotheses lack independent corroboration, and for nuclear rRNA typically lack resolution for deep relationships. As an alternative approach, we analyzed the mitochondrial (mt) genomes of anisakids (~14 kb) from different fish hosts in multiple countries, in combination with those of other ascaridomorphs available in the GenBank database. The circular mt genomes range from 13,948-14,019 bp in size and encode 12 protein-coding genes, 2 ribosomal RNAs and 22 transfer RNA genes. Our analysis showed that the Pseudoterranova decipiens complex consists of at least six cryptic species. In contrast, the hypothesis that Contracaecum ogmorhini represents a complex of cryptic species is not supported by mt genome data. Our analysis recovered several fundamental and uncontroversial ascaridomorph clades, including the monophyly of superfamilies and families, except for Ascaridiidae, which was consistent with the results based on nuclear rRNA analysis. In conclusion, mt genome analysis provided new insights into the phylogeny and taxonomy of ascaridomorph nematodes.
Highlights
Parasitic nematodes cause diseases associated with serious morbidity and mortality in animals and substantial reductions in crop yields[1]
Most mt genes are much more variable in sequence than nuclear ribosomal RNA genes[18,19], and recent studies have shown that amino acid sequences inferred from mt genome sequences are very useful for species identification as well as for investigating their inter-relationships[20,21,22,23,24]
Each of the 8 mt genomes contains 36 genes, consisting of 12 protein-encoding, 2 rRNA, and 22 tRNA genes (Fig. 1), which is typical for chromadorean nematodes
Summary
Parasitic nematodes (roundworms) cause diseases associated with serious morbidity and mortality in animals and substantial reductions in crop yields[1]. Some species of ascaridoids can be readily identified and distinguished based on morphological features (e.g., Toxocara cati, Toxocara canis and Toxascaris leonina), molecular data have revealed cryptic species complexes within other morphological species (e.g., Anisakis simplex, Contracaecum rudolphii and Pseudoterranova decipiens) Some of these cryptic species show considerable differences in host and geographical ranges[7,8,9]. Some studies[27,28] indicated that Anisakidae is more closely related to Ascarididae than to Toxocaridae, others[29,30] have argued the opposite, and some have suggested that Anisakidae is sister to Toxocaridae +Ascarididae[31,33] These results reflect inadequate resolution at higher levels for nuclear ribosomal data, conflicts between different inference methods, impacts of taxon sampling and limited phylogenetic signal from small data sets[34,35]. Analyses of mt genome sequences in the present study have provided novel insights into the phylogenetic relationships of several major lineages of ascaridomorphs
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