Introduction Subulura chinensis Schwartz, 1926 is a hitherto poorly known nematode species. The morphology of S. chinensis has not been sufficiently well described. In addition, the molecular data from species of the Subuluroidea are extremely limited. Methods The detailed morphology of S. chinensis was studied using light microscopy and, for the first time, scanning electron microscopy, based on newly collected specimens from the little owl Athene noctua (Scopoli) (Strigiformes: Strigidae) in China. The internal transcribed spacer (ITS) of ribosomal DNA and mitochondrial cytochrome c oxidase subunit 1 (cox1) target regions of S. chinensis were first amplified by polymerase chain reaction (PCR), then sequenced and analysed for the molecular identification of this species. Results Our SEM observations showed for the fist time the detailed morphology of the cephalic extremity, precloacal pseudo-sucker, caudal papillae, gubernaculum, phasmids and vulva of S. chinensis, and also determined the presence of a small, single medio-ventral precloacal papilla in the male. Moreover, we detected the presence of 0.08-0.40% and 0-1.30% nucleotide divergence among different individuals of S. chinensis in the ITS and cox1 regions, respectively. The supplementary morphological characters and genetic data will be very useful for the diagnosis of this poorly known species.
Background
The family Toxocaridae is a group of zooparasitic nematodes of veterinary, medical and economic significance. However, the evolutionary relationship of Porrocaecum and Toxocara, both genera currently classified in Toxocaridae, and the monophyly of the Toxocaridae remain under debate. Moreover, the validity of the subgenus Laymanicaecum in the genus Porrocaecum is open to question. Due to the scarcity of an available genetic database, molecular identification of Porrocaecum nematodes is still in its infancy.
Methods
A number of Porrocaecum nematodes collected from the Eurasian marsh harrier Circus aeruginosus (Linnaeus) (Falconiformes: Accipitridae) in the Czech Republic were identified using integrated morphological methods (light and scanning electron microscopy) and molecular techniques (sequencing and analyzing the nuclear 18S, 28S and ITS regions). The complete mitochondrial genomes of the collected nematode specimens and of Porrocaecum (Laymanicaecum) reticulatum (Linstow, 1899) were sequenced and annotated for the first time. Phylogenetic analyses of ascaridoid nematodes based on the amino acid sequences of 12 protein-coding genes of mitochondrial genomes were performed using maximum likelihood and Bayesian inference.
Results
A new species of Porrocaecum, named P. moraveci n. sp., is described based on the morphological and genetic evidence. The mitogenomes of P. moraveci n. sp. and P. reticulatum both contain 36 genes and are 14,517 and 14,210 bp in length, respectively. Comparative mitogenomics revealed that P.moraveci n. sp. represents the first known species with three non-coding regions and that P. reticulatum has the lowest overall A + T content in the mitogenomes of ascaridoid nematodes tested to date. Phylogenetic analyses showed the representatives of Toxocara clustered together with species of the family Ascarididae rather than with Porrocaecum and that P. moraveci n. sp. is a sister to P. reticulatum.
Conclusions
The characterization of the complete mitochondrial genomes of P. moraveci n. sp. and P. reticulatum is reported for the first time. Mitogenomic phylogeny analyses indicated that the family Toxocaridae is non-monophyletic and that the genera Porrocaecum and Toxocara do not have an affinity. The validity of the subgenus Laymanicaecum in Porrocaecum was also rejected. Our results suggest that: (i) Toxocaridae should be degraded to a subfamily of the Ascarididae that includes only the genus Toxocara; and (ii) the subfamily Porrocaecinae should be resurrected to include only the genus Porrocaecum. The present study enriches the database of ascaridoid mitogenomes and provides a new insight into the systematics of the superfamily Ascaridoidea.
Graphical Abstract
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