Phylogenetic affinities and systematic position of Entomelas sylvestris Baker, 1982 (Nematoda: Rhabdiasidae), a parasite of Breviceps sylvestris FitzSimons (Amphibia: Brevicipitidae) in South Africa

Molecular phylogeny of the family Rhabdiasidae (Nematoda: Rhabditida), with morphology, genetic characterization and mitochondrial genomes of Rhabdias kafunata and R. bufonis

BACKGROUND

The family Rhabdiasidae (Nematoda: Rhabditida) is a globally distributed group of nematode parasites, with over 110 species parasitic mainly in amphibians and reptiles. However, the systematic position of the family Rhabdiasidae in the order Rhabditida remains unsolved, and the evolutionary relationships among its genera are still unclear. Moreover, the present knowledge of the mitochondrial genomes of rhabdiasids remains limited.

METHODS

Two rhabdiasid species: Rhabdias kafunata Sata, Takeuchi & Nakano, 2020 and R. bufonis (Schrank, 1788) collected from the Asiatic toad Bufo gargarizans Cantor (Amphibia: Anura) in China, were identified based on morphology (light and scanning electron microscopy) and molecular characterization (sequencing of the nuclear 28S and ITS regions and mitochondrial cox1 and 12S genes). The complete mitochondrial genomes of R. kafunata and R. bufonis were also sequenced and annotated for the first time. Moreover, phylogenetic analyses based on the amino acid sequences of 12 protein-coding genes (PCGs) of the mitochondrial genomes were performed to clarify the systematic position of the family Rhabdiasidae in the order Rhabditida using maximum likelihood (ML) and Bayesian inference (BI). The phylogenetic analyses based on the 28S + ITS sequences, were also inferred to assess the evolutionary relationships among the genera within Rhabdiasidae.

RESULTS

The detailed morphology of the cephalic structures, vulva and eggs in R. kafunata and R. bufonis was revealed using scanning electron microscopy (SEM) for the first time. The characterization of 28S and ITS regions of R. kafunata was reported for the first time. The mitogenomes of R. kafunata and R. bufonis are 15,437 bp and 15,128 bp long, respectively, and both contain 36 genes, including 12 PCGs (missing atp8). Comparative mitogenomics revealed that the gene arrangement of R. kafunata and R. bufonis is different from all of the currently available mitogenomes of nematodes. Phylogenetic analyses based on the ITS + 28S data showed Neoentomelas and Kurilonema as sister lineages, and supported the monophyly of Entomelas, Pneumonema, Serpentirhabdias and Rhabdias. Mitochondrial phylogenomic results supported Rhabdiasidae as a member of the superfamily Rhabditoidea in the suborder Rhabditina, and its occurrance as sister to the family Rhabditidae.

CONCLUSIONS

The complete mitochondrial genome of R. kafunata and R. bufonis were reported for the first time, and two new gene arrangements of mitogenomes in Nematoda were revealed. Mitogenomic phylogenetic results indicated that the family Rhabdiasidae is a member of Rhabditoidea in Rhabditina, and is closely related to Rhabditidae. Molecular phylogenies based on the ITS + 28S sequence data supported the validity of Kurilonema, and showed that Kurilonema is sister to Neoentomelas. The present phylogenetic results also indicated that the ancestors of rhabdiasids seem to have initially infected reptiles, then spreading to amphibians.

New host record, Sclerophrys regularis (Bufonidae), for Rhabdias africanus (Rhabdiasidae, ) based on molecular and morphological evidence

With diversity of hosts range, most identified nematode species still lack the crucial connection between morphological and molecular make-up, which is important for precisely classifying the specimens. The present study provides the complete description of Rhabdias africanus in the new host record, Sclerophrys regularis. Fifty toad specimens were collected, and a high prevalence of R. africanus infection (74%) was observed. Morphology and ultrastructure were observed using light and scanning electron microscopes. Morphological characteristics, including peculiarities of the head, the shape and position of the lips, and the number of labial papillae, were described. The length of the body, the esophageal length, the distance from an anterior end to the nerve ring, and the tail length were reduced in the studied samples relative to previously described specimens. Furthermore, some variable matrices that have not previously been described, e.g., ovarian part widening, the nerve ring and its location, and eggs with different stages of larvae, were included in the present study. Genus and species identification was confirmed by comparing partial 12S (619 bp) and ITS (878 bp) gene sequences to those of Rhabdias species deposited in GenBank. The studied species showed a 99.34% resemblance to R. africanus from South Africa. We assume our findings will aid in the molecular identification of adult and larval stages of this genus in amphibians. We strongly recommend further studies on the environmental factors that promote Rhabdias infection and transmission.

Phylogeography of Rhabdias spp. (Nematoda: Rhabdiasidae) collected from Bufo species in Honshu, Shikoku, and Kyushu, Japan including possible cryptic species

The genus Rhabdias Stiles & Hassall, 1905 comprises lung parasites of amphibians and reptiles worldwide. In Japan, 9 species have been recorded, including Rhabdias incerta Wilkie, 1930 which has been reported only in Bufo species. In this study, to assess the diversity of R. incerta, we performed molecular analyses of Rhabdias species sampled from three species/subspecies of Japanese toads namely Bufo japonicus, B. japonicus formosus, and B. torrenticola, collected in various regions of Honshu, Shikoku, and Kyushu, Japan. DNA sequence divergence was compared using mtDNA (COI) and nuclear DNA (28S) to identify possible cryptic species. Morphological analysis was performed through light microscopy and scanning electron microscopy (SEM). The results revealed that Bufo spp. serve as hosts for at least three Rhabdias species. Morphologically, most samples were identified as R. incerta but with a longer body and larger buccal cavity than originally described. Rhabdias incerta appears to be specific to the genus Bufo and is further subdivided into two or three phylogroups based on subspecies divisions and biogeography of their host. Some Rhabdias specimens collected in this study resemble R. tokyoensis Wilkie, 1930, parasitic in hosts from the order Caudata, which suggests host switching. Both molecular and morphological analyses suggested the presence of undescribed and cryptic Rhabdias species within toads collected in Japan. This study was the first to molecularly characterize Rhabdias species in Japan, including novel sequences of R. incerta and two undescribed species.

Description and molecular characterisation of a new nematode species parasitic in the lungs of Strongylopus grayii (Smith) (Anura: Pyxicephalidae) in South Africa

Rhabdias delangei n. sp. (Nematoda: Rhabdiasidae) is described from the lungs of the clicking stream frog Strongylopus grayii (Smith) in the Western Cape Province of South Africa. The new species is characterised by the nerve-ring being located just posterior to the mid-length of the oesophagus, a pre-equatorial vulva, as well as a thin-walled and apparently underdeveloped rectum. Pairwise analyses based on ITS-28S and partial cox1 gene fragments showed significant differences between R. delangei n. sp. and its congeners from the Afrotropical Realm. Rhabdias delangei n. sp. is the 12th species of Rhabdias Stiles & Hassall, 1905 recorded in Afrotropical amphibians and the second species of the genus found in pyxicephalid hosts. Our phylogenetic analysis based on the ITS-28S region showed that the new species is closely related to the European species R. rubrovenosa (Schneider, 1866). Furthermore, both species are similar with respect to the underdeveloped rectum and pre-equatorial position of vulva.

Negative effects of parasitic lung nematodes on the fitness of a Neotropical toad (Rhinella horribilis)

Pathogens are increasingly implicated in amphibian declines but less is known about parasites and the role they play. We focused on a genus of nematodes (Rhabdias) that is widespread in amphibians and examined their genetic diversity, abundance (prevalence and intensity), and impact in a common toad (Rhinella horribilis) in Panama. Our molecular data show that toads were infected by at least four lineages of Rhabdias, most likely Rhabdias pseudosphaerocephala, and multiple lineages were present in the same geographic locality, the same host and even the same lung. Mean prevalence of infection per site was 63% and mean intensity of infection was 31 worms. There was a significant effect of host size on infection status in the wild: larger toads were more likely to be infected than were smaller conspecifics. Our experimental infections showed that toadlets that were penetrated by many infective Rhabdias larvae grew less than those who were penetrated by few larvae. Exposure to Rhabdias reduced toadlet locomotor performance (both sustained speed and endurance) but did not influence toadlet survival. The effects of Rhabdias infection on their host appear to be primarily sublethal, however, dose-dependent reduction in growth and an overall impaired locomotor performance still represents a significant reduction in host fitness.

Molecular and morphological characterisation of Rhabdias picardiae Junker, Lhermitte-Vallarino et Bain, 2010 (Nematoda: Rhabdiasidae) from Delaland's River Frog, Amietia delalandii (Duméril et Bibron, 1841) (Amphibia: Pyxicephalidae) in South Africa

Rhabdias picardiae previously known from the Guttural Toad, Sclerophrys gutturalis (Bufonidae) is described from a new host, Delaland's River Frog, Amietia delalandii (Pyxicephalidae). The species identification is confirmed by analysis of the COI gene sequence and morphological comparison with the original description. Morphological details of apical structures in adult parasitic R. picardiae, namely, the shape and position of lips, the number and position of external labial papillae, and the buccal capsule morphology are described using light and scanning electron microscopy. The buccal capsule width and length, the distance from anterior end to nerve ring and excretory pore, and ratios of distance to vulva to body length and body length to width were identified as less variable metrical characters in the studied sample of 30 specimens. Adult specimens demonstrated differences in the position of lips in relation to the oral opening and position of the buccal capsule in relation to the oesophagus depending on the size (age) of worms. The infective larvae of R. picardiae are described for the first time and characterised by two lateral alae consisting of two ridges, triangular pseudolabia with rounded tops, and presence of ornamentation on the rounded tail tip.