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Ascaridoids are among the commonest groups of zooparasitic nematodes (roundworms) and occur in the alimentary canal of all major vertebrate groups, including humans. They have an extremely high diversity and are of major socio-economic importance. However, their evolutionary history remains poorly known. Herein, we performed a comprehensive phylogenetic analysis of the Ascaridoidea. Our results divided the Ascaridoidea into six monophyletic major clades, i.e., the Heterocheilidae, Acanthocheilidae, Anisakidae, Ascarididae, Toxocaridae, and Raphidascarididae, among which the Heterocheilidae, rather than the Acanthocheilidae, represents the sister clade to the remaining ascaridoids. The phylogeny was calibrated using an approach that involves time priors from fossils of the co-evolving hosts, and dates the common ancestor of the Ascaridoidea back to the Early Carboniferous (approximately 360.47–325.27 Ma). The divergence dates and ancestral host types indicated by our study suggest that members of the Ascaridoidea first parasitized terrestrial tetrapods, and subsequently, extended their host range to elasmobranchs and teleosts. We also propose that the fundamental terrestrial-aquatic switches of these nematodes were affected by changes in sea-level during the Triassic to the Early Cretaceous.

Species of Baylisascaris (Nematoda: Ascarididae) are of great veterinary and zoonotic significance, owing to cause Baylisascariosis or Baylisascariasis in wildlife, captive animals and humans. However, the phylogenetic relationships of the current 10 Baylisascaris species remain unclear. Moreover, our current knowledge of the detailed morphology and morphometrics of the important zoonotic species B. procyonis is still insufficient. The taxonomical status of B. procyonis and B. columnaris remains under debate. In the present study, the detailed morphology of B. procyonis was studied using light and scanning electron microscopy based on newly collected specimens from the raccoon Procyon lotor (Linnaeus) in China. The results of the ASAP analysis and Bayesian inference (BI) using the 28S, ITS, cox1 and cox2 genetic markers did not support that B. procyonis and B. columnaris represent two distinct species. Integrative morphological and molecular assessment challenged the validity of B. procyonis, and suggested that B. procyonis seems to represent a synonym of B. columnaris. Molecular phylogenetic results indicated that the species of Baylisascaris were grouped into 4 clades according to their host specificity. The present study provided new insights into the taxonomic status of B. procyonis and preliminarily clarified the phylogenetic relationships of Baylisascaris species.

The acanthocephalans are characterized by a retractible proboscis, armed with rows of recurved hooks, which serves as the primary organ for attachment of the adult worm to the intestinal wall of the vertebrate definitive host. Whilst there is a considerable variation in the size, shape and armature of the proboscis across the phylum, intraspecific variation is generally regarded to be minimal. Consequently, subtle differences in proboscis morphology are often used to delimit congeneric species. In this study, striking variability in proboscis morphology was observed among individuals of Neorhadinorhynchus nudus (Harada, 1938) collected from the frigate tuna Auxis thazard Lacépède (Perciformes: Scombridae) in the South China Sea. Based on the length of the proboscis, and number of hooks per longitudinal row, these specimens of N. nudus were readily grouped into three distinct morphotypes, which might be considered separate taxa under the morphospecies concept. However, analysis of nuclear and mitochondrial DNA sequences revealed a level of nucleotide divergence typical of an intraspecific comparison. Moreover, the three morphotypes do not represent three separate genetic lineages. The surprising, and previously undocumented level of intraspecific variation in proboscis morphology found in the present study, underscores the need to use molecular markers for delimiting acanthocephalan species.

Acanthocephalans of the order Polymorphida mainly parasitic in birds and mammals, are of veterinary, medical and economic importance. However, the evolutionary relationships of its 3 families (Centrorhynchidae, Polymorphidae and Plagiorhynchidae) remain under debate. Additionally, some species of Polymorphida (i.e. Bolbosoma spp. and Corynosoma spp.) are recognized as zoonotic parasites, associated with human acanthocephaliasis, but the mitochondrial genomes for representatives of Bolbosoma and Corynosoma have not been reported so far. In the present study, the complete mitochondrial genomes B. nipponicum and C. villosum (Acanthocephala: Polymorphidae) are reported for the first time, which are 14 296 and 14 241 bp in length, respectively, and both contain 36 genes [including 12 PCGs, 22 tRNA genes and 2 rRNA genes] and 2 non-coding regions (NCR1 and NCR2). The gene arrangement of some tRNAs in the mitogenomes of B. nipponicum and C. villosum differs from that found in all other acanthocephalans, except Polymorphus minutus. Phylogenetic results based on concatenated amino acid (AA) sequences of the 12 protein-coding genes (PCGs) strongly supported that the family Polymorphidae is a sister to the Centrorhynchidae rather than the Plagiorhynchidae, and also confirmed the sister relationship of the genera Bolbosoma and Corynosoma in the Polymorphidae based on the mitogenomic data for the first time. Our present findings further clarified the phylogenetic relationships of the 3 families Plagiorhynchidae, Centrorhynchidae and Polymorphidae, enriched the mitogenome data of the phylum Acanthocephala (especially the order Polymorphida), and provided the resource of genetic data for diagnosing these 2 pathogenic parasites of human acanthocephaliasis.

A new species of Moniliformis, M. tupaia n. sp. is described using integrated morphological methods (light and scanning electron microscopy) and molecular techniques (sequencing and analysing the nuclear 18S, ITS, 28S regions and mitochondrial cox1 and cox2 genes), based on specimens collected from the intestine of the northern tree shrew Tupaia belangeri chinensis Anderson (Scandentia: Tupaiidae) in China. Phylogenetic analyses show that M. tupaia n. sp. is a sister to M. moniliformis in the genus Moniliformis, and also challenge the systematic status of Nephridiacanthus major. Moniliformis tupaia n. sp. represents the third Moniliformis species reported from China.

Baylisascaris transfuga (Rudolphi, 1819) is a common parasitic nematode in the digestive tract of various species of bears worldwide, with great veterinary significance. However, our present knowledge on the morphology of B. transfuga remains insufficient. In the present study, the detailed morphology of B. transfuga was studied using light and scanning electron microscopy (SEM), based on specimens collected from the polar bear Ursus maritimus Phipps (Carnivora: Ursidae) in the Shijiazhuang Zoo, China. The results revealed some morphological and morphometric variation between the present specimens and some of those from previous studies, including oesophageal length of female, number and morphology of postcloacal papillae and morphology of tail of males. Present SEM observations clearly showed the detailed morphology of lips, cervical alae, cloacal ornamentation, precloacal medioventral papilla, phasmids and tail tip. These supplementary morphological and morphometric data enable us to identify this ascaridid nematode more accurately.

Species of Seuratascaris Sprent, 1985 are a rarely reported group of ascaridoid nematodes, parasitising various frogs and toads. In the present study, a new subspecies of Seuratascaris , S. physalis bazhaiensis n. subsp. was described using integrated taxonomic methods, based on specimens collected from Odorrana graminea (Anura: Ranidae) in Guangxi Zhuang Autonomous Region, China. Results of the Assemble Species by Automatic Partitioning (ASAP) and Bayesian inference based on the mitochondrial cox 1, cox 2 and rrnS data all supported S. physalis bazhaiensis representing a distinct taxon from the nominate subspecies S. physalis physalis . Supplementary morphometric and genetic data of S. phy. physalis are presented based on newly collected material from Odorrana tiannanensis (Anura: Ranidae) and Rhacophorus sp. (Anura: Rhacophoridae) in Yunnan Province, China. A key to species of Seuratascaris is provided. The complete mitochondrial genome of S. physalis bazhaiensis was sequenced and annotated, and represents the first mitogenomic data for the genus Seuratascaris . This mitogenome has only 13,628 bp (including 12 protein-coding genes, 22 tRNA genes, 2 ribosomal RNAs, and only 1 non-coding region), and is the smallest of the reported ascaridoid mitogenomes so far. Les espèces de Seuratascaris Sprent, 1985 sont un groupe rarement signalé de nématodes ascaridoïdes, parasitant diverses grenouilles et crapauds. Dans la présente étude, une nouvelle sous-espèce de Seuratascaris , S. physalis bazhaiensis n. subsp. a été décrite à l’aide de méthodes taxonomiques intégrées, basées sur des spécimens collectés chez Odorrana graminea (Anura : Ranidae) dans la région autonome Zhuang du Guangxi, en Chine. Les résultats de l’assemblage d’espèces par partitionnement automatique (ASAP) et de l’inférence bayésienne basée sur les données mitochondriales cox 1, cox 2 et rrnS ont tous montré que S. physalis bazhaiensis représentait un taxon distinct de la sous-espèce nominale S. physalis physalis . Des données morphométriques et génétiques supplémentaires pour S. phy. physalis sont présentées sur la base de matériel nouvellement collecté à partir d’ Odorrana tiannanensis (Anura : Ranidae) et de Rhacophorus sp. (Anura : Rhacophoridae) dans la province du Yunnan, en Chine. Une clé des espèces de Seuratascaris est fournie. Le génome mitochondrial complet de S. physalis bazhaiensis a été séquencé et annoté, et représente les premières données mitogénomiques pour le genre Seuratascaris . Ce mitogénome ne compte que 13,628 pb (dont 12 gènes codant pour des protéines, 22 gènes d’ARNt, 2 ARN ribosomiques et seulement 1 région non codante), et est le plus petit parmi les mitogénomes des ascaridoïdes rapportés jusqu’à présent.

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. Graphical Abstract

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

Acanthocephalans of the genus Rhadinorhynchus parasitize various marine fishes worldwide. However, the true diversity of Rhadinorhynchus is still unclear. In this study, we found an example of phenotypic variation in trunk spines of the poorly known rhadinorhynchid species R. cololabis Laurs & McCauley, 1964. According to the number and distribution of trunk spines, the present specimens of R. cololabis can be divided into two distinct morphotypes, which may erroneously be recognized as distinct taxa in the absence of molecular data. However, the Assemble Species by Automatic Partitioning (ASAP) and Bayesian inference (BI) analyses based on different nuclear and mitochondrial sequence data, all confirm that the two distinct morphotypes are conspecific, and do not represent two separate genetic lineages. Our ASAP and BI results of cox1 data also suggest that is R. villalobosi Martínez-Flores et al. , 2025 is a synonym of R. trachinoti Grano-Maldonado et al. , 2025, and challenge the validity of R. dorsoventrospinosus Amin et al. , 2011, and R. hiansi Soota & Bhattacharya, 1981. The present findings also indicate that the number and distribution of trunk spines vary markedly in some species of Rhadinorhynchus , and care must be taken when differentiating Rhadinorhynchus species based on this feature. Additionally, the complete mitogenome of R. cololabis is presented for the first time, which has only 13,567 bp, and displays a very high level of similarity with the mitogenome of R. laterospinosus in both nucleotide sequences (94.6%) and amino acid sequences of 12 protein-coding genes (93.8%). However, comparative mitogenomics support R. cololabis and R. laterospinosus representing two separate taxa. Les Acanthocéphales du genre Rhadinorhynchus parasitent divers poissons marins du monde entier. Cependant, la véritable diversité de Rhadinorhynchus reste encore peu connue. Dans cette étude, nous avons trouvé un exemple de variation phénotypique des épines du tronc de l’espèce de Rhadinorhynchidae peu connue R. cololabis Laurs & McCauley, 1964. D’après le nombre et la répartition des épines du tronc, nos spécimens de R. cololabis peuvent être divisés en deux morphotypes distincts, qui pourraient être confondus avec des taxons distincts en l’absence de données moléculaires. Cependant, les analyses ASAP et BI basées sur différentes données de séquences nucléaires et mitochondriales confirment toutes que les deux morphotypes distincts sont conspécifiques et ne représentent pas deux lignées génétiques distinctes. Nos résultats ASAP et BI des données sur cox1 suggèrent également de traiter R. villalobosi Martínez-Flores et al. , 2025 comme un synonyme de R. trachinoti Grano-Maldonado et al. , 2025, et remettent en question la validité de R. dorsoventrospinosus Amin et al. , 2011, et R. hiansi Soota & Bhattacharya, 1981. Les présents résultats indiquent également que le nombre et la distribution des épines du tronc varient considérablement chez certaines espèces de Rhadinorhynchus , et que la prudence doit être de mise lors de la différenciation des espèces de Rhadinorhynchus sur la base de cette caractéristique. De plus, le mitogénome complet de R. cololabis est présenté pour la première fois. Il ne compte que 13 567 pb et présente une très forte similarité avec celui de R. laterospinosus , tant au niveau des séquences nucléotidiques (94,6%) que des séquences d’acides aminés de 12 gènes codant pour des protéines (93,8%). Cependant, la mitogénomique comparative suggère que R. cololabis et R. laterospinosus représentent deux taxons distincts.