Characterization and comparative analysis of the complete mitochondrial genome of Azygia hwangtsiyui Tsin, 1933 (Digenea), the first for a member of the family Azygiidae

Phylogeny of two accacoeliid species (Digenea: Hemiuroidea) ex Mola mola (Linnaeus, 1758) (Tetraodontiformes: Molidae) from Northwest Pacific, with first molecular data on Odhnerium Yamaguti, 1934

Accacoeliid digeneans associated with fish of the family Molidae exhibit enigmatically high taxonomic diversity. However, the phylogenetic relationships between species within this digenean taxon are poorly understood. In the present study, the first nuclear 28S rRNA gene, ITS2 region of nuclear DNA, and mitochondrial cox1 gene sequence datasets were obtained for two members of the Accacoeliidae, a type and only species of the genus Odhnerium Yamaguti 1934 and an unidentified Accacladocoelium sp. collected from Mola mola (Linnaeus, 1758) off Iturup Island. Analyses of molecular differentiation and phylogenetic relationships indicate that Accacladocoelium sp. is a sister species to Accacladocoelium nigroflavum (Rudolphi, 1819). The genus Odhnerium is closely related to Tetrochetus Looss, 1912, on the 28S rRNA gene-based phylogenetic tree. Results of phylogenetic analysis based on both the mitochondrial cox1 gene and the concatenated ribosomal ITS2 region and cox1 gene of mtDNA show that the genus Odhnerium is close to the A. nigroflavum + Accacladocoelium sp. clade. In turn, the genus Accacladocoelium does not have monophyletic status in the trees reconstructed from these data.

Mitogenomic analysis of the position of the Azygiidae and constituent genera, with a new species of Azygia

The Azygiidae Looss, 1899 is a family of digeneans with a Holarctic distribution in which members of some genera mature in marine elasmobranchs while others occur only in freshwater teleosts. Some have questioned whether the marine genus Otodistomum Stafford, 1904 indeed belongs to the same family as the freshwater azygiid genera, namely Azygia Looss, 1899, Proterometra Horsfall, 1933, and Leuceruthrus Marshall and Gilbert, 1905. We present phylogenetic analyses based on mitochondrial genomes, rDNA operons, and partial cytochrome c oxidase I (Cox1) sequences from North American and Asian species that support the monophyly of Azygiidae, and placement of Azygioidea in the suborder Hemiurata, in contrast to recently published mitochondrial genome phylogenies. All phylogenies indicate that Azygia includes Leuceruthrus, which we therefore propose to be a junior synonym, together with suppression of the Leuceruthrinae. The status of Proterometra was equivocal, with support in some but not all analyses of 28S, but not in other markers. We describe a new species of Azygia from northeastern North America. Our results confirm the morphological variability of adults in Azygia, with worm size positively correlated with host size in Azygia angusticauda. Phylogenies suggest a marine origin for the Azygiidae, and a Palearctic origin for freshwater azygiids, with a single trans-Atlantic radiation to the Nearctic.

Complete mitochondrial genome and phylogenetic analysis of Dollfustrema vaneyi (Trematoda: Bucephalidae)

BACKGROUND

The Bucephalidae is a large family of digenean trematodes but most previous analyses of its phylogenetic position have relied on a single mitochondrial gene or morphological features. Mitochondrial genomes (mitogenomes) remain unavailable for the entire family. To address this, we sequenced the complete mitogenome of Dollfustrema vaneyi and analyzed the phylogenetic relationships with other trematodes.

RESULTS

The circular genome of Dollfustrema vaneyi spanned 14,959 bp and contained 12 protein-coding genes, 22 transfer RNA genes, 2 ribosomal RNA genes, and a major non-coding region. We used concatenated amino acid and nucleotide sequences of all 36 genes for phylogenetic analyses, conducted using MrBayes, IQ-TREE and PhyloBayes. We identified pronounced topological instability across different analyses. The addition of recently sequenced two mitogenomes for the Aspidogastrea subclass along with the use of a site-heterogeneous model stabilized the topology, particularly the positions of Azygiidae and Bucephalidae. The stabilized results indicated that Azygiidae was the closest lineage to Bucephalidae in the available dataset, and together, they clustered at the base of the Plagiorchiida.

CONCLUSIONS

Our study provides the first comprehensive description and annotation of the mitochondrial genome for the Bucephalidae family. The results indicate a close phylogenetic relationship between Azygiidae and Bucephalidae, and reveal their basal placement within the order Plagiorchiida. Furthermore, the inclusion of Aspidogastrea mitogenomes and the site-heterogeneous model significantly improved the topological stability. These data will provide key molecular resources for future taxonomic and phylogenetic studies of the family Bucephalidae.

First Azygiid Reported from Africa or a Characiform Fish: Plenivitellinum kifi n. gen., n. sp. (Digenea: Azygiidae) Infecting the Gut of African Tigerfish, Hydrocynus vittatus Castelnau, 1861 (Characiformes: Alestidae) from the Kavango River

PURPOSE

This paper aims to describe Plenivitellinum kifi n. gen., n. sp. (Digenea: Azygiidae) infecting the gastrointestinal tract of the African tigerfish, Hydrocynus vittatus Castelnau, 1861 (Characiformes: Alestidae) in the Kavango River, Namibia. We revise the diagnosis of Azygiidae Lühe, 1909 to accommodate this new species.

METHODS

The worm was heat-killed, fixed in 10% neutral buffered formalin, stained in Van Cleave's and Ehrlich's hematoxylins, cleared in clove oil, and mounted on glass slide using Canada balsam.

RESULTS

The new azygiid resembles species of Otodistomum Stafford, 1904 by having an elongate body, a ventral sucker that is wider than the oral sucker and that is in the anterior half of the body, a pre-testicular ovary, a uterus that primarily occupies the inter-caecal space between the ovary and the ventral sucker, and a vitellarium that is restricted to the hindbody and that is confluent posteriorly in the post-testicular region. The new genus differs from all species of Azygiidae by having the combination of a ventral sucker that is wider than the oral sucker (vs. narrower), an elongate prostatic sac that extends posteriad to near the posterior margin of the ventral sucker (vs. an ovoid prostatic sac that is wholly anterior to or slightly overlaps the anterior margin of the ventral sucker), a vitellarium that nearly fills the post-testicular space and that extends posteriad beyond the caecal tips (vs. a vitellarium that is separate posteriorly or that is restricted to the anterior half of the post-testicular space and does not extend posteriad beyond the caecal tips), and an I-shaped excretory bladder (vs. Y-shaped).

CONCLUSION

This study documents the first record of an azygiid from Africa and the first record of an azygiid infecting a characiform fish.

A report on the complete mitochondrial genome of the trematode Azygia robusta Odhner, 1911, its new definitive host from the Russian Far East, and unexpected phylogeny of Azygiidae within Digenea, as inferred from mitogenome sequences

New data on the complete mitochondrial genome of Azygia robusta (Azygiidae) were obtained by the next-generation sequencing (NGS) approach. The mitochondrial DNA (mtDNA) of A. robusta had a length of 13 857 bp and included 12 protein-coding genes, two ribosomal genes, 22 transfer RNA genes, and two non-coding regions. The nucleotide sequences of the complete mitochondrial genomes of two A. robusta specimens differed from each other by 0.12 ± 0.03%. Six of 12 protein-coding genes demonstrated intraspecific variation. The difference between the nucleotide sequences of the complete mitochondrial genomes of A. robusta and Azygia hwangtsiyui was 26.95 ± 0.35%; the interspecific variation of protein-coding genes between A. robusta and A. hwangtsiyui ranged from 20.5 ± 0.9% (cox1) to 30.7 ± 1.2% (nad5). The observed gene arrangement in the mtDNA sequence of A. robusta was identical to that of A. hwangtsiyui. Codon usage and amino acid frequencies were highly similar between A. robusta and A. hwangtsiyui. The results of phylogenetic analyses based on mtDNA protein-coding regions showed that A. robusta is closely related to A. hwangtsiyui (belonging to the same suborder, Azygiida) that formed a distinct early-diverging branch relative to all other Digenea. A preliminary morphological analysis of paratypes of the two azygiid specimens studied showed visible morphological differences between them. The specimen extracted from Sakhalin taimen (Parahucho perryi) was most similar to A. robusta. Thus, we here provide the first record of a new definitive host, P. perryi, for A. robusta and also molecular characteristics of the trematode specimens.

Characterization of complete mitochondrial genome and ribosomal operon for Carassotrema koreanum Park, 1938 (Digenea: Haploporidae) by means of next-generation sequencing data

We obtained new data on the complete mitochondrial DNA (mtDNA) and the ribosomal operon of the trematode Carassotrema koreanum (Digenea: Haploporata: Haploporidae), an intestinal parasite of Carassius auratus, using next-generation sequencing. The mtDNA of C. koreanum contained 13,965 bp, including 12 protein-coding genes, two ribosomal genes, 22 transport RNA (tRNA) genes and a non-coding region. The ribosomal operon of C. koreanum was 10,644 bp in length, including ETS1 (1449 bp), 18S ribosomal RNA (rRNA) gene (1988 bp), ITS1 ribosomal DNA (rDNA) (558 bp), 5.8S rRNA gene (157 bp), ITS2 rDNA (274 bp), 28S rRNA gene (4152 bp) and ETS2 (2066 bp). Phylogenetic analysis based on mtDNA protein-coding regions showed that C. koreanum was closely related to Parasaccocoelium mugili, a species from the same suborder Haploporata. Bayesian phylogenetic tree topology was the most reliable and confirmed the validity of the Haploporata. The results of sequence cluster analysis based on codon usage bias demonstrated some agreement with the results of the phylogenetic analysis. In particular, Schistosoma spp. were differentiated from the other members of Digenea and the members of Pronocephalata were localized within the same cluster. Carassotrema koreanum and P. mugili fell within different clusters. The grouping of C. koreanum and P. mugili within the same cluster was obtained on the basis of frequencies of 13 specified codons, of which three codon pairs were degenerate. A similarity was found between two haploporid species and two Dicrocoelium spp. in the presence of TTG start codon of the mitochondrial nad5 gene. Our results confirmed the taxonomical status of the Haploporata identified in the previous studies and revealed some characteristic features of the codon usage in its representatives.

A new species of hemiuroidean trematode from Hatcheria macraei (Siluriformes, Trichomycteridae) and Heleobia hatcheri (Gastropoda, Cochliopidae) in a Patagonian River

A new hemiuroidean species, Genarchella pichileufuensis n. sp. (Derogenidae: Halipeginae), was found in the stomach of the siluriform freshwater fish, Hatcheria macraei (Girard, 1855), in the Pichileufu River, Patagonia, Argentina. Its rediae with immature cystophorous cercariae were found in the snail Heleobia hatcheri (Pilsbry, 1911) in the same site. The present new species is morphologically featured by having a cyclocoel in the hindbody unlike the other species of the genus. The characteristics of this species allowed us to amend the diagnosis of the genus Genarchella as follows: cyclocoel present or absent; testes symmetrical to tandem; ootype pouch present. In the phylogenetic analysis, G. pichileufuensis forms a well-supported clade with Genarchella spp. recovered from Mexican freshwater fishes. This clade is included in the cluster of representatives of the subfamily Halipeginae. So far, three hemiuroidean species, Thometrema patagonica (Szidat, 1956), Derogenes lacustris Tsuchida, Flores, Viozzi, Rauque et Urabe, 2021 and G. pichileufuensis n. sp., have been reported from freshwater fishes in Argentinean Patagonia.

First next-generation sequencing data for Haploporidae (Digenea: Haploporata): characterization of complete mitochondrial genome and ribosomal operon for Parasaccocoelium mugili Zhukov, 1971

The first data on a whole mitochondrial genome of Haploporidae, Parasaccocoelium mugili (Digenea: Haploporata: Haploporidae) was generated using the next-generation sequencing (NGS) approach. We sequenced the complete mitochondrial DNA (mtDNA) and ribosomal operon of Parasaccocoelium mugili, intestine parasite of mullet fish. The mtDNA of P. mugili contained 14,021 bp, including 12 protein-coding genes, two ribosomal genes, 22 tRNA genes, and non-coding region. The ribosomal operon of P. mugili was 8308 bp in length, including 18S rRNA gene (1981 bp), ITS1 rDNA (955 bp), 5.8S rRNA gene (157 bp), ITS2 rDNA (268 bp), 28S rRNA gene (4180 bp), and ETS (767 bp). We used the mtDNA protein-coding regions to make phylogenetic reconstructions of Haploporidae. Additionally, we performed the sequence cluster analysis based on codon usage bias of most of currently available mitochondrial genome data for trematodes. The observed gene arrangement in mtDNA sequence of P. mugili is identical to those of Plagiorchis maculosus (Rudolphi, 1802). Results of maximum likelihood (ML) phylogenetic analysis showed that P. mugili was closely related to Paragonimus species from the suborder Xiphidiata. The results of sequence cluster analysis based on codon usage bias showed that P. mugili has the highest similarity with Plagiorchis maculosus (Xiphidiata). Our results do not contradict to proposing a new suborder for Haploporoidea-Haploporata. On the basis of obtained results, the relationship between mitochondrial protein-coding gene rearrangements and synonymous nucleotide substitutions in mitochondrial genomes has been suggested.

Complete mitochondrial genome sequence of Lepus yarkandensis Günther, 1875 (Lagomorpha, Leporidae): characterization and phylogenetic analysis

Lepusyarkandensis is a national second-class protected animal endemic to China and distributed only in the hot and arid Tarim Basin in Xinjiang. We sequenced and described the complete mitogenome of L.yarkandensis to analyze its characteristics and phylogeny. The species’ DNA is a 17,047 bp circular molecule that includes 13 protein-coding genes (PCGs), two rRNA genes, 22 tRNA genes, and one control region. The overall base composition was as follows: A, 31.50%; T, 29.40%; G, 13.30% and C, 25.80%, with a high A+T bias of 60.9%. In the PCGs, ND6 had deviation ranges for AT skew (–0.303) and GC skew (0.636). The Ka/Ks values of ND1 (1.067) and ND6 (1.352) genes were >1, indicating positive selection, which might play an important role in the adaptation of L.yarkandensis to arid and hot environments. The conserved sequence block, the central conserved domain, and the extended termination-associated sequences of the control region and their features were identified and described. The phylogenetic tree based on the complete mitogenome showed that L.yarkandensis was closely related to the sympatric Lepustibetanuspamirensis. These novel datasets of L.yarkandensis can supply basic data for phylogenetic studies of Lepus spp., apart from providing essential and important resource for further genetic research and the protection of this species.