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Characterization of the complete mitochondrial genomes of Corynosoma bullosum (von Linstow, 1892) and C. evae Zdzitowiecki, 1984 (Acanthocephala: Polymorphida), and the phylogenetic implications

Published online by Cambridge University Press:  10 February 2025

Y.-Y. Xie ,
H.-X. Chen ,
T. A. Kuzmina ,
O. Lisitsyna  and
L. Li Open the ORCID record for L. Li [Opens in a new window]
Y.-Y. Xie
Affiliation:
Hebei Key Laboratory of Animal Physiology, Biochemistry and Molecular Biology; College of Life Sciences, Hebei Normal University, 050024 Shijiazhuang, Hebei Province, P. R. China Hebei Collaborative Innovation Center for Eco‐Environment; 050024 Shijiazhuang, Hebei Province, P. R. China
H.-X. Chen
Affiliation:
Hebei Key Laboratory of Animal Physiology, Biochemistry and Molecular Biology; College of Life Sciences, Hebei Normal University, 050024 Shijiazhuang, Hebei Province, P. R. China Hebei Collaborative Innovation Center for Eco‐Environment; 050024 Shijiazhuang, Hebei Province, P. R. China
T. A. Kuzmina
Affiliation:
I. I. Schmalhausen Institute of Zoology National Academy of Sciences of Ukraine, 15, Bogdan Khmelnytsky Street, Kyiv, 01030, Ukraine Institute of Parasitology, Slovak Academy of Sciences, Hlinkova 3, Kosice, 04001, Slovakia
O. Lisitsyna
Affiliation:
I. I. Schmalhausen Institute of Zoology National Academy of Sciences of Ukraine, 15, Bogdan Khmelnytsky Street, Kyiv, 01030, Ukraine
L. Li*
Affiliation:
Hebei Key Laboratory of Animal Physiology, Biochemistry and Molecular Biology; College of Life Sciences, Hebei Normal University, 050024 Shijiazhuang, Hebei Province, P. R. China Hebei Collaborative Innovation Center for Eco‐Environment; 050024 Shijiazhuang, Hebei Province, P. R. China
*
Corresponding author: L. Li; Email: liangliangex369@126.com
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Abstract

Species of the genus Corynosoma (Acanthocephala: Polymorphida) mainly parasitize marine mammals and rarely marine birds, and are of veterinary and medical importance due to causing corynosomiasis in wildlife and humans. However, the current knowledge of the mitochondrial genomes and mitogenomic phylogeny of this group remains very insufficient. In the present study, the complete mitochondrial genomes of C. bullosum (von Linstow, 1892) and C. evae Zdzitowiecki, 1984 were sequenced and annotated for the first time. Both mitogenomes comprise 12 protein-coding genes (missing atp8), 22 tRNA genes, and 2 ribosomal RNAs (rrnS and rrnL), plus 2 non-coding regions (NCR1 and NCR2). Corynosoma bullosum has the largest mitogenome (14,879 bp) of any polymorphid species reported so far, while C. evae has the smallest (13,947 bp), except for Sphaerirostris lanceoides (Petrochenko, 1949). Comparative mitogenomic analysis also revealed the presence of distinct discrepancies in A + T content and gene rearrangement across the families Polymorphidae, Centrorhynchidae, and Plagiorhynchidae. Moreover, phylogenetic analyses based on the concatenated amino acid sequences of 12 protein-coding genes strongly supported the monophyly of the order Polymorphida and a close affinity between the families Polymorphidae and Centrorhynchidae in Polymorphida. The present mitogenomic phylogeny provides additional evidence for a sister relationship between the genera Corynosoma and Bolbosoma and demonstrated that C. evae has a closer relationship with C. villosum than C. bullosum in the genus Corynosoma.

Keywords

AcanthocephalaPolymorphidaeCorynosomamitochondrial genomephylogeny

Information

Type
Research Paper
Information
Journal of Helminthology , Volume 99 , 2025 , e22
Copyright
© The Author(s), 2025. Published by Cambridge University Press

Introduction

The phylum Acanthocephala is an important group of zooparasites, with over 1,300 species occurring in the alimentary canal of all major vertebrate groups worldwide (Amin Reference Amin2013; Nickol Reference Nickol, Crompton and Nickol1985; Perrot-Minnot et al. Reference Perrot-Minnot, Cozzarolo, Amin, Barčák, Bauer, Filipović Marijić, García-Varela, Servando Hernández-Orts, Yen Le, Nachev, Orosová, Rigaud, Šariri, Wattier, Reyda and Sures2023; Petrochenko Reference Petrochenko1956; Yamaguti Reference Yamaguti1963). The Polymorphidae is the largest family in the order Polymorphida, comprising 12 genera and more than 120 described species mainly parasitizing marine mammals, fish-eating marine birds, and waterfowls (Amin Reference Amin2013; Aznar et al. Reference Aznar, de León and Raga2006; García-Varela et al. Reference García-Varela, de León, Aznar and Nadler2011, Reference García-Varela, Pérez-Ponce De León, Aznar and Nadler2013; Hernández-Orts et al. Reference Hernández-Orts, Brandão, Georgieva, Raga, Crespo, Luque and Aznar2017; Laskowski and Zdzitowiecki Reference Laskowski, Zdzitowiecki, Klimpel, Kuhn and Mehlhorn2017; Ru et al. Reference Ru, Yang, Chen, Kuzmina, Spraker and Li2022). In the family Polymorphidae, species of the genus Corynosoma are of veterinary and medical importance due to causing corynosomiasis in wildlife and humans (Kennedy Reference Kennedy2006; Mathison et al. Reference Mathison, Mehta and Couturier2021). To date, there have been at least 4 cases of human corynosomiasis caused by C. villosum van Cleave, 1953; C. validum van Cleave, 1953; C. strumosum (Rudolphi, Reference Rudolphi1802); and Corynosoma sp. (Fujita et al. Reference Fujita, Waga, Kitaoka, Imagawa, Komatsu, Takanashi, Anbo, Anbo, Katuki, Ichihara, Fujimori, Yamasaki, Morishima, Sugiyama and Katahira2016; Schmidt Reference Schmidt1971; Takahashi et al. Reference Takahashi, Ito, Sato, Goto, Kawamoto, Fujinaga, Yanagawa, Saito, Nakao, Hasegawa and Fujiya2016; Yagi et al. Reference Yagi, Ohbuchi, Uraguchi, Akao, Sakai, Nonaka and Oku2002).

Recently, many mitochondrial genomes of acanthocephalans have been reported, which were also successfully used for investigating the phylogenetic relationships of the higher taxa in Acanthocephala (García-Gallego et al. Reference García-Gallego, Raga, Fraija-Fernández and Aznar2023; Gazi et al. Reference Gazi, Sultana, Min, Park, García-Varela, Nadler and Park2012, Reference Gazi, Kim and Park2015; Li et al. Reference Li, Yang, Chen, Kuzmina, Spraker and Li2024; Sarwar et al. Reference Sarwar, Zhao, Kibet, Sitko and Nie2021; Xie et al. Reference Xie, Chen, Kuzmina, Lisitsyna and Li2024; Zhao et al. Reference Zhao, Yang, Lü, Ru, Wayland, Chen, Li and Li2023). However, the current knowledge of the mitochondrial genomes for the polymorphid acanthocephalans remains insufficient. To date, a total of 7 species belonging to 4 genera (Bolbosoma, Corynosoma, Polymorphus, Southwellina) in the Polymorphidae have had their mitochondrial genomes sequenced (García-Gallego et al. Reference García-Gallego, Raga, Fraija-Fernández and Aznar2023; Gazi et al. Reference Gazi, Kim and Park2015; Li et al. Reference Li, Yang, Chen, Kuzmina, Spraker and Li2024; Sarwar et al. Reference Sarwar, Zhao, Kibet, Sitko and Nie2021). In the genus Corynosoma, the only species to have had its mitochondrial genome sequenced is C. villosum (Li et al. Reference Li, Yang, Chen, Kuzmina, Spraker and Li2024).

In the present study, in order to enrich the mitogenomic data of the genus Corynosoma, the complete mitochondrial genomes of C. bullosum (von Linstow, 1892) and C. evae Zdzitowiecki, 1984 were sequenced and annotated for the first time. Moreover, phylogenetic analyses were performed based on the concatenated amino acid sequences of the 12 protein-coding genes (PCGs) of mitogenomes of Acanthocephala to further clarify the evolutionary relationships of the three families within the order Polymorphida and the phylogenetic position of the genus Corynosoma in the Polymorphidae.

Material and methods

Collection of acanthocephalans

Acanthocephalans of C. bullosum and C. evae were collected from the intestine of Antarctic black rockcod Notothenia coriiceps Richardson (Perciformes: Nototheniidae) off Galindez Island, Argentine Islands, West Antarctica during the 19th Ukrainian Antarctic expedition to the Ukrainian Antarctic Station (UAS) ‘Akademik Vernadsky’ in April 2014–February 2015. The acanthocephalans were collected manually, washed in distilled water for 1–3 hours to evaginate the proboscis if needed, and fixed in 70% ethanol for further study. Later, all acanthocephalans were identified on the basis of morphological characters, and voucher specimens of C. bullosum and C. evae were deposited in the I. I. Schmalhausen Institute of Zoology, National Academy of Sciences of Ukraine, Kyiv, Ukraine (NC-49-2014-Acanth; NC-63-2014-Acanth; NC-72-2014-Acanth) and the College of Life Sciences, Hebei Normal University, Hebei Province, China (HBNU-A-F20241110CL).

Mitochondrial genome sequencing, assembly, and annotation

For mitogenome sequencing, the genomic DNA was extracted using a modified CTAB (pH 8.0)-based DNA extraction protocol from single cystacanth female for each species of Corynosoma bullosum and C. evae. The genomic DNA library was constructed, and a total of 50 GB of clean data for each sample of C. bullosum and C. evae were generated using the Pair-End 150 sequencing method on the Illumina NovaSeq 6000 platform by Novogene (Tianjin, China). The mitochondrial genomes were assembled using GetOrganelle v1.7.2a (Jin et al. Reference Jin, Yu, Yang, Song, dePamphilis, Yi and Li2020). Annotation of the mitochondrial genomes, including the protein-coding genes (PCGs), transfer RNA (tRNA), and ribosomal RNA (rRNA), was performed using the MitoS web server (http://mitos.bioinf.uni-leipzig.de/index.py) and MitoZ v3.6 (Meng et al. Reference Meng, Li, Yang and Liu2019). The open reading frames (ORFs) of the PCGs were manually confirmed using the ORF finder (https://www.ncbi.nlm.nih.gov/orffinder/) with the invertebrate mitochondrial genetic code chosen and were identified by comparison with the reported mitochondrial genomes of acanthocephalans. The ‘lost’ tRNA genes ignored by both MitoS and MitoZ were identified using BLAST based on the database of the existing tRNA sequences of Acanthocephala. The secondary structures of tRNAs were predicted using the ViennaRNA module (Gruber et al. Reference Gruber, Bernhart and Lorenz2015), with corrections using MitoS2 (Bernt et al. Reference Bernt, Merkle, Ramsch, Fritzsch, Perseke, Bernhard, Schlegel, Stadler and Middendorf2007) and RNA structure v6.3 (Reuter et al. Reference Reuter and Mathews2010). The CGView online server V1.0 (http://stothard.afns.ualberta.ca/cgview_server/) was utilized for visualizing gene organization features. Base composition, amino acid usage, and relative synonymous codon usage (RSCU) were calculated using a Python script (Supplementary file 1), with reference to the codon adaptation index (CAI) (Cock et al. Reference Cock, Antao, Chang, Chapman, Cox, Dalke, Friedberg, Hamelryck, Kauff, Wilczynski and de Hoon2009). Strand asymmetry was assessed using the formulae: AT skew = (A − T) / (A + T); GC skew = (G − C) / (G + C). The complete mitochondrial genomes of C. bullosum and C. evae obtained in the present study have been deposited in the GenBank database (https://www.ncbi.nlm.nih.gov) under the accession numbers PQ516697 (C. bullosum) and PQ516696 (C. evae).

Phylogenetic analyses

Phylogenetic analyses were conducted based on the concatenated amino acid (AA) sequences of the 12 PCGs of mitochondrial genomes of Acanthocephala using maximum likelihood (ML) and Bayesian inference (BI) methods, respectively. Rotaria rotatoria and Philodina citrina (Rotifera: Bdelloidea) were chosen as the out-group. The in-group included 42 acanthocephalan species belonging to 15 families. Detailed information on the representatives of Acanthocephala included in the present phylogeny was provided in Table 1. Fasta files with the amino acid sequences for all 12 PCGs were extracted from the GenBank files by PhyloSuite v1.2.3 (Zhang et al. Reference Zhang, Gao, Jakovlić, Zou, Zhang, Li and Wang2020). The alignment of extracted amino acid sequences was done using MAFFT v7.313 under the iterative refinement method of E-INS-I (Katoh and Standley Reference Katoh and Standley2013). For Bayesian inference, the VT+F+I+G4 model was identified as the best-fit substitution model according to the Bayesian Information Criterion (BIC) using ModelFinder (Kalyaanamoorthy et al. Reference Kalyaanamoorthy, Minh, Wong, Von Haeseler and Jermiin2017). Bayesian inference used MrBayes v3.2, which was run for 5 × 106 MCMC generations. The maximum likelihood (ML) inference was performed using IQTREE v2.1.2, while the other parameters were kept at default (Golombek et al. Reference Golombek, Tobergte and Struck2015; Minh et al. Reference Minh, Schmidt, Chernomor, Schrempf, Woodhams, Von Haeseler and Lanfear2020). The mtInv+F+I+I+R6 model was identified as the best-fit substitution model according to the Akaike Information Criterion (AIC) using ModelFinder. Node support in ML inference was evaluated using 1,000 bootstrap (BS) replicates. The phylogenetic trees were visualized using iTOL v6.1.1 (Letunic and Bork Reference Letunic and Bork2021).

Table 1.

Detailed information on the representatives of Acanthocephala included in the present phylogeny

Results

General characterization of the complete mitogenomes

The complete mitogenomes of C. bullosum and C. evae are 14,879 and 13,947 bp long, respectively, and both contain 36 genes, including 12 PCGs (cox1–3, nad1–6, nad4L, cytb, and atp6; missing atp8), 22 transfer RNAs (tRNAs), and 2 ribosomal RNAs (rrnS is 584 bp in C. bullosum vs, 583 bp in C. evae, both located between trnM and trnF; rrnL is 919 bp in C. bullosum vs. 915 bp in C. evae, both located between trnY and trnL1), plus 2 non-coding regions (NCR1 is 1,312 bp in C. bullosum vs. 373 bp in C. evae, both located between trnW and trnV; NCR2 is 318 bp in C. bullosum vs. 324 bp in C. evae, both located between trnI and trnM) (Figure 1, Table 2). All genes of the mitogenomes of C. bullosum and C. evae are transcribed from the same DNA strand in the same direction. The nucleotide composition of C. bullosum and C. evae mitogenomes is biased toward A and T (Table 3), with an overall A + T content of 63.8% for C. bullosum (23.8% A, 40.0% T, 29.0% G, and 7.20% C) and 61.6% for C. evae (21.3% A, 40.3% T, 31.0% G, and 7.40% C). The nucleotide composition of C. bullosum and C. evae exhibited distinct skew away from A/C in favor of T/G (C. bullosum: AT skew=−0.25, GC skew=0.60; C. evae: AT skew = −0.31, GC skew = 0.61).

Figure 1.

Gene maps of the mitochondrial genomes of Corynosoma bullosum and C. evae. All 22 tRNA genes are nominated by the one-letter code with numbers differentiating each of the two tRNAs, serine and leucine. All genes are transcribed in the clockwise direction on the same strand. The green circle shows GC skew, and the black circle shows GC content.

Figure 2.

Relative synonymous codon usage (RSCU) of Corynosoma bullosum and C. evae. The codon families (in alphabetical order) were labelled on the x-axis. Values on the top of each bar representing amino acid usage in percentage.

The 12 PCGs in the mitogenomes of C. bullosum and C. evae have 10,418 bp and 10,389 bp (excluding termination codons) encoded 3,472 and 3,462 amino acids, respectively (The inconsistent base pair-amino acid relations are probably due to overlapping ORFs). The size of each 12 PCGs in the mitogenomes of C. bullosum and C. evae is provided in Table 2. Among the 12 PCGs of C. bullosum, 9 genes (cox1, cox2, cox3, nad1, nad2, nad3, nad4L, nad5, and cytb) used GTG as the start codon, while 3 genes (atp6, nad4, and nad6) used ATG. There are 5 genes (cox2, cox3, cytb, nad2, and nad4L) used TAA as the termination codon, and 5 genes (cox1, atp6, nad1, nad4, and nad5) used TAG as the termination codon. The remaining 2 genes (nad3 and nad6) are inferred to terminate with incomplete stop codon T (Table 2). Among the 12 PCGs of C. evae, 9 genes (cox1, cox2, cox3, nad1, nad2, nad5, nad6, atp6, and cytb) used GTG as the start codon, while 3 genes (nad3, nad4, and nad4L) used ATG. There are 5 genes (cox1, cox2, cox3, atp6, and nad4L) used TAA as the termination codon, and 4 genes (nad1, nad2, nad5, and cytb) used TAG as the termination codon. The remaining 3 genes (nad3, nad4, and nad6) are inferred to terminate with incomplete stop codon T (Table 2). The codon usage in the mitogenomes of C. bullosum and C. evae is presented in Figure 2.

Table 2.

Organization of the mitochondrial genomes of Corynosoma bullosum and C. evae. ‘Ini/Ter cod’ and ‘Int seq’ indicates initial/terminal codons and the length of intergenic sequences, respectively

Table 3.

Base composition and skewness in the mitogenomes of Corynosoma bullosum and C. evae

In the mitogenomes of C. bullosum and C. evae, a total of 22 tRNAs were identified, ranging from 52 bases to 61 bases in size (Table 2). Among the 22 tRNAs of C. bullosum and C. evae, four tRNAs (trnR, trnN, trnW, and trnV) were predicted to be folded into a typical cloverleaf secondary structure. Two tRNAs (trnQ and trnS1) lack (DHU) arm. The remaining tRNAs lack a TψC (T) arm (Figures 3 and 4).

Figure 3.

The predicted secondary structures of 22 tRNAs in the mitogenome of Corynosoma bullosum (Watson-Crick bonds indicated by lines, GU bonds indicated by dots, grey bold bases representing anticodons). The tRNAs were labelled with the abbreviations of their corresponding amino acids according to the IUPAC-IUB code.

Figure 4.

The predicted secondary structures of 22 tRNAs in the mitogenome of Corynosoma evae (Watson-Crick bonds indicated by lines, GU bonds indicated by dots, grey bold bases representing anticodons). The tRNAs were labelled with the abbreviations of their corresponding amino acids according to the IUPAC-IUB code.

The 36 gene arrangement in the mitogenomes of C. bullosum and C. evae were identical, both in the following order: cox1, trnG, trnQ, trnY, rrnL, trnL1, nad6, trnD, atp6, nad3, trnW, trnV, trnK, trnE, trnT, trnS2, nad4L, nad4, trnH, nad5, trnL2, trnP, cytb, nad1, trnI, trnM, rrnS, trnF, cox2, trnC, cox3, trnA, trnR, trnN, trnS1, and nad2 (Figures 1 and 5).

Figure 5.

Comparison of the linearized mitochondrial genome arrangement for acanthocephalans species. All genes are transcribed in the same direction from left to right. The tRNAs were labelled by a single-letter code for the corresponding amino acid. Corynosoma bullosum and C. evae were indicated using asterisk (*).

Phylogenetic analyses

The BI and ML trees are nearly identical in topology, both showing that the Acanthocephala comprise three major monophyletic clades (clade I, II, and III) (Figure 6). Clade I, comprising species of the orders Oligacanthorhynchida and Moniliformida, represents the class Archiacanthocephala. Clade II includes the representatives of the orders Gyracanthocephala and Neoechinorhynchida of the class Eoacanthocephala, together with the class Polyacanthocephala. In clade II, both the families Quadrigyridae and Neoechinorhynchidae do not appear monophyletic due to the representatives of Polyacanthocephala, Quadrigyridae, Tenuisentidae, and Neoechinorhynchidae mixed together. Clade III contains species of the orders Echinorhynchida and Polymorphida, representing the class Palaeacanthocephala. In the clade III, the family Polymorphidae showed a sister relationship with the Centrorhynchidae. Moreover, the representative of Corynosoma clustered together with species of Bolbosoma in the Polymorphidae, and C. evae displayed a closer relationship with C. villosum than C. bullosum in Corynosoma.

Figure 6.

Phylogenetic analyses of Acanthocephala inferred from ML and BI methods based on concatenated amino acid sequences of 12 protein-coding genes (PCGs) of mitochondrial genome. Rotaria rotatoria and Philodina citrina were treated as the out-group. Bootstrap values ≥ 70 and Bayesian posterior probabilities values ≥ 0.90 were shown in the phylogenetic trees. Corynosoma bullosum and C. evae are indicated using asterisk (*).

Discussion

In the order Polymorphida, only 13 species representing 7 genera, belonging to 3 different families, have had their mitogenomes sequenced (García-Gallego et al. Reference García-Gallego, Raga, Fraija-Fernández and Aznar2023; Gazi et al. Reference Gazi, Kim and Park2015, Reference Gazi, Kim, Park, Littlewood and Park2016; Li et al. Reference Li, Yang, Chen, Kuzmina, Spraker and Li2024; Muhammad et al. Reference Muhammad, Suleman, Ma, Khan, Li, Zhao, Ahmad and Zhu2019a, Reference Muhammad, Suleman, Ma, Khan, Wu, Zhu and Li2019b, 2020b, 2020c; Sarwar et al. Reference Sarwar, Zhao, Kibet, Sitko and Nie2021), and over 70% genera in Polymorphida still have all representatives with unknown mitogenomic data. In the present study, the complete mitochondrial genomes of C. bullosum and C. evae were provided for the first time.

Comparative mitochondrial genomic analysis indicated that the size of the mitogenome of C. bullosum (14,879 bp) is larger than those of all species of the three classes Archiacanthocephala, Eoacanthocephala, and Polyacanthocephala (13,269–14,282 bp) (see Table 1 for details). In the class Palaeacanthocephala, the mitogenome of C. bullosum is also larger than those of echinorhynchid species with the exception of Micracanthorhynchina dakusuiensis (16,309 bp) and Pseudoacanthocephalus sp. (14,883 bp) (Gao et al. Reference Gao, Yuan, Wu, Xiang, Xie, Song, Chen, Wu and Ou2022), and other polymorphid species (14,241–14,742 bp) reported so far (García-Gallego et al. Reference García-Gallego, Raga, Fraija-Fernández and Aznar2023; Gazi et al. Reference Gazi, Kim and Park2015; Li et al. Reference Li, Yang, Chen, Kuzmina, Spraker and Li2024; Sarwar et al. Reference Sarwar, Zhao, Kibet, Sitko and Nie2021), but smaller than that of Plagiorhynchus transversus (15,477 bp) belonging to the family Plagiorhynchidae, and some species of the family Centrorhynchidae (i.e., Centrorhynchus clitorideus, C. aluconis, and Sphaerirostris picae) (15,144–15,844 bp). Conversely, C. evae has the smallest mitogenome of any polymorphid species reported so far, with the exception of the centrorhynchid species Sphaerirostris lanceoides (13,478 bp) (Muhammad et al. Reference Muhammad, Suleman, Ahmad, Li, Zhao, Ullah, Zhu and Ma2020b).

The composition and 36 gene rearrangement of C. bullosum and C. evae are identical to all of the known mitogenomes of the Polymorphidae, except for Southwellina hispida [some tRNA gene rearrangement events occurred in the mitogenome of this species] (Figure 5) (Gazi et al. Reference Gazi, Kim and Park2015). The overall A + T content in the mitogenomes of C. bullosum and C. evae are also similar to that of the known polymorphid mitogenomes, all displaying a strong A+T bias (García-Gallego et al. Reference García-Gallego, Raga, Fraija-Fernández and Aznar2023; Gazi et al. Reference Gazi, Kim and Park2015; Li et al. Reference Li, Yang, Chen, Kuzmina, Spraker and Li2024; Sarwar et al. Reference Sarwar, Zhao, Kibet, Sitko and Nie2021). However, comparative mitogenomic analysis revealed the presence of significant discrepancy in A + T content and gene rearrangement across the families Polymorphidae, Centrorhynchidae, and Plagiorhynchidae within Polymorphida. The overall A + T content in the mitogenomes of Polymorphidae and Plagiorhynchidae (60.9–64.4%) is distinctly higher than that of Centrorhynchidae (vs 54.5–58.1%). Moreover, the arrangement of the 36 genes in the mitogenomes of the Centrorhynchidae and Plagiorhynchidae is coincident but different from that of the Polymorphidae (Figure 5).

Phylogenetic relationships of three orders Heteramorphida, Echinorhynchida, and Polymorphida within the class Palaeacanthocephala, and the systematic position of some families within Echinorhynchida, are still not well resolved due to the scarcity and inaccessibility of suitable material or genetic data for some taxa. In contrast, the molecular phylogenetics of Polymorphida have been relatively well studied. Some previous studies supported the monophyly of Polymorphida and revealed the phylogenetic relationships among its included three families Plagiorhynchidae, Centrorhynchidae, and Polymorphidae (Dai et al. Reference Dai, Yan, Li, Zhang, Liu, Gao, Ohiolei, Wu, Guo, Fu and Jia2022; Gao et al. Reference Gao, Yuan, Wu, Xiang, Xie, Song, Chen, Wu and Ou2022; Gazi et al. Reference Gazi, Kim, Park, Littlewood and Park2016; Li et al. Reference Li, Yang, Chen, Kuzmina, Spraker and Li2024; Muhammad et al. Reference Muhammad, Suleman, Ma, Khan, Li, Zhao, Ahmad and Zhu2019a, Reference Muhammad, Suleman, Ma, Khan, Wu, Zhu and Li2019b; Song et al. Reference Song, Zhang, Gao, Cheng, Xie, Li and Wu2019; Xie et al. Reference Xie, Chen, Kuzmina, Lisitsyna and Li2024; Zhao et al. Reference Zhao, Yang, Lü, Ru, Wayland, Chen, Li and Li2023). The present mitogenomic phylogenetic results agreed well with these previous studies and provide further evidence for a sister relationship between the Polymorphidae and Centrorhynchidae (Dai et al. Reference Dai, Yan, Li, Zhang, Liu, Gao, Ohiolei, Wu, Guo, Fu and Jia2022; Gao et al. Reference Gao, Yuan, Wu, Xiang, Xie, Song, Chen, Wu and Ou2022; Gazi et al. Reference Gazi, Kim, Park, Littlewood and Park2016; Muhammad et al. Reference Muhammad, Suleman, Ma, Khan, Li, Zhao, Ahmad and Zhu2019a, Reference Muhammad, Suleman, Ma, Khan, Wu, Zhu and Li2019b; Song et al. Reference Song, Zhang, Gao, Cheng, Xie, Li and Wu2019; Zhao et al. Reference Zhao, Yang, Lü, Ru, Wayland, Chen, Li and Li2023). Our phylogenetic results also suggested a close affinity between Bolbosoma and Corynosoma, both being parasites of marine mammals, which are accordant with the previous phylogenies (García-Varela et al. Reference García-Varela, Pérez-Ponce De León, Aznar and Nadler2013; Presswell et al. Reference Presswell, García-Varela and Smales2018; Ru et al. Reference Ru, Yang, Chen, Kuzmina, Spraker and Li2022).

Supplementary material

The supplementary material for this article can be found at http://doi.org/10.1017/S0022149X2500001X.

Authors contribution

Y.-Y.X., H.-X.C., and L.L. contributed to the study design, annotated mitogenome, and conducted the phylogenetic analyses. T.A.K. and O.L. collected and identified acanthocephalan specimens. Y.-Y.X. and L.L. wrote the manuscript. All authors read and approved the final manuscript.

Financial support

This study was supported by the National Natural Science Foundation of China (Grant No. 31872197) and the National Key R&D Program of China (Grant No. 2022YFC2601200). The participation of Olga Lisitsyna and Tetiana Kuzmina in this study was partially supported by the National Antarctic Scientific Center, Ministry of Education and Science of Ukraine (project number H/01-2024).

Competing interest

The authors declare that they have no competing interests.

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Figure 0

Table 1. Detailed information on the representatives of Acanthocephala included in the present phylogeny

Figure 1

Figure 1. Gene maps of the mitochondrial genomes of Corynosoma bullosum and C. evae. All 22 tRNA genes are nominated by the one-letter code with numbers differentiating each of the two tRNAs, serine and leucine. All genes are transcribed in the clockwise direction on the same strand. The green circle shows GC skew, and the black circle shows GC content.

Figure 2

Figure 2. Relative synonymous codon usage (RSCU) of Corynosoma bullosum and C. evae. The codon families (in alphabetical order) were labelled on the x-axis. Values on the top of each bar representing amino acid usage in percentage.

Figure 3

Table 2. Organization of the mitochondrial genomes of Corynosoma bullosum and C. evae. ‘Ini/Ter cod’ and ‘Int seq’ indicates initial/terminal codons and the length of intergenic sequences, respectively

Figure 4

Table 3. Base composition and skewness in the mitogenomes of Corynosoma bullosum and C. evae

Figure 5

Figure 3. The predicted secondary structures of 22 tRNAs in the mitogenome of Corynosoma bullosum (Watson-Crick bonds indicated by lines, GU bonds indicated by dots, grey bold bases representing anticodons). The tRNAs were labelled with the abbreviations of their corresponding amino acids according to the IUPAC-IUB code.

Figure 6

Figure 4. The predicted secondary structures of 22 tRNAs in the mitogenome of Corynosoma evae (Watson-Crick bonds indicated by lines, GU bonds indicated by dots, grey bold bases representing anticodons). The tRNAs were labelled with the abbreviations of their corresponding amino acids according to the IUPAC-IUB code.

Figure 7

Figure 5. Comparison of the linearized mitochondrial genome arrangement for acanthocephalans species. All genes are transcribed in the same direction from left to right. The tRNAs were labelled by a single-letter code for the corresponding amino acid. Corynosoma bullosum and C. evae were indicated using asterisk (*).

Figure 8

Figure 6. Phylogenetic analyses of Acanthocephala inferred from ML and BI methods based on concatenated amino acid sequences of 12 protein-coding genes (PCGs) of mitochondrial genome. Rotaria rotatoria and Philodina citrina were treated as the out-group. Bootstrap values ≥ 70 and Bayesian posterior probabilities values ≥ 0.90 were shown in the phylogenetic trees. Corynosoma bullosum and C. evae are indicated using asterisk (*).

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