Comparative mitogenomics of two parasitic isopods, Ione cornuta and Pseudione longicauda, with putatively linear mitochondrial genomes

Researchers have characterized the mitochondrial genomes of two obligately parasitic isopods, Ione cornuta and Pseudione longicauda , finding compact, A + T-rich sequences with conserved protein-coding gene order and evidence consistent with putatively linear genome organization. The assembled mitochondrial genomes were 14,773 base pairs long in I. cornuta and 14,438 base pairs in P. longicauda . Each genome was assembled as a single contig, and all positions were covered by mapped Illumina reads.

Each also contained 13 protein-coding genes, two ribosomal RNA genes and 21 annotated transfer RNA genes. The evidence for linear organization came from restriction digestion using an enzyme predicted to have a single recognition site. In each species, digestion produced two distinguishable fragments.

On the basis of this result, the researchers conservatively regarded both mitochondrial genomes as putatively linear. The test did not resolve their precise topology or terminal structures, so the authors did not treat linearity as definitively established. Comparative analysis showed that the order of the 13 protein-coding genes was conserved between I. cornuta and P. longicauda , as well as among the sampled closely related epicaridean taxa.

Positional variation was concentrated mainly in transfer RNA genes rather than protein-coding genes. Phylogenetic analyses placed P. longicauda as the sister species of Gyge ovalis , with that clade further grouped with Epipenaeon fissurae . In another inferred relationship, I. cornuta formed a sister association with Pleurocryptella shinkai .

The study also estimated protein-coding gene evolution using pairwise nonsynonymous-to-synonymous substitution ratios, known as Ka/Ks. Twelve of the 13 protein-coding genes had Ka/Ks estimates below 1, while atp8 had an elevated estimate of 3.35. Because atp8 was short in the analyzed sequences and the pairwise comparison included only two species, the researchers treated the result as a preliminary indication of accelerated sequence divergence rather than definitive evidence of positive selection.

Taken together, the mitochondrial genome sequences expand the genomic resources available for Bopyroidea and provide comparative information on topology, structural variation, gene arrangement, phylogenetic relationships and protein-coding gene evolution in these parasitic isopods.

Subject of Research: Comparative mitochondrial genomics of two obligately parasitic isopods

Article Title: Comparative mitogenomics of two parasitic isopods, Ione cornuta and Pseudione longicauda, with putatively linear mitochondrial genomes

Article References: Ma, X., Chen, L., Huang, T., Huang, L., Cheng, D., Huang, Y., Pengsakul, T., Hu, Y., Yang, D., & Qiao, Y. (2026). Comparative mitogenomics of two parasitic isopods, Ione cornuta and Pseudione longicauda, with putatively linear mitochondrial genomes. BMC Genomics. https://doi.org/10.1186/s12864-026-13453-9

Image Credits: AI Generated

DOI: 10.1186/s12864-026-13453-9

Keywords: mitochondrial genomics, parasitic isopods, Ione cornuta, Pseudione longicauda, Bopyroidea, genome organization, mitochondrial phylogenetics, transfer RNA genes, protein-coding evolution, Comparative, mitogenomics, parasitic