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

Bopyroidea comprises a diverse group of obligate parasitic isopods, but available mitochondrial genomic data remain limited. This study characterized the mitochondrial genomes of Ione cornuta and Pseudione longicauda and examined their genome organization, phylogenetic positions, and protein-coding gene evolution. The mitochondrial genomes of I. cornuta and P. longicauda were 14,773 and 14,438 bp in length, respectively. Each was assembled as a single contig, and all positions were covered by mapped Illumina reads. Restriction digestion using an enzyme predicted to have a single recognition site produced two distinguishable fragments in each species. Based on the available evidence, both mitogenomes were conservatively regarded as putatively linear, although their precise topology and terminal structures remain unresolved. Both genomes were compact and A + T-rich and contained 13 protein-coding genes, two rRNA genes, and 21 annotated tRNA genes. Several tRNAs exhibited predicted noncanonical secondary structures. The order of the 13 protein-coding genes was conserved between the two species and among the sampled closely related epicaridean taxa, whereas positional variation was mainly concentrated in tRNA genes. Phylogenetic analyses placed P. longicauda as the sister species of Gyge ovalis , with this clade further grouped with Epipenaeon fissurae , whereas I. cornuta formed a sister relationship with Pleurocryptella shinkai . Pairwise Ka/Ks analysis showed that 12 protein-coding genes had ω estimates below 1, whereas atp8 had an elevated estimate of 3.35. Given the short length of atp8 and the comparison of only two species, this result was interpreted as a preliminary indication of accelerated sequence divergence rather than definitive evidence of positive selection. These mitochondrial genomes expand the genomic resources available for Bopyroidea and provide comparative information on mitochondrial genome topology, structural variation, gene arrangement, phylogenetic relationships, and protein-coding gene evolution in parasitic isopods.