Collembola are minute, wingless hexapods that inhabit soil and surface litter environments. Owing to their limited dispersal capacity, populations on oceanic islands are likely to experience geographic isolation, promoting genetic structuring and potentially lineage divergence. We investigated patterns of genetic differentiation and phylogenetic relationships in the surface-dwelling species Tomocerus cf. ocreatus across the Ryukyu Archipelago of southwestern Japan.
To assess population structure, we analyzed six mitochondrial and nuclear loci from specimens collected on multiple islands spanning approximately 800 km, from Yonaguni Island to Amami-Oshima. Phylogenetic reconstruction showed that the Ryukyu populations form a well-supported monophyletic lineage that is sister to a mainland Chinese clade comprising T. qinae and T. pseudocreatus. Levels of sequence divergence among islands suggest that the Ryukyu populations may represent two or three independently evolving lineages. Furthermore, genetic distances at individual loci were positively correlated with geographic separation among sampling sites.
Complete mitochondrial genomes were additionally obtained from two populations on Okinawa Island, sampled from its northern and southern regions. Each mitogenome comprised the typical 37 genes (13 protein-coding genes, 2 rRNA genes, and 22 tRNA genes). Comparative analyses revealed mean sequence identities of 92.5% in protein-coding regions, 96.0% in rRNA genes, and 98.0% in tRNA genes, indicating substantial intraspecific divergence even within a single island. This high level of mitochondrial variability suggests that the cytochrome c oxidase subunit I (COI) gene, widely used for DNA barcoding in animals, may have limited resolution for delimiting conspecific populations in this taxon.
By the time of presentation, additional sequence data from a more northern island (Yakushima) will be incorporated to further evaluate latitudinal patterns of diversification. Overall, our results demonstrate pronounced genetic structuring both within and among islands in the Ryukyu Archipelago, consistent with ongoing or historical processes of species differentiation in this surface-dwelling collembolan.