Potential cryptic speciation in the Japanese populations of the pelagic tunicate, Oikopleura dioica

Oikopleura dioica is a small, pelagic tunicate that lives inside an intricate, filter-feeding, cellulose structure known as a house. It is commonly reported to inhabit the neritic zone of all oceans. Despite its ubiquitous distribution, it is classified as one single species based on morphological features. However, considering some of its biological characteristics such as a short life span (approximately 5 days at 20°C in laboratory), limited mobility (i.e., dispersal dependent on local currents and tides), and lack of the non-homologous end-joining pathway, its genome may be rapidly evolving, which may in turn lead to widespread speciation events. Our crossing experiment of two different laboratory strains of O. dioica in Japan, Okinawa and Osaka, revealed fertilization incompetence. We observed differences in temperature preference in culture. Comparison of COX1 sequences between these two populations showed only 88.3% identity. Moreover, whole genome sequencing revealed significant structural differences at genomic level. In order to further investigate population-wise variabilities in genetic structures, O. dioica was sampled from 6 different regions on both sides of the Kuroshio current along the Ryukyu Archipelago and Southern Japan during the summer of 2019. Genomic DNA was extracted from a single individual using a modified salting-out method, and later sequenced with ligation kit using Nanopore technology. Although O. dioica maintains strong morphological similarities among populations worldwide, further investigation is necessary for detailed morphological comparison between biologically and genetically distinct populations. Combined investigation of O. dioica from multiple aspects in biology will help us grasp the scope of complexity in community structure, biological as well as genetic diversity of this ecologically important pelagic tunicate, which once thought was a single species.