Molecular phylogeny implies exclusive, repetitive, convergent establishment of cyanobacterial symbiosis in didemnid ascidians (Tunicata: Ascidiacea: Didemnidae).

Despite the great diversity of the phylum Chordata, lifelong obligate symbiosis with cyanobacteria is only known in certain species belonging to four genera of didemnid ascidians: Didemnum, Diplosoma, Lissoclinum, and Trididemnum. This photosymbiosis in didemnid ascidians exhibits diversity in photosymbiont taxa, symbiont location in the host ascidian, and modes of vertical transmission of symbionts. While the family Didemnidae comprises over 600 colonial species, approximately 30 symbiotic species are known from tropical waters, alongside many non-symbiotic congeners. This study aimed to elucidate the evolutionary history of this unique photosymbiosis in the family Didemnidae by constructing molecular phylogenetic trees based on the partial sequences of two regions in the mitochondrial cytochrome c oxidase subunit I gene (COI) and mapping the character states of photosymbiosis in each ascidian species harboring obligate photosymbionts onto the trees. The molecular phylogenetic trees generally supported the monophyly of Didemnum, Diplosoma, and Lissoclinum, but not that of Trididemnum. In contrast, the phylogenetic tree did not support the monophyly of the following character states of obligate photosymbiosis in ascidians: photosymbiont taxa (Prochloron, Synechocystis, and filamentous species), photosymbiont location (tunic, tunic cells, and peribranchial/common cloacal cavities), and modes of vertical transmission of photosymbionts in which the symbionts are attached to the larval trunk, collected by the rastrum, or transferred by tunic cells. These results suggest that obligate photosymbiosis with similar character states may have been exclusively established multiple times within the family Didemnidae. Because the modes of vertical transmission are closely associated with symbiont location, similar modes of vertical transmission may have evolved multiple times under similar colony structures and reproductive modes.