This is the only lifelong, obligate photosymbiosis in chordates thus far described. For host species in which cyanobacterial cells are distributed in the cavities and/or tunic, this association is a form of obligate symbiosis: the host ascidians always associate with cyanobacterial cells. Approximately 30 species from four genera of the family Didemnidae harbor cyanobacterial cells on the colony surface, in the common cloacal cavities, and/or the tunic (Fig. Other than the association between green algae and the spotted salamander embryo (Kerney et al., 2011), photosymbiosis is known only in colonial ascidians inhabiting coral reefs (e.g., Kott, 1982 Lewin and Cheng, 1989). © 2014 Wiley Periodicals, Inc.Īlthough symbiosis involving photosynthetic organisms has been documented in various metazoan taxa, such as Porifera, Cnidaria, Acoelomorpha, and Mollusca, its occurrence is rare in the phylum Chordata. Ascidian photosymbiosis is discussed from the viewpoint of evolution and diversification of vertical transmission. Why obligate photosymbiosis was established exclusively in these didemnid ascidians remains uncertain. Accordingly, host species of distinct lineages likely acquired similar modes of transmission convergently. Species of different genera have similar modes of transmission wherein they harbor cyanobacterial cells at similar sites within colonies. In other photosymbiotic species, the mode is constrained by the distribution pattern of photosymbionts in the host colony. In Diplosoma species, the mode is constrained phylogenetically all photosymbiotic Diplosoma have the same mode of transmission using a unique organ known as the rastrum. Ascidian species exhibit several modes of cyanobacterial transmission. Here we review the diversity of modes of cyanobacterial transmission to shed a light on the evolutionary history of ascidian photosymbiosis. Photosymbionts are transferred from the maternal colony to embryos or prehatching larvae brooded in the colonies. This is the only lifelong, obligate photosymbiosis reported in chordates. Some tropical ascidians of the family Didemnidae invariably harbor cyanobacterial cells in the common cloacal cavities and/or tunic.
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