The Oikopleura coenocyst, a unique chordate germ cell permitting rapid, extensive modulation of oocyte production

Byline: Philippe Ganot, Jean-Marie Bouquet, Torben KallesA[cedilla]e, Eric M. Thompson Keywords: Vasa; Nuage; Gamete; Larvacean; Germline cyst; Intercellular bridge; Clutch manipulator; Ascidian; Prophase I; Endoreplication Abstract: The ability to adjust reproductive output to environmental conditions is important to the fitness of a species. The semelparous, chordate, Oikopleura dioica, is particularly adept in producing a highly variable number of oocytes in its short life cycle. Here we show that this entails an original reproductive strategy in which the entire female germline is contained in a single multinucleate cell, the 'coenocyst'. After an initial phase of syncytial nuclear proliferation half of the nuclei entered meiosis whereas the other half became highly polyploid. The inner F-actin network, with associated plasma membranes, formed a highly ramified infrastructure in which each meiotic nucleus was contained in a pseudo-compartmentalized pro-oocyte linked to the common cytoplasm via ring canals. At a set developmental time, a subset of the pro-oocytes was selected for synchronous growth and the common coenocyst cytoplasm was equally partitioned by transfer through the ring canals. Examination of related species indicated that the coenocyst arrangement is a conserved feature of Appendicularian oogenesis allowing efficient numerical adjustment of oocyte production. As Appendicularia are the second most abundant class of zooplankton, with a world-wide distribution, the coenocyst is clearly a common and successful reproductive strategy on a global scale. Author Affiliation: Sars Centre for Marine Molecular Biology, Bergen High Technology Centre, University of Bergen, ThormA[cedilla]hlensgate 55, N-5008 Bergen, Norway Article History: Received 20 June 2006; Revised 25 September 2006; Accepted 14 October 2006