The Oikopleura coenocyst, a unique chordate germ cell permitting rapid, extensive modulation of oocyte production.
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.
Ganot
Philippe
P
Sars Centre for Marine Molecular Biology, Bergen High Technology Centre, University of Bergen, Thormøhlensgate 55, N-5008 Bergen, Norway.
Bouquet
Jean-Marie
JM
Kallesøe
Torben
T
Thompson
Eric M
EM
D000199:Actins / Q000502:physiology; D000818:Animals; D002462:Cell Membrane / Q000502:physiology; D002467:Cell Nucleus / Q000648:ultrastructure; D003593:Cytoplasm / Q000502:physiology; D005260:Female; D008540:Meiosis; D008954:Models, Biological; D009865:Oocytes / Q000502:physiology* / Q000648:ultrastructure; D009866:Oogenesis; D011123:Polyploidy; D014561:Urochordata / Q000502:physiology* / Q000648:ultrastructure; D015048:Zooplankton
D000199:Actins