SYNCHRONOUS SPERMATOGENESIS IN APPENDICULARIANS

Spermatogenesis was studied in appendicularians belonging to the families Fritillariidae (Fritillaria borealis, Fritillaria pellucida) and Oikopleuridae (Oikopleura dioica, Oikopleura albicans, Oikopleura longicauda). Particular attention was given to specimen of Fritillaria borealis collected in the fjord of Bergen (Norway) and to Oikopleura dioica reared in a laboratory at the Station Zoologique in Villefranche-sur-Mer. In all species, spermatogenesis proceeds in a highly synchronized way in the whole testis, thanks to the presence of syncytial structures. Two models were identified: 1) In Fritillaria borealis, the syncytium contains organelles and two populations of large and small nuclei. The large somatic nuclei produce "nuage" and occupy the center of wide cytiplasmic areas, at the periphery of which the small nuclei of the germ cells lie. As spermatogenesis progresses, large nuclei and syncytial cytoplasm reduce volume, while germ cells greatly proliferate and undergo meiosis and spermiogenesis. Lastly, the large nuclei degenerate while late spermatids, all possessing a single mitochondrion and a flagellum, segregate from the syncytium. 2) In Oikopleura dioica, the large cytoplasmic area contains most of the organelles and germ cell nuclei hanging at the periphery. Germ cells undergo series of mitosis, meiosis and spermiogenesis, whereas mitochondria migrate from the common cytoplasmic area through bridges. Lastly, the syncytial area is markedly reduced, and spermatozoa segregate and mature all together. In all species, spermatozoa are small and flagellate. They mature in great numbers and are appropriately arranged in the testis, thus exploiting to fulest extent the space for the imminent spawning.