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Relations between sexual and asexual reproduction concerning the Phylactolaemata


Relations between sexual and asexual reproduction concerning the Phylactolaemata



Ann Soc Roy Zool Belgique 86(2): 169-189, 1955/



The purpose of this study is to discover the relationship between sexual reproduction (gameto-genesis) and the 2 modes of asexual reproduction (statoblast formation and colonial budding) in the Phylactolaematous Bryozoa in general and Plumatella fungosa in particular. Phylactolaemata overwinter in statoblast form. During the remainder of the year they exist as budding colonies. P. fungosa colonies were collected weekly over an extended period and in all stages of development, near Brussels, because they could not be satisfactorily reared in the laboratory. Sexual reproduction results in the formation of eggs, sperms and larvae. The ovary develops on the body wall above each bud of the colony. The spermary develops on the funiculus of each bud. The larva develops (never more than 1/zoid) in the incubating pouch or ooecium which is on the body wall above each ovary. The funiculus, a tissue strand connecting the zoid stomach to the body wall has 2 physiologically different zones: the peduncular and the statoblastic. Spermatogenesis occurs on the peduncular zone, near the gut. Statoblasts develop along the statoblastic zone, near the body wall. Gametogenesis occurs earlier in the life of the zoid than does statoblast formation. In early April the winter statoblasts hatch, exposing a developing bud A, from whose wall already is forming a graded series of smaller buds: primordial bud B, adventive bud B', doublement bud C, etc. Already upon hatching spermatogenesis is in progress on the various bud funiculi and oogenesis in the bud ovaries. So, gametogenesis is not the exclusive property of adulthood but is found in young, immature buds. It continues at the same time as the colony is reproducing asexually by budding. In colonies colonial budding (the sprouting of new zoids from the body wall) continues throughout the entire growing season. Gametogenesis continues from early April to June, then gradually wanes while statoblast formation becomes dominant. The gametic phase is thus assumed to inhibit the statoblastogenetic phase in the earlier part of the season. The ovaries and spermatic follicles dehisce into the body cavity. Self-fertilization evidently occurs but may not result in the production of larvae unless other conditions are met, such as the accessibility of an ooecium and the incubation of the oocyte within it. Histolysis or regression of dehisced products and of parental polypides in whose walls the larvae are developing follows. The older more basal zooecia gradually die, leaving only those at the distal tips alive. Colonies hatched from winter statoblasts under natural conditions exhibit gametogenesis. Colonies developed from summer statoblasts or from winter statoblasts hatched in the laboratory or out of season do not show gametogenesis. Whether the factors responsible for this early gametogenesis are environmental or hereditary is unknown.

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