Oogenesis in the viviparous phoronid, Phoronis embryolabi.

Abstract

The study of gametogenesis is useful for phylogenetic analysis and can also provide insight into the physiology and biology of species. This report describes oogenesis in the Phoronis embryolabi, a newly described species, which has an unusual type of development, that is, a viviparity of larvae. Phoronid oogonia are described here for the first time. Yolk formation is autoheterosynthetic. Heterosynthesis occurs in the peripheral cytoplasm via fusion of endocytosic vesicles. Simultaneously, the yolk is formed autosynthetically by rough endoplasmic reticulum in the central cytoplasm. Each developing oocyte is surrounded by the follicle of vasoperitoneal cells, whose cytoplasm is filled with glycogen particles and various inclusions. Cytoplasmic bridges connect developing oocytes and vasoperitoneal cells. These bridges and the presence of the numerous glycogen particles in the vasoperitoneal cells suggest that nutrients are transported from the follicle to oocytes. Phoronis embryolabi is just the second phoronid species in which the ultrastructure of oogenesis has been studied, and I discuss the data obtained comparing them with those in Phoronopsis harmeri. Finally, I discuss the distribution of reproductive patterns across both, molecular and morphological phylogenetic trees in Phoronida proving that parental care has evolved independently several times in this phylum.