Oogenesis in the viviparous phoronid, Phoronis embryolabi
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.