Morphogenesis in Hyalophysa chattoni (Ciliophora: Apostomatida): Reduction of Oral Polykineties in the Encysted Phoront

Abstract

An ultrastructural study of Hyalophysa's phoront revealed that ciliary absorption and kinetosomal absorption are two distinct events. During the transformation from the tomite (microstome) to the meridional phoront stage, the ogival field (OG) loses its cilia. Ciliary loss involves an in situ disassembly of the structure and does not involve the retraction of axonemes into the cytoplasm or the detachment of cilia from the cell surface. The end result of the deciliation is a bald kinetosomal field. Pellicular folds, perhaps exaggerated to accommodate newly available membrane, are pronounced. Following deciliation, the kinetosomes of the OG are absorbed. The kinetosomes are disassembled in situ as well, as they are always located at the inner membrane of the pellicle and never detached from the cell surface or within autophagic vacuoles. Ciliary loss and kinetosomal absorption also occur in the falciform fields (FFs). Deciliation begins first, followed by the loss of two rows of kinetosomes in FF9. The end result is a FF9 and FF8 that are two kinetosomes wide and lack cilia. The mechanism of ciliary and kinetosomal loss is the same as in the OG. The reorganization of the OG and FFs is a necessary event in the metamorphosis of the crawling tomite stage (microstome) to the trophont (macrostome), as the ventral surface of the trophont develops an extensive extended cytostome. The reduction of the OG and FFs possibly provides tubulin and membrane reserves for the development of the somatic ciliature and extended cytostome in the trophont.