Ultrastructure of selected life history stages of the parasitic dinoflagellate Euduboscquella cachoni

Euduboscquella species differ from most other syndinean dinoflagellates by having mononucleate trophonts, but resemble species of Amoebophrya and Sphaeripara by episome-hyposome differentiation and cortical complexity. Cytology and development of Euduboscquella species are well characterized, but their ultrastructure remains essentially unexplored. Transmission electron microscopy of Euduboscquella cachoni trophonts, tomont, and sporocytes revealed previously unrecognized structures. Initially dense, fibrous chromosomes uncoiled during early infection, with condensed chromosomes absent over much of the growth cycle recondensing at trophont maturity. The hyposomal amphiesma was two appressed membranes, the episomal cortex was alveolate, and a supraepisomal cavity limited by membrane enclosed the episome. Pseudopod-like extensions of the hyposome during mid infection may facilitate osmotrophic nutrition. The pharyngeal lamina appears to lack ingestatory function; however, transcortical transport of particles occurred via the supraepisomal cavity and episomal micropores. Microtubules originating from the electron-opaque perinema bordering the episome, formed an episomal skeleton hypothesized to function with the pharyngeal lamina, perinema, and the paired membranes of the supraepisomal cavity to effect parasite egress and ingestion of host material. Trichocysts absent during early infection developed during late infection and reached maturity during sporogenesis, suggesting functional importance in spore survival or infection.

Automated summary

Euduboscquella species differ from most other syndinean dinoflagellates by their mononucleate trophonts, but share similarities with species of Amoebophrya and Sphaeripara in terms of episome-hyposome differentiation and cortical complexity. While the cytology and development of Euduboscquella species have been well characterized, their ultrastructure has been largely unexplored. Transmission electron microscopy revealed previously unrecognized structures in Euduboscquella cachoni trophonts, tomonts, and sporocytes, providing new insights into their cellular organization and life cycle. The study suggests the presence of unique mechanisms for parasite egress and host material ingestion.