Ejectosome biogenesis and assembly in the raphidophycean alga Gonyostomum semen

The raphidophycean flagellate Gonyostomum semen (Ehrenberg) Diesing is a stramenopile protist whose ejectosomes - rod-like organelles at the cell periphery - are discharged as mucilaginous threads, which kill some zooplankton grazers. Ejectosomes are formed from fibrils, which are first distinguishable in the Golgi, where they can reach 650 nm in length, have a diameter of 21-27 nm, and possess a narrow electron-transparent lumen. Fusion of Golgi vesicles forms a post-Golgi structure containing many fibrils, which gives rise to an immature ejectosome in which some fibrils crisscross at an angle of approximately 45 degrees to the longitudinal axis of the ejectosome and other fibrils are approximately parallel to the longitudinal axis of the ejectosome. The arrangement of fibrils is not discernible in the mature ejectosome, where they form a densely stained core; between the core and the ejectosome membrane lies a less-stained layer. The anterior end of the core terminates in a domed structure that is in contact with the ejectosome membrane. Mature ejectosomes, which are approximately 1.8 to 3.1 μm long by 0.5-0.6 μm in diameter, migrate to the cell periphery and discharge in response to chemical or physical stimuli. This paper describes the assembly of ejectosomes in G. semen and presents a structure somewhat different from that previously reported.