Ultrastructure of cystocarp development in Gelidium robustum (Gelidiaceae: Gelidiales: Rhodophyta)

Abstract

The ultrastructure of cystocarp development is described for the red alga Gelidium robustum (Gardn.) Hollenb and Abbott. The external cortical cells of the cystocarp remain uninucleate and unmodified, while the internal ones are binucleate, producing large mucilage sacs that discharge their contents into the cystocarpic cavity. Crystalline inclusions occur in both kinds of cells. Nutritive filaments, up to five cells long, are produced. Microtubules are associated with lipid droplets and occur in cells of the third-order filament, which probably function as food suppliers and food-conducting cells for the nutritive filaments. The younger, terminal cells of the nutritive filaments are electron-dense, having a large nucleus and lipid droplets and probably function as nutritive suppliers of the terminal gonimoblast cells. The older cells of the nutritive filaments are electron-light-possessing mucilage sacs, which function as mucilage suppliers of the cystocarpic cavity. Carpospore differentiation proceeds through three developmental stages. The youngest carpospores have an ovoid shape, possessing a large nucleus, proplastids with the peripheral thylakoid and some mucilage sacs, while they are surrounded by the carposporangial mucilage. Starch is polymerized in the perinuclear cytoplasmic area of the later stages of young carpospores. Intermediate-aged carpospores continue their starch polymerization and thylakoid development of plastids. Curved dictyosomes produce vesicles that contribute carpospore wall material. Mature carpospores have a nucleus, fully developed chloroplasts, numerous starch granules and adhesive vesicles, while they are surrounded by a two-layered wall and compressed mucilage, which constitutes a carposporangial wall.