Observations on hypnospores in Ulothrix zonata (Chlorophyceae)

Abstract

The formation, structure, and development of hypnospores in Ulothrix zonata are described. Hypnospores are formed by cleavages similar to those in zoosporogenesis, but not all of the parent cell cytoplasm is included in the hypnospores. The hypnospores lack flagella or an eyespot and possess a thick, dense cell wall. This cell wall, the pyrenoid, and the nuclear morphology of hypnospores closely resemble those of vegetative cells. The number and size of the hypnospores produced varies within a filament from 2 to 4 large ones per cell to as many as 32 smaller ones per cell. The hypnospores are retained within the parent cell where they germinate into multicellular filaments whose subsequent growth causes a rupture in the parent cell wall. The new filaments are released and settle on the substrate to continue growth. The presence of hypnospores in U. zonata represents a unique form of asexual reproduction among members of the genus Ulothrix.