Detailed analyses on the parthenospore formation in Closterium moniliferum (Zygnematophyceae, Charophyta)

A parthenosporic strain of Closterium moniliferum (strain izu76-1) was isolated from a rice paddy field in Japan. Molecular phylogenic analysis was used to verify the initial species determination based on its morphology, and it was confirmed to be closely related to another identified strain of the species. By using time-lapse photography, the process of parthenospore formation was observed and categorised into the following stages: (1) cell division, resulting in the formation of two sister sporangial cells from one vegetative cell; (2) the formation of conjugation papillae via elongation of the cell wall; and (3) formation of the parthenospore through cytoplasm condensation without pairing with other cells. Callose accumulation was observed during the process of parthenospore formation. An appropriate cell density was critical for parthenospore formation to progress in the culture. Parthenospore formation was promoted in conditioned medium in which the cells had been previously cultured, suggesting the presence of a parthenospore formation-promoting factor. These findings provide insights into the mechanisms of cell-cell interactions in parthenosporic strains and the evolution of conjugation styles.