Cyst formation in a freshwater strain of the choanoflagellate Desmarella moniliformis Kent

Cyst formation in a freshwater strain of the colonial freshwater choanoflag ellate Desmarella moniliformis Kent (Protozoa: Choanoflagellida) has been s tudied with light and electron microscopy for the first Lime. Batch culture s inoculated with motile vegetative cells start to produce cysts within 3 d ays during the exponential phase of growth. Cyst production proceeds until in late stationary phase there is a preponderance of cysts. Transfer of cys ts to fresh medium results in limited excystment. Encystment involves the p roduction of electron-dense fibrillar wall material, firstly around the nec k of the cell and then around the posterior end. As the wall material is de posited the neck of the cell elongates and the dictyosome rotates from the horizontal to vertical plane. The number of mitochondrial profiles seen in individual sections of cells increases. Finally the neck of the cell is ret racted, the flagellum and collar tentacles are withdrawn, and the bottom of the neck of the cyst wall is sealed with a diaphragm of wall material. Exc ystment, which has nut been observed directly, appears to involve the disru ption of the wall at the base of the neck, the remainder of the cyst wall r emains intact. Comparisons ale made between encystment in Desmarella and cy st development in other protists.