An ultrastructural study of planozygotes and encystment of a marine dinoflagellate,Scrippsiellasp

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Abstract
The fine structure of the planozygotes and encystment of Scrippsiella sp. have been studied by scanning and transmission electron microscopy. The planozygote is armoured and has two longitudinal flagella and one transverse flagellum. Unlike the vegetative cell, the planozygote has two pairs of basal bodies, large numbers of starch granules and many vesicles containing electron-dense particles. The build-up of storage materials and the reduction in size and number of the organelles in the cytoplasm, which are necessary processes for encystment, take place in the planozygote. The main feature of encystment is the formation of the thick cyst wall. The theca is shed at a very early stage of encystment before wall formation. During encystment the cell is surrounded by three layers c membranes. The cyst wall is constructed between the middle and the inner membrane and thickens due to the deposition of wall material on the inward side of the middle membrane. Mucofibrous material is seen between the outer and the middle membranes. Bundles of the mucofibrous material are raised to form protrusions which are calcified as encystment proceeds. The mucofibrous material may play an important part in the formation of the protrusions, perhaps acting as a skeleton, while the outer membrane may provide protection for the formation and calcification of these protrusions.