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Chloroplast continuity during the formation of the tetraspore in antithamnion subulatum Burton, Arthur Hugh Scott

Abstract

The development of the tetrasporangium of Antithamnion subulatum (Harvey) J.G. Agardh was studied using light and electron microscopy in order to elucidate the origin of proplastids, and the continuity of chloro-plasts during the production of the tetraspore. The results show that proplastids arise through a "blebbing" process of the mature chloroplasts.This results in the production of proplastids which are identical to those found free in the cytoplasm of the tetra-sporangial mother cell, and are in most respects similar to proplastids observed by others in the apical regions of other red algae. The inclusion of a single DNA-containing genophore within the forming proplastids strongly suggests that each of the scattered genophores in the mature chloroplasts contains at least one complete genome. Division of mature chloroplasts was not seen within the tetra-sporangial mother cell or the tetrasporangial initial. However, within the young tetrasporangium the mature chloroplasts appear to undergo several simultaneous divisions resulting in numerous smaller discoid plastids. It is these plastids which, through a succession of growth and division phases, make the major contribution to the continuity of chloroplasts in the formation of the tetraspore, rather than the proplastids which have been produced in low numbers throughout its development. The colourless nature of the young tetrasporangium is not due to the presence of a high concentration of proplastids, but rather is related to the high frequency of chloroplast division, resulting in membrane production being much more rapid than phycobilisome formation.

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