ON THE COVER

Vol. 68   No.2 June 2003

Technical note

     Chloroplasts of Glaucocystophyta, like Cyanophora paradoxa, possess the most primitive features among the known plastids, resembling cyanobacteria in the presence of' a surrounding peptidoglycan layer, pigment composition and morphology, and hence they are called 'cyanelles'. Recently electron microscopic observations have revealed that the cyanelle division of C. paradoxa represents an intermediate stage between cyanobacterial and plastid division. The cyanelle division involves ingrowth of the peptidoglycan-containing septum at the cleavage site (arrowhead). On one hand, only a single plastid dividing-ring (cyanelle ring) (arrow) is formed on the stromal face of the inner envelope membrane at the isthmus, but cytosolic plastid dividing-rings, which are common in the ordinary plastids, are not detectable on the outer envelope membrane. The absence of the cytosolic plastid dividing-ring may be accounted for by the septum-based division mechanism that does not require external mechanical force. According to the prevailing hypothesis that primary endosymbiosis occurred only once, the mechanism of the plastid division may have evolved via that of the cyanelle division of Glaucocystophyta, like C. paradoxa (see Hashimoto, H. 2003: Int. Rev. Cytol., 222: 63-98., Iino, M. and H. Hashimoto 2003: J. Phycol., 39: 561 -569. ).

     (Haruki Hashimoto, Department of Life Sciences, Graduate School of' Arts and Sciences, The University of Tokyo, Komaba, Meguro-ku, Tokyo, 153-8902, Japan)

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ON THE COVER

Vol. 68 No.2 June 2003
Technical note

Chloroplasts of Glaucocystophyta, like Cyanophora paradoxa, possess the most primitive features among the known plastids, resembling cyanobacteria in the presence of' a surrounding peptidoglycan layer, pigment composition and morphology, and hence they are called 'cyanelles'. Recently electron microscopic observations have revealed that the cyanelle division of C. paradoxa represents an intermediate stage between cyanobacterial and plastid division. The cyanelle division involves ingrowth of the peptidoglycan-containing septum at the cleavage site (arrowhead). On one hand, only a single plastid dividing-ring (cyanelle ring) (arrow) is formed on the stromal face of the inner envelope membrane at the isthmus, but cytosolic plastid dividing-rings, which are common in the ordinary plastids, are not detectable on the outer envelope membrane. The absence of the cytosolic plastid dividing-ring may be accounted for by the septum-based division mechanism that does not require external mechanical force. According to the prevailing hypothesis that primary endosymbiosis occurred only once, the mechanism of the plastid division may have evolved via that of the cyanelle division of Glaucocystophyta, like C. paradoxa (see Hashimoto, H. 2003: Int. Rev. Cytol., 222: 63-98., Iino, M. and H. Hashimoto 2003: J. Phycol., 39: 561 -569. ). (Haruki Hashimoto, Department of Life Sciences, Graduate School of' Arts and Sciences, The University of Tokyo, Komaba, Meguro-ku, Tokyo, 153-8902, Japan)
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