Turning points in evolutionary pathways of the plastid division

Abstract

Summary: Cyanelles of glaucocystophytes are probably the most primitive plastid known because of their peptidoglycan content and the sequence phylogeny of cyanelle DNA. Cyanelle division involves ingrowth of the septum at the cleavage site with the inner envelope membrane invaginating at the leading edge and the outer envelope membrane invaginating behind the septum. In dividing cyanelles, a single electron-dense ring(cyanelle ring)is present on the stromal face of the inner envelope membrane at the isthmus, but no electron-dense annular structures are detectable on the outer envelope membrane. Thus a single, stromal cyanelle ring such as this is quite unique and also distinct from FtsZ rings, which are not detectable by transmission electron microscopy. These features suggest that the cyanelle division of glaucocystophytes represent an intermediate stage between cyanobacterial and plastid division. If monophyly of all plastids is true, the cyanelle ring and the homologous inner PD-ring might have evolved earlier than the outer PD-ring. In Nannochloropsis oculata(Eustigmatophyta), the outermost membrane of the secondary plastids merges with the outer membrane of the nuclear envelope, forming a Nucleus-Plastid Continuum(NPC). As the true plastids surrounded by the double envelope complete to divide, the inner nuclear envelope divides by binary fission in advance of the outer one. This allows to maintain continuity of the plastidal outermost membrane and the outer membrane of the nuclear envelope throughout the cell division cycle. Finally the closed sac composed of the plastidal outermost membrane and the nuclear outer envelope membrane divides into two halves, giving rise to two daughter sets of the NPC. The NPC may have evolved during the establishment of the eukaryote-eukaryote endosymbiogenesis as a mechanism for division and partitioning of the secondary plastids under control of the secondary host nucleus.