PHRAGMOPLAST‐MEDIATED CYTOKINESIS IN NON‐CHAROPHYCEAN GREEN ALGAE— IS IT HOMOLOGOUS TO THE PROCESS IN STREPTOPHYTES?

Abstract

Chapman, R. L.1, Henk, M. C.1, Borkhsenious, O.2 & Waters, D. A1 1Department of Biological Sciences, Louisiana State University, Baton Rouge, LA 70803 USA; 2School of Veterinary Medicine, Louisiana State University, Baton Rouge, LA 70803 USAPhragmoplast‐mediated cytokinesis, basic to all land plants, occurs in three orders of charophycean algae–the Coleochaetales, the Charales, and the Zygnematales–but not in the Chlorokybales and Klebsormidiales. Phragmoplast‐mediated cytokinesis has also been found in two genera of the Trentepohliales, an anomaly for chlorophycean algae. Within the green algal orders where phragmoplasts have been found, not all genera studied have this mode of cytokinesis. Among the genera that do have it, there is significant variation in the mechanisms involved. In the Trentepohliales, the temporal separation of karyokinesis and cytokinesis, the massive “double‐cone” microtubule array, and the centripetal fusion of cell plate vesicles are distinct features that indicate that on a structural basis the phragmoplast‐mediated cytokinesis is not homologous to that in the charophycean green algae or the land plants. However, given the structural differences in this process among the charophycean green algae and between them and the land plants, one wonders if structural variations are relevant to the question of homology. We propose that in the Trentepohliales, differences in the details of the vegetative cytokinetic processes and the microtubular structures from those found in the streptophytes are great enough to indicate parallel evolution of phragmoplast‐mediated cytokinesis in these groups. The putative combination of cell wall infurrowing with a central phragmoplast in Phycopeltis can be interpreted as support for the nonhomology of the Trentepohlialean phragmoplast‐mediated cytokinesis and that in the streptophytes. Evolutionary and developmental studies within the chlorophycean and streptophyte evolutionary lineages are needed.