PHRAGMOPLAST OF THE GREEN-ALGA SPIROGYRA IS FUNCTIONALLY DISTINCT FROM THE HIGHER-PLANT PHRAGMOPLAST

Cytokinesis in the green alga Spirogyra (Zygnemataceae) is characteriz ed by centripetal growth of a septum, which impinges on a persistent, centrifugally expanding telophase spindle, leading to a phragmoplast-l ike structure of potential phylogenetic significance (Fowke, L. C., an d J. D. Pickett-Heaps. 1969. J. Phycol. 5:273-281). Combining fluoresc ent tagging of the cytoskeleton in situ and video-enhanced differentia l interference contrast microscopy of live cells, the process of cytok inesis was investigated with emphasis on cytoskeletal reorganization a nd concomitant redistribution of organelles. Based on a sequence of cy toskeletal arrangements and the effects of cytoskeletal inhibitors the reon, cytokinetic progression could be divided into three functional s tages with respect to the contribution of microfilaments (MFs) and mic rotubules (MTs): (1) Initiation: in early prophase, a cross wall initi al was formed independently of MFs and MTs at the presumptive site of wall growth. (2) Septum ingrowth: numerous organelles accumulated at t he cross wall initial concomitant with reorganization of the extensive peripheral interphase MF array into a distinct circumferential MF arr ay. This array guided the ingrowing septum until it contacted the expa nding interzonal MT array. (3) Cross wall closure: MFs at the growing edge of the septum coaligned with and extended along the interzonal MT s toward the daughter nuclei. Thus, actin-based transportation of smal l organelles during this third stage occurred, in part, along a scaffo ld previously deployed in space by MTs. Displacement of the nuclei-ass ociated interzonal MT array by centrifugation and depolymerization of the phragmoplast-like structure showed that the success of cytokinesis at the third stage depends on the interaction of both MF and MT cytos keletons. Important features of the phragmoplast-like structure in Spi rogyra were different from the higher plant phragmoplast: in particula r, MFs were responsible for the positioning of organelles at the fusio n site, contrary to the proposed role of MTs in the higher plant phrag moplast.