Based on molecular, ultrastructural, and biochemical data, the charophycean green algae are considered to be the closest living relatives of plants. Hence, studies of these algae may increase our understanding of ancestral characteristics in the streptophyte lineage (charophyceans + embryophytes). The complex process of cytokinesis has special significance in plants, due to the impact of cell wall placement on plant form. New cell walls synthesized between daughter cells during plant cytokinesis become part of a permanent support system that is built upon as the plant continues to grow. The morphologically diverse genus Coleochaete presents a unique opportunity for studying the evolution of cytokinesis. It has long been known that cytokinesis in the morphologically complex circular thalloid forms C. scutata and C. orbicularis is associated with a phragmoplast. The cytoskeletal basis of cell division has not previously been examined in the morphologically simpler forms. Here we report the presence of phragmoplasts in the loosely branched form C. irregularis. This finding supports the hypothesis that phragmoplasts arose in a common ancestor of Coleochaetales, Charales and embryophytes. We also report evidence that cytokinesis in the genus Coleochaete can resemble the model of polarized cytokinesis recently described in vacuolate cells of Arabidopsis (SR Cutler and DW Ehrhardt. 2002. PNAS 99(5): 2812-2817). Hence, in Coleochaete it appears that in at least some cases, the new cell plate attaches to one side of the parent wall first, and then extends to the other side of the highly vacuolate cell. Our evidence suggests that polarized cytokinesis may have arisen in a common ancestor of Coleochaete and embryophytes.

Key words: charophycean algae, Coleochaete, cytokinesis, evolution, phragmoplast