Based on molecular, morphological, and biochemical data, the charophycean green algae are considered to be the closest living relatives of plants. Hence study of these algae may reveal how or when various features of plants evolved. Transmission electron microscopy was used to examine plasmodesmal ultrastructure in the advanced charophycean alga Coleochaete orbicularis. Three methods of preservation were employed: standard chemical fixation, microwave-enhanced chemical fixation, and high pressure freezing followed by freeze substitution. With all three methods, similar features were observed in plasmodesmata of Coleochaete. A central structure is seen in cross-sectional views of some plasmodesmata, and appears to be connected to the plasma membrane via spoke-like structures. In longitudinal section, connections between the endoplasmic reticulum and the plasmodesmata were sometimes observed. Spoke-like structures radiate away from the plasma membrane and in some cases connect with ring-like wall specializations. Wall specializations are found in some plant plasmodesmata, but were not observed in a previous study of plasmodesmata in another advanced charophycean alga, Chara zeylanica. Generally however, the plasmodesmata of Coleochaete and Chara are similar to each other and to the plasmodesmata of plants. Given that the central structure of charophycean plasmodesmata appears to be less commonly observed than is the desmotubule of plant plasmodesmata, it is possible that the plasmodesmata of charophyceans are less specialized than those of plants, or that they have been modified after charophyceans diverged from a common ancestor with plants.

Key words: charophycean algae, Coleochaete, evolution, plasmodesmata, ultrastructure