Structure and asexual reproduction of the enigmatic charophycean green alga Entransia fimbriata (Klebsormidiales, Charophyceae)

As the closest relatives of embryophytes, the charophycean green algae (sensu Mattox and Stewart) may reveal the evolutionary history of characters in this lineage. Recent molecular phylogenetic analysis indicates that the little-known species Entransia fimbriata Hughes is a member of the charophycean order Klebsormidiales. In this study LM and EM were used to identify and describe additional structural characters of Entransia so that comparisons could be made with Klebsormidium and with other charophycean algae outside the order Klebsormidiales. Features that Entransia shares with various members of the genus Klebsormidium include cylindrical cells in unbranched filaments that may spiral, parietal chloroplasts that cover only part of the circumference of the cell, H-shaped cross walls, and vegetative reproduction by both fragmentation and formation of zoospores or aplanospores. Among the characteristics that distinguish Entransia from Klebsormidium are a highly lobed chloroplast with multiple pyrenoids; a single large vacuole; short cells that die and collapse, apparently facilitating filament fragmentation; and germinating filaments with condensed adhesive at the base and a tapering spine at the tip. Although Entransia has sometimes been tentatively considered to be a member of the Zygnemataceae, the presence of a flagellate life history stage distinguishes Entransia from this group. The pyrenoids of Entransia are typical of those of charophycean algae in having traversing membranes and surrounding starch. Presence of multiple such pyrenoids in each chloroplast of Entransia supports the hypothesis that the common ancestor of charophycean algae and embryophytes had a single chloroplast with multiple pyrenoids.