ULTRASTRUCTURE OF THE ENIGMATIC CHAROPHYCEAN GREEN ALGA ENTRANSIA FIMBRIATA

Cook, M. E. Department of Biological Sciences, Illinois State University, Normal, IL 61790‐4120 USA Based on molecular, ultrastructural, and biochemical data, the charophycean green algae are considered to be the closest living relatives of plants. Hence, comparative studies of these algae may increase our understanding of ancestral characteristics in the streptophyte lineage (charophyceans + embryophytes). Recent molecular phylogenetic analysis suggests that Entransia fimbriata is a deeply branching charophycean that is perhaps affiliated with the order Klebsormidiales (Karol and Delwiche unpublished). Transmission electron microscope examination of Entransia reveals that cytokinesis occurs by means of furrowing that does not appear to involve microtubules, a characteristic also reported in cytokinesis of Klebsormidium flaccidum. Both taxa are unbranched filaments of cells with a single starch‐filled chloroplast that contains a pyrenoid with traversing membranes. No plasmodesmata are found in either organism. Thus, preliminary TEM findings support the molecular evidence indicating that Entransia may be affiliated with Klebsormidiales. Among the characteristics that distinguish Entransia from Klebsormidium are a highly lobed chloroplast and some unusual wall features. In many cases, the ingrowing septum edges of Entransia do not meet in the most direct way, forming cross walls with large protuberances. Thick cross walls in Entransia are apparently also created by short cells that die and collapse, functioning as necridia.