STRUCTURAL AND EVOLUTIONARY ASPECTS OF THE FLAGELLAR APPARATUS IN GREEN ALGAE AND LAND PLANTS

Abstract

Ultrastructural features of the flagellar apparatus in green algae and land plants seem to be of considerable phylogenetic significance. Based on functional considerations details of the flagellar apparatus can be used to construct tentative lines of evolution within the green algae. An “ancestral” asymmetric zooflagellate possibly gave rise to a green unicell after a primary endosymbiotic event involving a green prokaryote. During the transition from phagotrophic to phototrophic nutrition in most green algae (the Chlorophyceae and Ulvaphyceae sensu Stewart & Mattox and most Prasinophyceae sensu Christensen) the original asymmetric flagellar apparatus was transformed into a 180° rotational symmetric flagellar apparatus with 4 flagella and a flagellar root system of the X‐2‐X‐2‐type. Most significant in this transition was presumably the evolution of the phototactic apparatus and of an apical flagellar groove. In the Charophyceae sensu Stewart & Mattox (including those green algae related to the ancestry of the archegoniate land plants) the early transition towards a terrestrial life, lack of an eyespot apparatus and development of a special type of oogamy possibly led to the evolution of the unilateral flagellar root in these organisms. It is suggested that the evolution of the unilateral MLS‐root in the Charophyceae sensu Stewart & Mattox can be traced back to an “ancestral green flagellate” before the evolution of the typical X‐2‐X‐2 flagellar root system. A tentative evolutionary scheme of the green algae mainly based on motile cell structure and mitotic and cytokinetic mechanisms is presented.