BioSystems, 10 (1978) 145--152 145 © Elsevier/North-Holland Scientific Publishers Ltd. STRUCTURAL EVOLUTION IN THE FLAGELLATED CELLS OF GREEN ALGAE AND LAND PLANTS* KENNETH D. STEWART and KARL R. MATTOX Department of Botany, Miami University, Oxford, Ohio 45056, U.S.A. 1. Introduction Greenwood, 1955; Manton, 1964) and later, in more detail, for Chlamydomonas by Comparative morphological studies employ- Ringo (1967). This arrangement is often ing the electron microscope and modem referred to as a "cruciate" rootlet system. improvements of light microscopy have recently provided a more substantial basis for interpreting the phylogeny of green algae (Mattox and Stewart, 1977; Molnar, Stewart and Mattox, 1975,Pickett-Heaps and Merchant, 1972; Pickett-Heaps, 1975; Stewart and Mattox, 1975a). T!he events of cell division and the micromorphology of zoospores or flagellated gametes have been the most re- vealing features, and it is now clear that there are at least 2 distinct groups of advanced (i.e.~ A colonial, filamentous or parenchymatous) green algae, the Chlorophyceae and Charo- phyceae (Pickett-Heaps, 1975; Stewart and Mattox, 1975a), ~md that the ancestors of B both lines were scale~overed green flagellates (Mattox and Stewart, 1977; Stewart and c Mattox, 1975a; Stewart, Mattox and Chandler, 1974). Although the features of the 2 groups are distinct and concerned with such reliable and Fig, 1. The diagram outlines how a Pyramimonas- conservative features as mitosis and the like cell (A) might have given rise to a Klebsormidium flagellar apparatus, a very perplexing problem or Coleochaete-like zoospore (B) by "halving," the has been our ignorance of the function and conversion of one rhizoplast to an MLS, and the loss evolutionary origin: of some of the most of both the pit and the microtubular rootlets that ascend the sides of the pit. C represents a cross important characteristics. The flagellar appar- section of the cell shown in B. A zoospore of Trente- atus of one group, the Chlorophyceae, usually pohlia (D) could have arisen either by a doubling of has 4 or more relatively narrow microtubular the cell shown in B or from a "whole" cell by the rootlets descending from the region of the conversion of both rhizoplasts of A to MLS's. The basal bodies (Fig. 2D) in a manner first diagrams are not exact representations because a single plane would not include 2 microtubular root- described for zoosr~res (Manton, Clarke and lets and both rhizoplasts of Pyramimonas. In Pyram- imonas the nucleus is shown in its prophase position. * This work was supported by National Science D was drawn after Graham and McBride (1975). Foundation Grant DEB76-08822 and by a special mb, microtubular band; mi, microbody; mls, multi- allocation from Miami University for the purchase of layered structure ; mr, microtubular rootlet; n, nucleus; an electron microscope. rh, rhizoplast. 146 The Chlorophyceae are further characterized structure of the spermatozoids of lower by a mitotic spindle whose interzonal region archegoniates and demonstrates that Charo- collapses during telophase and is replaced by phyceae includes the algae most closely re- a phycoplast, a system of microtubules which lated to the ancestry of land plants. Phyco- lies between the daughter nuclei in the plane logists and other botanists have long ex- of cytokinesis. Members of the Charophy- ercised their imaginations as to the identity, ceae have a rootlet system consisting of a growth form and life cycle of those green single broad band of microtubules (2 in algae which gave rise to the land flora, but Trentepohlia, Graham and McBride, 1975), modern discussion of the significance of the associated at its proximal end with a complex structure of flagellated cells to this problem lamellate structure (Fig. 1B). The broad band began recently with Manton's (1965) paper of microtubules and the associated structure on the phylogeny of flagellar structure. Of are commonly called the "multilayered particular interest here is her conviction that structure" (MLS). Although the taxonomic the asymmetrical flagellated gametes of lower value of the mentioned characteristics has not land plants indicate that the flagellated unicell- yet been questioned, they have been difficult ular ancestors of land plants, and by implica- to explain for the following reasons: (1) an tion the zoospores of the later filamentous MLS is not known to occur in the scaly green ancestors, were asymmetrical and unlike flagellates presumed to be relatively primitive Chlamydomonas and the zoospores of advanc- and related to the ancestry of the Charophy- ed chlorophycean algae. The conspicuously ceae and other green algae; (2) a descending lateral attachment of the flagella in archegon- rootlet system comparable to that of Chlamy- iates also stands in contrast to the anterior domonas and other chlorophycean algae does insertion of the Chlorophyceae. Manton's not occur in Platymonas, a scaly green flagel- study of scaly green flagellates ("prasino- late considered to be a primitive chlorophy- phytes") appeared to strengthen this view, cean alga because it has a phycoplast; (3) because some of those organisms are asym- there has been no detailed explanation or metrical and appear to be rather primitive hypothesis regarding the circumstances lead- green algae. The later discoveries of scales on ing to the evolution of the phycoplast, a the asymmetrical spermatozoids of Chara structure unknown in organisms other than (Pickett-Heaps, 1968) and zoospores of chlorophycean algae. Coleochaete (McBride, 1971) had a number We have very recently presented evidence of significant implications that became clearer and a rationale for the concept that the with further studies. Most importantly, the phycoplast and collapsing telophase spindle scales indicate that the monadal ancestors of evolved in response to a restriction of cell Coleochaete and Chara were scaly rather elongation during mitosis caused by the than walled like Chlamydomonas. Further- evolution of cell walls from scales in the more, the presence of a rootlet system like flagellated unicellular ancestors of the Chloro- that of the spermatozoids of land plants phyceae (Mattox and Stewart, 1977). The indicated that that line of descent from details cannot be repeated here, and the scaly flagellates led not only to Coleochaete body of this paper will be concerned with the and Chara but to land plants as well. The first 2 of the 3 problems mentioned above presence of an MLS in Klebsormidium (Mar- and with some other aspects of the structure chant, Pickett-Heaps and Jacobs, 1973) and evolution of green algal flagellated cells. further demonstrated that some of the relatively primitive filamentous forms of that 2. Cell symmetry and the flagellar apparatus line are extant. Finally, evidence accumulated that all advanced green algae, both Charophy- The MLS of the zoospores and flagellated ceae and Chlorophyceae had scaly ancestors. gametes of charophycean algae matches the Scales on Platymonas, a flagellate with a 147 phycoplast (Stewart, Mattox and Chandler, We believe that a consideration of the 1974), and scales on the zoospores of Pseuden- position, nature and distribution of some doclonium, in which the rootlet system is components of the flageUar apparatus in cruciate (Mattox and Stewart, 1973), point scaly green flagellates provides some strong out that it is the MLS, not scales, that truly hints toward an answer to the questions posed distinguishes the flagellated cells of the above. Although scaly green flagellates have Cha~ophyceae from those of the Chloro- microtubular rootlets similar in structure to phyceae. those of other green algae except charophytes, The ancestral position of scaly green flagel- their orientation is different. The microtubular lates and the occurrence of some asymmetrical rootlets of chlorophytes extend posteriorly genera among them have seemed to strongly from the basal bodies (Fig. 2B) whereas those support Manton's opinion that the unicellular of some scaly green flagellates do not descend ancestors of land ]plants were asymmetrical (see Birkbeck, Stewart and Mattox, 1974; Mattox and Stewart, 1973; Pickett-Heaps, 1975, Chap. 8; Stewart and Mattox, 1975b). -mr The fact remains, however, that no known scaly green flagellate has an external form -rh that resembles the zoospores of Klebsormid- ium and Coleochaete, where the flagella are inserted a very sho3.~ distance from the acute \? anterior of the cell (see Fig. 1B); in scaly \ green flagellates the flagella are usually in- serted in a pit of depression and in the asym- metrical forms, proportionately farther from the cell's anterior. Even more serious are the facts that no known scaly green flagellate has \\ .~ a rootlet system even remotely similar to B D the MLS of charophytes and archegoniates and that the zoospores of Trentepohlia are c symmetrical by virtue of having 2 MLS's (Fig. 1D). With the information available, it seems to be a reasonable inference that the distinctive characteristics of charophycean zoospores never existed in unicellular flagel- Fig. 2. The diagram outlines how a phycoplast-utiliz- ing Platymonas-like cell might have given rise to a lated organisms but arose with the transition Pleurastrum-like zoospore (B) by the conversion of from unicellular flagellated organisms to the rhizoplasts to microtubular rootlets and the loss flagellated reproductive cells of filamentous of both the pit and the microtubular rootlets that lie forms. If so, the questions to be answered along the sides of the pit. C represents a cross section are: (1) what happened to the flagellar pits of the cell in B. The cell in D has 4 cruciately arrang- ed, microtubular rootlets similar to Chlamydomonas during the transition? (2) how and from what and many flagellated reproductive cells of chloro- did the MLS evolve', and (3) were the ancestral phycean algae; it could have arisen by a doubling of forms distinctively asymmetrical scaly flagel- the rootlet system in a cell like B or by a replication lates like Heteromastix, Micromonas and of rhizoplasts in a cell like Platymonas (A) before the Mesostigma, as following Manton's argument, rhizoplasts were transformed to microtubular rootlets.