Assembly and Motility of Eukaryotic Cilia and Flagella. Lessons fromChlamydomonas reinhardtii 

Cilia and flagella are among the most ancient cellular organelles, providing motility for primitive eukaryotic cells living in an aqueous environment. During adaptation to life on land, some groups of organisms, including advanced fungi, red algae, cellular slime molds, conifers, and angiosperms, lost the ability to assemble flagella (Raven et al., 1999). The centriole or basal body, which organizes the assembly of flagella, also is absent in these groups. In other lineages, flagella were retained only on gametic cells. Land plants are believed to have arisen from one group of green algae, the charophytes (for review, see Bhattacharya and Medlin, 1998; Qiu and Palmer, 1999), in which the only flagellated cells are motile sperm. The first land plants, bryophytes, which are thought to be the ancestors of higher plants, also produce flagellated sperm cells that require water to swim to the egg. Ultrastructural features of the basal body apparatus in the flagellated cells have provided important morphological data for phylogenetic studies of algae and bryophytes.