The flagellate cytoskeleton: introduction of a general terminology for microtubular flagellar roots in protists
ABSTRACT The cytoskeleton of protists may consist of a few microtubules beneath the plasma membrane in small and apparently reduced organisms. In other species, however, it may comprise highly complex systems of microtubules and fibrous components. The 9+2 flagellum/cilium is believed to have arisen only once during the evolution of the eukaryotic cell. The thesis of the present paper is that not only the 9+2 axoneme and the 9x3 basal body but also other structures of the flagellar apparatus are homologous. A common feature of most eukaryote cells is the presence of microtubular roots, believed to play a range of functions in the cell. Roots associated with different flagella are usually different. However, the discovery of flagellar transformation, a process during which a flagellum passes through a maturation process before taking up its final position in the cell one or more generations later, has made it possible to identify the mature (final) position of each flagellum. This allows comparison of roots associated with mature flagella in different organisms. Using such information, a comparative study of the microtubular roots has been made on representatives from the major taxonomic groups of protists. It has resulted in the proposal of a common numbering (identification) system for microtubular roots, which may hopefully replace the many labelling systems presently in use in the different groups of protists.