Ultrastructures and evolutionary modalities of flagellar and ciliary systems in protists

Abstract

Summary— As both the ultrastructure and function of flagella and cilia have been for the main part remarkably conserved during Eukaryote, evolution, the question arises as to whether the variations observed at the organite ultrastural level, or at the level of the development of a flagellar or ciliary cellular system could be considered as systematic or phylogenetic criteria.With regard to the fundamental structure, the known variations concern: 1) the kinetosome (length, position and number of cartwheels, number of triplets, and respective lengths of the microtubules); 2) the transition zone (various structural types); 3) the axoneme (number of doublets, central tubules, arms); 4) the paraxonemal formations (presence, position, structure); 5) the membrane (intramembranous particles; intramembranous particles; addition of components, mastigonemes, scales); 6) the fibres associated with the kinetosomes. Some of these variations are characteristic of taxa, and are considered as phylogenetic markers.Regarding the variations in the number of ciliary or flagellar units per cell, the following can be distinguished: cells with only one kinetosome (Ks), carrying one flagellum; cells with 2 neighbouring Ks (primary Ks = 2A), only one Ks, or the 2 Ks, bearing a flagellum; cells with numerous flagella or cilia. We consider that this configuration can result from: a) addition of new Ks around the primary Ks, forming a primary group (PG): either by replication of the primary couple (2 × 2A in Karotomorpha; 2 × 2A in Phaeopolykrikos; 4(2 × 2A) in Polkrikos), or by addition of new Ks (N) without copy or replication of the primary Ks (2A + 2N in Polytomella; 2A +3N in Tetratrichomonas; 2A + 3N in Tetratrichomonas; 2A + 4N in Hexamastix; 2A + 6N in Pyramimonas octopus). b) amplification of each constituent of the primordial couple in opalinids. c) amplification of the primary group [yx(AA + 2N) with y = 2 in diplomonadida and y > 2 in calonymphidae. d) appearance in morphogenetic fields situated outside the primary group (but possibly related to it) of new Ks which then form secondary groups (SG) where they are arranged in polarized rows composed of juxtaposed cortical units (monokinetids). In this case, PG can be of type 2A+2N (1PG+1SG in lophomonadida; 1PG+2SG in some trichonymphina), or can lose some Ks, resulting in reduced PG (4 reduced PG+4GS in Staurojoenina, n reduced PG + nGS in spirotrichonymphina). The PG may even disappear; either partially leaving remnants in the form of MTOCs in Stephanopogon, or completely, as in cilliates where the general ciliature would represent on the SG.From the above, it is seen that: a) the pluriflagellar state in protists depends on only a small number of factors; b) the same factors are used in different groups; c) different states have been obtained within different groups. An analysis of the respective evolution of kinetosomes and centrioles is proposed.