MOLECULAR PHYLOGENY OF CILIATES - WHAT DOES IT TELL US ABOUT THE EVOLUTION OF THE CYTOSKELETON AND OF DEVELOPMENTAL STRATEGIES

An rRNA phylogeny of 22 species of ciliates belonging to seven of Small an Lynn's eight classes has been obtained by distance and parsimony methods. It displays good congruence with classical systematics at low taxonomic levels and several major surprises at higher levels: 1) The species analyzed group into five major branches, four of which emerge almost simultaneously: hypotrichs, oligohymenophorans, litostomes, and nassophoreans corresponding to four of Small and Lynn's classes. The simultaneous emergence of these groups contradicts the long accepted view that litostomes (a group with simple", symmetrical, apical oral apparatus) are primitive," while hypotrichs are "highly evolved." 2) Heterotrichs group with a karyorelictid, together forming the first emerging branch. While this supports the view that karyorelictids may be early-emerging ciliates, it completely explodes the traditional "spirotrichs" taxon, which united heterotrichs and hypotrichs. Instead, this reinforces the concept af Postciliodesmatophora and suggests that asymmetric oral apparatuses (i.e., with distinct paroral and adoral ciliatures) may be primitive in ciliates. The global topology of the tree therefore does not fit with the classical views af ciliate evolution, from simple" oral apparatus and stomatogenesis to complex" ones. Instead, a rather striking agreement with the strategy adopted to construct the cortical framework was disclosed. We noted that the cytoskeletal elements used to strengthen the cell surface could be subdivided into four main types: epiplasm, filaments, continuous microtubules, or basal body derived fibers. These four types fitted quite well with the major evolutionary lines disclosed by the molecular phylogeny. We therefore discuss unorthodox hypotheses assuming an early explosive radiation of ciliates into a small number of major lineages differing essentially in the solution adopted to subtend the cell surface and anchor the infraciliature.