Molecular evolution inferred from small subunit rRNA sequences: What does it tell us about phylogenetic relationships and taxonomy of the parabasalids?

The Parabasala ore a primitive group of protists divided into two classes: the trichomonads and the hypermasigids. Until recently, phylogeny and toxon omy of parabasalids were mainly based on the comparative analysis of morpho logical characters primarily linked to the development of their cytoskeleto n. Recent use of molecular markers, such as small subunit (SSU) rRNA has le d to new insights into the systematics of the Parabasala and other groups o f protists. An updated phylogeny based on SSU rRNA is provided and compared to that inferred from ultrastructural data. The SSU rRNA phylogeny contrad icts the dogma equating simple characters with primitive characters. Hyperm astigids, possessing a hyperdeveloped cytoskeleton, exhibit the mosi basal emergence in the parabasalid lineage. Other observations emerge from the SS U rRNA analysis, such as the secondary loss of some cytoskeleton structures in all representatives of the Monocercomonadidoe, the existence of seconda rily free-living taxa (reversibility of parasitism) and the evidence agains t the co-evolution of the endobiotic parabasalids and their animal hosts. A ccording to phylogenies based on SSU rRNA, ail the trichomonad families ore not monophyletic groups, puffing into question the validity of current tax onomic assignments. The precise branching order of some taxo remains unclea r, but this issue can possibly be addressed by the molecular analysis of ad ditional parabasalids. The goal of such additional analyses would be to pro pose, in a near future, a revision of the toxonomy of this group of protist s that takes into account both molecular and morphological data.