The three taxa emerging at the base of the eukaryotic ribosomal RNA phylogenetic tree (Diplomonadida, Microspora, and Parabasalia) include a wide array of parasitic species, and some free-living organisms that appear to be derived from a parasitic ancestry. The basal position of these taxa, which lack mitochondria, has recently been questioned. I sequenced most of the ribosomal RNA gene cluster of the free-living diplomonad Trepomonas agilis and a secondary structure model was reconstructed for the SSU rRNA. I conducted a RASA matrix analysis to identify, independently from tree reconstruction, putative long branch attraction effects in the data matrix. The results show that each of the basal clades and the euglenozoan clade act, indeed, as long branches and may have been engaged in a process of accelerated rate of evolution. A nucleotide signature analysis was conducted in the conserved regions for positions defining the three great domains of life (Eubacteria, Archea, and Eukaryota). For the three basal taxa, this analysis showed the presence of a significant number of different non-eukaryotic nucleotides. A precise study of the nature and location of these nucleotides led to conclusions supporting the results of the RASA analysis. Altogether, these findings suggest that the basal placement of these taxa in the SSU ribosomal RNA phylogenetic tree is artifactual, and flawed by long branch attraction effects.
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