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with a partially edited cDNA. The driving force for the evolutionary fixation of these retroposition events was postulated to be the stochastic loss of entire minicircle sequence classes and their encoded guide RNAs upon segregation of the single kinetoplast DNA network into daughter cells at cell division. A large plasticity in the relative abundance of minicircle sequence classes has been observed during cell culture in the laboratory. Computer simulations provide theoretical evidence for this plasticity if a random distribution and segregation model of minicircles is assumed. The possible evolutionary relationship of the C to U and U-insertion editing systems is discussed.

The term RNA editing describes several types of posttranscriptional modifications of RNAs that involve either specific insertion/deletion or modifications of nucleotides (Smith et al., 1997). The uridine (U)-insertion/deletion type of editing has so far only been found to occur in the mitochondria of kinetoplastid protists (Alfonzo et al., 1997; Stuart et al., 1998). We recently showed that C to U nucleotide modification editing also occurs in the mitochondria of these cells (Alfonzo et al., 1999). The origin and evolution of these two genetic systems is the subject of this paper.

KINETOPLASTID PROTISTS CONSIST OF TWO MAJOR GROUPS: THE TRYPANOSOMATIDS AND THE BODONIDS

Kinetoplastid protists belonging to the Euglenozoa phylum, according to rRNA phylogenetic trees, represent one of the earliest mitochondrial-containing extant branches of the eukaryotic lineage (Cavalier-Smith, 1997). This view may change in the future, as protein-based phylogenies favor a later divergence of Euglenozoa (Budin and Philippe, 1998; Germot and Philippe, 1999; Philippe and Forterre, 1999). However, even in such a case, this phylum still demonstrates a long and independent evolutionary history and is well separated from other eukaryotic groups. Taxonomists previously have proposed the existence of two suborders in the Kinetoplastida, the Trypanosomatina and Bodonina. All of the pathogenic trypanosomatids belong to the suborder, Trypanosomatina, and to the single family, Trypanosomatidae. Phylogenetic reconstructions using nuclear SSU rRNA sequences have confirmed the separation of the trypanosomatids as a derived late-emerging group. The trypanosomes, which initially were thought to be paraphyletic, with Trypanosoma brucei as an early-diverging branch (Fernandes et al., 1993; Landweber and Gilbert, 1994; Maslov et al., 1994), are now thought more likely to be monophyletic (Alvarez et al., 1996; Lukes et al., 1997) (Fig. 1). There are two major clades of trypanosomatids, the trypanosomes and the clade of Leishmania,



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