varies in different species. For example, the ND7 gene in the trypanosomes, T. brucei and T. cruzi, is pan-edited in two domains, whereas in the Leishmania-Crithidia clade this gene is edited only at the 5′ end of each domain. The A6 gene in the trypanosomes is pan-edited, whereas in the Leishmania-Crithidia clade, the editing of the A6 gene shows a gradient of restriction to the 5′ end of the single domain, from L. tarentolae, to Herpetomonas muscarum, to Phytomonas serpens, and to Blastocrithidia culicis (Maslov et al., 1994).

To date, the deepest lineage in which U-insertion editing has been detected is the bodonid group. Because minicircles, which presumably encode gRNAs, are observed in B. caudatus, B. saltans, and C. helicis, this suggests that the free noncatenated state is a primitive feature. Catenation of minicircles to form the kDNA network probably arose in an ancestor of the trypanosomatids as a molecular mechanism designed to avoid minicircle losses by missegregation. Concatenation of minicircles in the 180-kb megacircle as observed in T. borreli might have independently arisen as another solution to the same problem. However, additional analyses of the kDNA structure in bodonids are required to shed more light on kDNA evolution.

The only mitochondrial gene isolated from the deeper branching Diplonema (Maslov et al., 1999) and Euglena gracilis (Tessier et al., 1997; Yasuhira and Simpson, 1997) is the COI gene, which is unedited. In addition, no evidence was obtained for small gRNA-like molecules in E. gracilis mitochondria by 5′ capping experiments (Yasuhira and Simpson, 1997). This preliminary evidence does not, of course, eliminate the possibility of editing in these cells, but the simplest hypothesis is that this type of editing evolved in the mitochondrion of an ancestral bodonid after the split from the euglenoid lineage.

The only species for which the entire complement of gRNAs is known (Maslov and Simpson, 1992) is the UC strain of L. tarentolae, which has been maintained as the promastigote form in culture in various laboratories for more than 60 years. There are 15 maxicircle-encoded gRNAs and 17 minicircle-encoded gRNAs in this strain. Five pan-edited genes (G1–G5) show a complete absence of productive editing in this strain, as evidenced by an inability to PCR-amplify mature edited transcripts by standard methods. These genes are productively edited in T. brucei. Two of the minicircle-encoded gRNAs in the L. tarentolae UC strain, gLt19 (=gG4-III) and gB4 (=gND3-IX), represent nonessential gRNAs for these nonfunctional editing cascades. This was determined by analyzing the minicircle-encoded gRNA complement of LEM125, a recently isolated