Analysis of Ribosomal RNA Genes Suggests That Trypanosomes Are Monophyletic

Abstract

To further investigate the phylogeny of protozoa from the order Kinetoplastida we have sequenced the small subunit (SSU) and a portion of the large subunit (LSU) nuclear rRNA genes. The SSU and LSU sequences were determined from a lizard trypanosome, Trypanosoma scelopori and a bodonid, Rhynchobodo sp., and the LSU sequences were determined from an insect trypanosomatid, Crithidia oncopelti, and a bodonid, Dimastigella trypaniformis. Contrary to previous results, in which trypanosomes were found to be paraphyletic, with Trypanosoma brucei representing the earliest-diverging lineage, we have now found evidence for the monophyly of trypanosomes. Addition of new taxa which subdivide long branches (such as that of T. brucei) have helped to identify homoplasies responsible for the paraphyletic trees in previous studies. Although the monophyly of the trypanosome clade is supported in the bootstrap analyses for maximum likelihood at 97% and maximum parsimony at 92%, there is only a small difference in ln-likelihood value or tree length between the most optimal monophyletic tree and the best suboptimal paraphyletic tree. Within the trypanosomatid subtree, the clade of trypanosomes is a sister group to the monophyletic clade of the nontrypanosome genera. Different groups of trypanosomes group on the tree according to their mode of transmission. This suggests that the adaptation to invertebrate vectors plays a more important role in the trypanosome evolution than the adaptation to vertebrate hosts.

Kinetoplastid phylogenetics, with special reference to the evolution of parasitic trypanosomes

“…E-mail: j.r.stevens@ex.ac.uk 2000) and even in those parasites of medical importance, it is little more than a decade since the widespread introduction of DNA sequencing technologies has allowed the evolution of parasitic trypanosomatids (and kinetoplastids in general) to be evaluated by formal phylogenetic analysis (Fernandes et al, 1993;Alvarez et al, 1996;Maslov et al, 1996;Lukes et al, 1997;Haag et al, 1998;Hannaert et al, 1998;Stevens et al, 1999Stevens et al, , 2001Wright et al, 1999;Hamilton et al, 2004;Moreira et al, 2004). To fully understand the evolutionary history of parasitic kinetoplastids and to understand the context within which the evolution of each parasite group has unfolded, an understanding not just of the parasites, but also of broader kinetoplastid phylogenetics is required.…”

“…A monophyletic group consists of taxa that are descended from a single common ancestor and contains all known descendants from that ancestor. The first molecular phylogenetic studies, based on comparisons of mitochondrial and nuclear ribosomal DNAs (rDNA), showed trypanosomes to be paraphyletic (Gomez et al, 1991;Fernandes et al, 1993;Maslov et al, 1996), while subsequent rDNA-based studies, which have included more taxa from a far broader range of host species (Lukes et al, 1997;Stevens et al, 1999Stevens et al, , 2001Wright et al, 1999;Simpson et al, 2002;Hamilton et al, 2004) (Fig. 2) supported monophyly.…”

The Evolution of Trypanosomes Infecting Humans and Primates

“…The ssu rRNA gene has become the marker of choice for evolutionary analyses of the kinetoplastid protozoa (e.g. Fernandes et al 1993, Maslov et al 1994, Marché et al 1995, Lukes et al 1997, Haag et al 1998.…”

“…However, it is only recently, with advances in molecular and phylogenetic methods, that significant progress towards resolving trypanosome evolutionary history is being made (e.g. Lake et al 1988, Fernandes et al 1993, Wiemer et al 1995, Maslov et al 1996, Lukes et al 1997, Haag et al 1998.…”

Future trypanosomatid phylogenies: refined homologies, supertrees and networks

“…Fernandes et al (1993), Maslov et al (1996) and Lukes et al (1997) utilised variation within the 18S to infer phylogeny within the kinetoplatids leading to the suggestion that Trypanosoma was monophyletic. Noyes (1998) expanded upon the taxonomic coverage of the 18S data and stimulated much debate both in biogeographic implications (Stevens & Gibson 1998) and methodology of inference from these data (Maslov & Lukes 1998, Noyes & Rambaut 1998.…”