Searching for a Tree That Can be Trusted

Abstract

In his Comment `Can Trypanosoma Trees be Trusted?' Noyes[1xNoyes, H. Parasitol. Today. 1998; 14: 49–50Abstract | Full Text | Full Text PDF | PubMed | Scopus (9)See all References[1]discusses possible reasons for a faster evolution in the Salivarian species compared to the remaining trypanosomes and other trypanosomatids, as it appears in the published rRNA phylogenetic trees[2xFernandes, A.P., Nelson, K., and Beverley, S.M. Proc. Natl. Acad. Sci. U. S. A. 1993; 90: 11608–11612Crossref | PubMed | Scopus (150)See all References, 3xBerchtold, M. et al. Parasitol. Res. 1994; 80: 672–679Crossref | PubMed | Scopus (12)See all References, 4xMarche, S. et al. Mol. Biochem. Parasitol. 1995; 71: 15–26Crossref | PubMed | Scopus (38)See all References, 5xMaslov, D.A. et al. Mol. Biochem. Parasitol. 1996; 75: 197–205Crossref | PubMed | Scopus (149)See all References], and also questions the reliability of such trees. This scepticism is well justified.It is known that unequal rates of sequence evolution in different lineages may result in the `branch attraction' artefact[6xFelsenstein, J. Syst. Zool. 1978; 27: 401–410CrossrefSee all References[6]. Speaking in terms of the trypanosome trees, accelerated rates of evolution in Salivarian trypanosomes would cause the long branches of an outgroup (the cryptobiid Trypanoplasma borreli) and Trypanosoma brucei to get together to the exclusion of all other species, thus rendering the entire genus Trypanosoma paraphyletic[2xFernandes, A.P., Nelson, K., and Beverley, S.M. Proc. Natl. Acad. Sci. U. S. A. 1993; 90: 11608–11612Crossref | PubMed | Scopus (150)See all References, 5xMaslov, D.A. et al. Mol. Biochem. Parasitol. 1996; 75: 197–205Crossref | PubMed | Scopus (149)See all References]. A known cure to this problem includes finding species which would `cut' the long branches[7xHendy, M.D. and Penny, D. Syst. Zool. 1989; 38: 297–309CrossrefSee all References[7]. We have recently attempted this kind of analysis[8xLukes, J. et al. J. Mol. Evol. 1997; 44: 521–527Crossref | PubMed | Scopus (84)See all References[8]. Subdivision of the long branches was done by using several other Salivarian species and a reptile trypanosome, as well as two additional bodonids (Dimastigella trypaniformis and Rhynchobodo sp.). Additional reduction of the homoplasy (similarity due to reversions or parallelism and not a common origin) was achieved by using only the most conserved sites of the rRNA alignment. Subsequently, we saw a bootstrap support for the monophyly of trypanosomes increasing to 97% and 92% levels for maximum likelihood and maximum parsimony analyses, respectively. This result is in agreement with the protein-based trees[9xWiemer, E.A.C. et al. J. Mol. Evol. 1995; 40: 443–454Crossref | PubMed | Scopus (39)See all References, 10xAlvarez, F. et al. Mol. Phylogenet. Evol. 1996; 5: 333–343Crossref | PubMed | Scopus (26)See all References]. However, the best monophyletic trees were not significantly shorter than the trees in which trypanosomes were `forced' to be paraphyletic. We interpreted these results as evidence for the victory of the monophyletic model, however, the margin is very narrow.Returning to the question asked by Noyes, we hope that including additional trypanosome sequences and finding closer bodonid outgroup species would result in the trypanosomatid trees finally become more trustable and provide us with a lot of interesting information on `evolutionary expansion'[11xVickerman, K. Int. J. Parasitol. 1994; 24: 1317–1331Crossref | PubMed | Scopus (118)See all References[11]of these rather successful parasites.