The evolution of salivarian trypanosomes.

CA Hoare (1972 The Trypanosomes of Mam-mals, Blackwell, Oxford, 749 pp.) divided mam-malian trypanosomes into two sections, theSalivaria and the Stercoraria. The Salivaria com-prises all the African tsetse-transmitted trypano-somes of mammals, while the Stercoraria includesmammalian trypanosomes transmitted by the pos-terior route. However, the status of other tsetse-transmitted species, e.g. Trypanosoma grayi fromcrocodiles, and of other trypanosomes transmittedby the anterior route, e.g. T. rangeli from SouthAmerica and leech transmitted trypanosomes, re-main under debate. To some extent such unknownsare linked to the limitations of the morphologicaland transmission characters used in early investi-gations of trypanosome taxonomy and evolution.With the wealth of molecular sequence data nowavailable, particularly for the phylogenetically ver-satile 18S rRNA gene (ML Sogin et al. 1986 ProcNatl Acad Sci USA 83: 1383-1387) some of thesequestions may now be fruitfully re-addressed.Accordingly, we undertook phylogenetic analysisof published ribosomal RNA 18S sequences,supplemented with data from our own studies, ouraim being to re-examine the evolutionary relation-ships of Salivarian trypanosomes of mammals andother vertebrates.Fourteen Trypanosoma species 18S rRNA se-quences were obtained from EMBL/GenBank forphylogenetic analysis: T. boissoni U39580; T.carassii L14841; T. rotatorium U39583; T. triglaeU39584; T. brucei brucei M12676; T. congolense(kilifi-type) U22317; T. congolense (forest-type)U22319; T. congolense (savannah-type) U22315;T. simiae U22320; T. vivax U22316; T. cruziX53917; T. cruzi M31432; T. avium U39578; T.scelopori U67182; together with three newly avail-able sequences (JR Stevens et al. 1999 Parasitol-ogy 118: 107-116.); T. grayi AJ005278; T. rangeliAJ009160; T. varani AJ005279. Five additional18S sequences from a range of other kinetoplastidspecies were included as outgroups: Crithidiafasciculata X03450; Leishmania amazonensisX53912; L. major X53915; Trypanoplasma borreliL14840; Bodo caudatus X53910. Sequences werealigned primarily on the basis of their secondarystructure (J-M Neefs et al. 1990 Nucleic Acids Res18: 2237-2243). Sub-sections of the alignment,between regions of high homology were sub-aligned using the program Clustal V (DG Higginset al. 1992 Comp Applns Biosci 8: 189-191), be-fore final adjustments were made by eye.Bootstrapped maximum parsimony analysis of the22 rRNA 18S sequences was performed with 100replicates using test version 4.0d63 of PAUP*,written by David L Swofford.The phylogenetic analysis (Figure) places theSalivarian trypanosomes in a monophyletic cladecomprising exclusively mammalian trypanosomesof African origin. Within the Salivarian group, thevarious types of T. congolense also constitute amonophyletic group. T. rangeli, T. grayi and try-panosomes of fish and amphibia are excluded fromthe Salivarian clade; T. rangeli is placed firmly ina separate clade with T. cruzi . A third major clade,comprising trypanosomes with aquatic hosts, formsa separate early branch within the monophyleticTrypanosoma which is not directly ancestral to ei-ther the Salivaria or the Stercoraria. Branches re-ceiving less than 50% bootstrap support are pre-sented as polytomies.In agreement with a number of previous stud-ies (F Alvarez et al. 1996 Mol Phylogen Evol 5:333-343, J Lukes et al. 1997 J Mol Evol 44: 521-527), our analysis confirms the monophyly of theAfrican Salivarian trypanosomes. Within theSalivaria, our results also indicate that the varioustypes of T. congolense (forest, kilifi, savannah)share common ancestry. Such a finding contrastswith the results of isoenzyme and RAPD basedstudies (I Sidibe, cited M Tibayrenc 1998 Int JParasitol 28: 85-104), in which it is suggested thatT. congolense may be polyphyletic. We proposethat this apparent difference in conclusions is due