Evidence for multiple origins of human infectivity in Trypanosoma brucei revealed by minisatellite variant repeat mapping

In recent years a wide variety of biochemical and molecular typing systems has been employed in the study of parasite diversity aimed at investigating the level of genetic diversity and delineating the relationship between di fferent species and subspecies. However, such methods have failed to differ entiate between two of the classically defined subspecies of the protozoan parasite Trypanosoma brucei: the human infective, T. b. rhodesiense, which causes African sleeping sickness, and the non-human infective T. b. brucei. This has led to the hypothesis that T, b. rhodesiense is a host range vari ant of T. b. brucei. In this paper we test this hypothesis by examining hig hly polymorphic tandemly repeated regions of the trypanosome genome, i.e., minisatellite loci. We have employed the technique of minisatellite variant repeat mapping by PCR (MVR-PCR), which determines the distribution of vari ant repeat units along the tandem array of:one minisatellite, MS42. The map s generated by this technique not only allow unequivocal allele identificat ion but also contain within them cladistic information which we used to det ermine the possible genetic relationship between the different subspecies o f T, brucei Our findings revealed that human infective (T. b. rhodesiense) isolates from Uganda are more closely related to the local non-human infect ive isolates (T. b. brucei) than they are to other human infective stocks f rom different regions, suggesting that human infectivity has originated ind ependently in these different geographical regions. This would infer that t he separate classification of all human infective stocks from East Africa i nto the subspecies T. b. rhodesiense is genetically inappropriate and it wo uld be better to consider geographically separate populations as host range variants of T. brucei brucei or perhaps as a series of different subspecie s. Based on these data, it is clear that MVR mapping is a very useful tool for the analysis of zoonotic eukaryotic pathogens where delineation of the origins of outbreaks of disease and definition of human infective strains a re key questions.