Isozyme variability of Trypanosoma brucei s.l.: genetic, taxonomic, and epidemiological significance.

Seventy-eight Trypanosoma brucei s.l. stocks from different hosts, representing the three Trypanosoma brucei subspecies and three Trypanosoma evansi stocks, were studied for variation at 18 polymorphic isozyme loci. The results were used to determine the genetic variability among stocks and to estimate gene flow among populations. Total genetic variability in T. brucei s.l. was less than that in Trypanosoma cruzi, the agent of Chagas' disease. Results support a clonal population structure in T. brucei, but do not preclude a hypothesis of occasional mating. However, some natural clones of T. brucei s.l. appear as genetically stable and should be considered as useful taxonomic units in applied studies. Greater genotypic diversity was observed in trypanosomes isolated from wild mammals. Numerical taxonomy methods identified a group of clones representing most of the human stocks from Central and West Africa. This group probably corresponds to Trypanosoma brucei gambiense "group I" (Gibson, Parasitology Today 2, 255-257, 1986). As reported elsewhere, genetic evidence of the subspecies Trypanosoma brucei brucei and Trypanosoma brucei rhodesiense was ambiguous, suggesting that these taxa represent more "nosodemes" rather than actual genetic clades.