Epidemiology, biochemistry and evolution of Trypanosoma cruzi lineages based on ribosomal RNA sequences.

A puzzle in Chagas disease research has been to correlate molecular markers with epidemiology since isolates of Trypanosoma cruzi exhibit a broad host range, induce distinct clinical presentations in patients and show great diversity in biological and biochemical characteristics. Early studies on population genetics revealed substantial isozymic variability among isolates of T. cruzi defining three major groups or zymodemes named Z1, Z2 and Z3 (Miles et al. 1978, 1980). Further analysis of 15 gene loci disclosed a greater heterogeneity determining the distribution of 121 T. cruzi isolates into 43 zymodemes that could not be grouped by the authors in few natural clusters (Tibayrenc & Ayala 1988). The main conclusion of these studies was that the population structure of this parasite is clonal rather than sexual, and, as a consequence, the present genetic and biological variability of T. cruzi is resultant from the independent evolution of clonal lines (Tibayrenc & Ayala 1988). Acceptance of clonal propagation of T. cruzi implies that the classical definition of species is not fully adequate to this parasite. It has been proposed that each distinct genotype should be studied individually and special attention given to the most represented ones (Tibayrenc 1995). On the other hand, other groups still believe that it is worth searching for genetically and ecologically coherent group-