Two types of ribosomal RNA genes in hybrid Trypanosoma cruzi strains.

Trypanosoma cruzi isolates can be divided into two major phylogenetic lineages-T. cruzi I and T. cruzi II. The population structure is predominantly clonal, with sexuality having no or limited influence on the evolution of the parasite. Isoenzymes and nuclear gene sequences have provided evidence that some T. cruzi strains are hybrids. Previous work of our group has shown that the putative hybrid strains designated as group 1/2 contain two types of rDNA units, corresponding to those found in T. cruzi I and T. cruzi II. In this study, the presence and transcription of the two types of ribosomal RNA (rRNA) cistrons were investigated in epimastigotes, metacyclic and tissue culture trypomastigotes of group 1/2 isolates. PCR and RT-PCR assays indicate that both types of cistrons are present in group 1/2 strains, but only type-2 genes are transcribed in all developmental stages. The structure of the promoter regions of group 1/2 was compared to reference T. cruzi I and T. cruzi II strains. In all cases, the transcription start point was mapped to a conserved A residue located approximately 1800 bp upstream the 18S rRNA gene. The distribution of rDNA clusters in chromosomal bands of group 1/2 was evaluated by pulsed-field gel electrophoresis (PFGE). The majority of type-2 rDNA genes are localized in a 1.5 Mbp band, whereas type-1 cistrons are mostly concentrated in a 1.1 Mbp band. The structural and functional studies of group 1/2 ribosomal cistrons described here may shed light on the evolutionary processes that took place during the generation of such hybrid organisms.