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Genetic variation

A nomenclature for T. cruzi has been adopted since 2009 and includes six discrete taxonomic units (DTUs), namely, T. cruzi I (TcI), T. cruzi II (TcII), T. cruzi III (TcIII), T. cruzi IV (TcIV), T. cruzi V (TcV), and T. cruzi VI (TcVI), based on different molecular markers and biological features.2 Recently, T. cruzi bat has been described as an independent DTU, based on phylogenetic and phylogeographical analyses, multiple molecular markers, and degrees of sequence divergence between T. cruzi bat and the six already reported DTUs.3

Many eukaryotic pathogenic microorganisms that were previously assumed to propagate clonally have retained cryptic sexual cycles. The T. cruzi parasite comprises a heterogeneous population that displays clonal propagation due to the different cycles of transmission, and also the possibility of recombination exchange that can be found in nature. Several reports indicate that natural hybridization in T. cruzi may be frequent, potentially involving independent exchange of kinetoplast and nuclear genetic material as well as canonical meiotic mechanisms.4

Different molecular markers including a 48 set of microsatellite loci have shown the great diversity in TcI.5—7 Primers designed based on the sequences of TcI confirmed the existence of three genotypes (Ia, Ib, and Id) and a new genotype found in the Southern Cone countries named as TcIe.8 Genetic variability has also been clearly demonstrated, reporting homogeneous (TcII) and heterogeneous groups considered hybrids due to recombination events (TcIII—TcVI).9,10 Hybrids are considered within T. cruzi showing TcIII and TcIV probably as a product of recombination of TcI and TcII and TcIV— TcVI as a product of recombination of T. cruzi II and T. cruzi III/T. cruzi IV,11 although this last statement is still controversial.

The molecular epidemiology and distribution of T. cruzi genotypes may have important implications on the disease features. However, a few correlations have been relating T. cruzi genetic variability and the disease outcome, showing T. cruzi I more related to patients with cardiomyopathy in Colombia and Venezuela and T. cruzi II—T. cruzi VI more related to patients with digestive syndrome.12

For triatomines, susceptibility or resistance to trypanosome infections seems to be modulated by the intestinal symbionts, which are vital for development. T. cruzi is considered to be subpathogenic to triatomines, whereas Trypanosoma rangeli is another species that commonly infects Rhodnius species and causes pathogenicity based on a reduction of the number of symbionts Some studies using different species of triatomines, such as R. pallescens, T. dimidiata, R. colombiensis, and P. geniculatus, have shown the affinity of TcI to infect these species in comparison with TcII transmitted by Triatoma infestans in the southern countries of South America.13

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