Sex, subdivision, and domestic dispersal of Trypanosoma cruzi lineage I in southern Ecuador

Abstract

Background: Molecular epidemiology at the community level has an important guiding role in zoonotic disease control programmes where genetic markers are suitably variable to unravel the dynamics of local transmission. We evaluated the molecular diversity of Trypanosoma cruzi, the etiological agent of Chagas disease, in southern Ecuador (Loja Province). This kinetoplastid parasite has traditionally been a paradigm for clonal population structure in pathogenic organisms. However, the presence of naturally occurring hybrids, mitochondrial introgression, and evidence of genetic exchange in the laboratory question this dogma.

Methodology/principal findings: Eighty-one parasite isolates from domiciliary, peridomiciliary, and sylvatic triatomines and mammals were genotyped across 10 variable microsatellite loci. Two discrete parasite populations were defined: one predominantly composed of isolates from domestic and peridomestic foci, and another predominantly composed of isolates from sylvatic foci. Spatial genetic variation was absent from the former, suggesting rapid parasite dispersal across our study area. Furthermore, linkage equilibrium between loci, Hardy-Weinberg allele frequencies at individual loci, and a lack of repeated genotypes are indicative of frequent genetic exchange among individuals in the domestic/peridomestic population.

Conclusions/significance: These data represent novel population-level evidence of an extant capacity for sex among natural cycles of T. cruzi transmission. As such they have dramatic implications for our understanding of the fundamental genetics of this parasite. Our data also elucidate local disease transmission, whereby passive anthropogenic domestic mammal and triatomine dispersal across our study area is likely to account for the rapid domestic/peridomestic spread of the parasite. Finally we discuss how this, and the observed subdivision between sympatric sylvatic and domestic/peridomestic foci, can inform efforts at Chagas disease control in Ecuador.

Figure 1. Distribution of genotypes in communities and transmission cycles in Loja Province.

Black dots indicate locations from which isolates were obtained. Colored circles indicate proportion of LOJA _Sylv (green) and LOJA _Dom/Peri (red) genotypes (see Figure 2) among T. cruzi isolates from foci designated as sylvatic (S) or domestic/peridomestic (D/P). Crossed circles indicate absence of isolates from a particular location.The area of each pie chart represents the number of samples. LOJA _Dom/Peri is more common among D/P foci, largely irrespective of community. However, some cross-propagation between transmission cycles is evident. Insert shows in gray the location of Loja province within Ecuador and of Ecuador within South America.

Figure 2. Neighbor-joining tree based on D _AS values from 10 microsatellite loci.

Continental-scale affiliations of Loja TcI strains are shown in the insert figure and the local subdivision between transmission cycles is shown in the main figure. T. cruzi D _AS values correspond to the mean of 1,000 random diploid re-samplings of the dataset. Solid grey triangles indicate samples from other regions of South America while colored triangles indicate Ecuadorian populations (LOJA _Dom/Peri and LOJA _Sylv). Branch color codes indicate capture environment. Blue: domicile; Red: peridomicile; Green: sylvatic. Sample codes were defined as follows: M: mammal; Dm: Didelphis marsupialis; Rr: Rattus rattus; Ss: Sciureus stramineus; V: vector; Pc: Panstrongylus chinai; Tc: Triatoma carrioni; Re: Rhodnius ecuadoriensis; A: adult; N: nymph. Values in italics correspond to bootstraps (%) over 10,000 trees drawn from 100 randomly sampled diploid datasets. Values in bold correspond to topological stability (percentage congruent trees) over 1,000 randomly sampled diploid datasets (see Methods ).

Figure 3. Spatial genetic analysis among T. cruzi isolates from LOJA _Sylv and LOJA _Dom/Peri.

A spatial structure was evident within LOJA _Sylv isolates, while no spatial genetic structure was found among LOJA _Dom/Peri isolates. Genetic (D _AS) and geographic (km) distance were compared. Closed circles and line correspond to samples from LOJA _Sylv (R_XY = 0.265, p = 0.000; Slope = 0.0015±0.0001 (SE)); open circles and dashed line correspond to samples from LOJA _Dom/Peri (R_XY = 0.100, p = 0.164; Slope = 0.0004±0.0004 (SE)). Equivalent statistics under δμ² are LOJA _Sylv R_XY = 0.177, p = 0.001; LOJA _Dom/Peri R_XY = −0.052, p = 0.384.