Genomic changes of Chagas disease vector, South America

Abstract

We analyzed the main karyologic changes that have occurred during the dispersion of Triatoma infestans, the main vector of Chagas disease. We identified two allopatric groups, named Andean and non-Andean. The Andean specimens present C-heterochromatic blocks in most of their 22 chromosomes, whereas non-Andean specimens have only 4-7 autosomes with C-banding. These heterochromatin differences are the likely cause of a striking DNA content variation (approximately 30%) between Andean and non-Andean insects. Our study, together with previous historical and genetic data, suggests that T. infestans was originally a sylvatic species, with large quantities of DNA and heterochromatin, inhabiting the Andean region of Bolivia. However, the spread of domestic T. infestans throughout the non-Andean regions only involved insects with an important reduction of heterochromatin and DNA amounts. We propose that heterochromatin and DNA variation mainly reflected adaptive genomic changes that contribute to the ability of T. infestans to survive, reproduce, and disperse in different environments.

Figure 1

Location of the collection sites of individual Triatoma infestans analyzed in this study. Dotted lines indicate T. infestans distribution during the 1980s. Full circles indicate Andean samples. Open squares indicate non-Andean samples. (See number identification of each population in Tables 1–3.)

Figure 2

Representative C-banding patterns observed in male Triatoma infestans: 2n = 22 (20 autosomes plus XY in males/ XX females) coming from non-Andean (A-C) and Andean regions (D-F). Scale bar = 10 μm. A: Spermatogonial mitotic prometaphase. This specimen from Argentina presents the lowest number of C-banded autosomes (four chromosomes). B: First meiotic metaphase. Only two heterochromatic bivalents, formed by the pairing of the four C-banded autosomes showed in 2A, are observed. The Y (heterochromatic) and X (euchromatic) chromosomes appear as univalents, as typically observed in hemipteran insects. C: Karyotype obtained from 2A. Heterochromatic C-bands are clearly detected in four autosomes and in the Y sex chromosome. D: Mitotic prometaphase in male specimen from Andean Bolivia. Almost all chromosomes present C-bands in one or both chromosomal ends. E: First meiotic metaphase of the same insect shown in 2D. All bivalents except one (arrowhead) are formed by chromosome with C-bands. As observed in other hemipterans, the bivalents form a ring with the univalent sex chromosomes (X and Y) in the center. F: Karyotype obtained from 2D. Chromosome size and C-banding pattern are clearly different from those observed in 2C. Heterochromatic blocks are localized in most autosomes and in both sex chromosomes.

Figure 3

Gonial mitotic prometaphases in male (A) and female (B) specimens of Triatoma infestans from non-Andean regions. Scale bar = 10 μm. A: Most common C-banding pattern detected in non-Andean region (BB BB AA). This pattern is constituted by four autosomes with a C-block in both chromosomal ends (B morph) and two chromosomes with a C-block in only one telomere (A morph) indicated by arrowheads. The Y chromosome appears C-heterochromatic. The other 14 autosomes and the X chromosome are C-negative (euchromatic). B: Females only have C-bands in the autosomes; sex chromosomes (XX) are euchromatic and indistinguishable from autosomes without heterochromatin.

Figure 4

Meiotic pairing in the experimental male hybrid progeny between Andean and non-Andean specimens of Triatoma infestans. Scale bar = 10 μm. A: First meiotic metaphase in an insect obtained by crossing a female from Andean region (with C-banded X chromosomes) with a male from non-Andean region. As expected, both sex chromosomes are heterochromatic. As observed in normal specimens, the ten bivalents form a ring with the univalent sex chromosomes in the middle. Chromosome pairing was normal even between chromosomes with great heterochromatic differences. B: Selected bivalent with a diagram of its mitotic and meiotic configuration. As generally observed in hemipteran insects, only a single chiasma is represented. Chromosomes involved in the pairing have different C-patterns: one has C-bands in both chromosomal ends (B-morph); the other one is completely euchromatic (C-morph); the resulting bivalent is asymmetric. C: Selected bivalent with a diagram of its mitotic and meiotic configuration. Chromosomes involved have different C-patterns: one has C-bands in only one chromosomal end (A-morph) while the other one is completely euchromatic (C-morph); the resulting bivalent is also asymmetric. D: First meiotic metaphase in an insect obtained by crossing a female from non-Andean region (with euchromatic X chromosomes) with a male from Andean region. As expected, the X chromosome appears euchromatic in the hybrid. Chromosome pairing was completely normal even between chromosomes with great heterochromatic differences.