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. 2018 Dec 17;100(2):303–305. doi: 10.4269/ajtmh.18-0075

Identification Key for the Chagas Disease Vectors of Five Brazilian States, Based on Cytogenetic Data

Kelly Cristine Borsatto 1, Maria Tercília Vilela Azeredo-Oliveira 1, Kaio Cesar Chaboli Alevi 1,*
PMCID: PMC6367600  PMID: 30560768

Abstract.

Chagas disease is a public health problem caused by the protozoan Trypanosoma cruzi that affects about 8 million people worldwide. The main form of transmission of T. cruzi is vectorial, through triatomines feces contaminated with the parasite. All species are considered as potential vectors of T. cruzi. The main identification keys of these vectors are based only on morphological characters. However, there are very similar or even same species (cryptic species) that may lead to wrong classification of the vectors. Therefore, we developed an identification key using cytogenetic data, to aid and help the correct classification of triatomines. From the cytogenetic characters, identification keys were created for the five Brazilian states (Alagoas, Amapá, Ceará, Roraima, and Santa Catarina). These data are important because the correct classification of triatomines helps directly the activity of the vector control programs.

Chagas disease is a public health problem in Latin America and is increasingly spreading to new geographic regions such as Europe, North America, Japan, and Australia, mainly associated with the migration of people infected by the protozoan Trypanosoma cruzi (Chagas, 1909) (Kinetoplastida, Trypanosomatidae), the etiologic agent of Chagas disease.1–3 Although it is estimated that eight million people are infected by T. cruzi and ten thousand chagasic patients die annually, approximately 70 million people are at risk of contracting Chagas disease, making it the main cause of death due to parasitic disease in Latin America and the main cause of infectious cardiomyopathy in the world.3–6

The main form of transmission of T. cruzi is vectorial, through the feces of triatomines contaminated with the parasite because these insects are hematophagous and have the habit of defecating during the blood repast.3,7 The triatomines belong to the order Hemiptera, suborder Heteroptera, family Reduviidae, and subfamily Triatominae.8 Currently, 152 species are admitted in the subfamily Triatominae, grouped into 18 genera and five tribes.9 Brazil presents a great diversity of species,10 that, as a result of anthropic actions (such as deforestation and burning), are migrating to the house environment, a process known as domiciliation.11

Although there are species with a greater or lesser degree of importance in the transmission of Chagas disease (with emphasis on Triatoma infestans [Klug, 1834], Rhodnius prolixus Stål, 1859, Triatoma dimidiata [Latreille, 1811], Panstrongylus megistus [Burmeister, 1835], and Triatoma brasiliensis Neiva, 1911, which are worldwide in the transmission of the disease, as well as T. infestans, T. brasiliensis, Triatoma pseudomaculata Corrêa & Espínola, 1964, Triatoma sordida [Stål, 1859] P. megistus, which present the greatest vectorial competence in Brazil),10 all triatomines of both sexes and at any stage of development are considered as potential vectors of this disease.

The taxonomy and systematics of triatomines are mainly based on morphological characters and the main identification keys of these vectors used only these characters.8,9,12 However, there are very similar or even same species (cryptic species) from the morphological point of view,13 hindering the activity of vector control programs. Thus, the use of new approaches is necessary to characterize a taxon, and cytogenetic14–20 and molecular21–25 analyses have been shown to be important tools to differentiate these hematophagous insects.

Based on this, we developed an identification key based on cytogenetic data, with the purpose of helping and facilitating the correct classification of triatomines.

Cytogenetic data available in the literature were grouped and used for the construction of the identification keys of the triatomines present in the Brazilian states.14–16 Because 60 species were never studied cytogenetically, we present only the keys referring to the states in which all the triatomines had already been analyzed, namely Alagoas, Amapá, Ceará, Roraima, and Santa Catarina (the geographical distribution of triatomines was based on Galvão10).

From the cytogenetic characteristics (karyotype, constitutive heterochromatin pattern in chromatin, and chromosomes and localization of 45S rDNA probes), identification keys were created for the five Brazilian states (Table 1).

Table 1.

Key for the Brazilian triatomines based on cytogenetic data

Identification Key
Alagoas State
1. Karyotype with 2n = 21 chromosomes (18A + X1X2Y) Panstrongylus megistus
2. Karyotype with 2n = 23 chromosomes (20A + X1X2Y) Triatoma tibiamaculata
3. Karyotype with 2n = 24 chromosomes (20A + X1X2X3Y) Panstrongylus lutzi
4. Karyotype with 2n = 25 chromosomes (22A + X1X2Y) Triatoma rubrofasciata
5. Karyotype with 2n = 22 chromosomes (20A + XY) 6
6a. Prophase without heterochromatin blocks dispersed inside the nucleus Psammolestes tertius
6b. Prophase with heterochromatic blocks dispersed inside the nucleus 7
7a. Heterochromatin in 3--4 pairs of autosomes Triatoma pseudomaculata
7b. Heterochromatin in the all autosomes Triatoma brasiliensis
Amapá State
1. Karyotype with 2n = 23 chromosomes (20A + X1X2Y) Panstrongylus geniculatus
2. Karyotype with 2n = 22 chromosomes (20A + XY) 3
3a. 45S rDNA probe located in X chromosome Rhodnius robustus
3b. 45S rDNA probe located in X and Y chromosomes Rhodnius pictipes
Ceará State
1. Karyotype with 2n = 21 chromosomes (18A + X1X2Y) Panstrongylus megistus
2. Karyotype with 2n = 23 chromosomes (20A + X1X2Y) Panstrongylus geniculatus
3. Karyotype with 2n = 24 chromosomes (20A + X1X2X3Y) Panstrongylus lutzi
4. Karyotype with 2n = 22 chromosomes (20A + XY) 5
5a. Prophase without heterochromatin blocks dispersed inside the nucleus Psammolestes tertius
5b. Prophase with heterochromatic blocks dispersed inside the nucleus 6
6a. 45S rDNA probe located in sex chromosome (X) Rhodnius nasutus
6b. 45S rDNA probe located in autosomes 7
7a. Heterochromatin in 3--4 pairs of autosomes Triatoma pseudomaculata
7b. Heterochromatin in the all autosomes Triatoma brasiliensis
Roraima State
1. Karyotype with 2n = 23 chromosomes (20A + X1X2Y) Panstrongylus geniculatus
2. Karyotype with 2n = 22 chromosomes (20A + XY) 3
3a. Prophase with heterochromatic blocks dispersed inside the nucleus Triatoma maculata
3b. Prophase without heterochromatin blocks dispersed inside the nucleus 4
4a. 45S rDNA probe located in X chromosome Rhodnius robustus
4b. 45S rDNA probe located in X and Y chromosomes Rhodnius pictipes
Santa Catarina State
1. Karyotype with 2n = 21 chromosomes (18A + X1X2Y) Panstrongylus megistus
2. Karyotype with 2n = 23 chromosomes (20A + X1X2Y) Triatoma tibiamaculata
3. Karyotype with 2n = 22 chromosomes (20A + XY) 4
4a. Prophase without heterochromatin blocks dispersed inside the nucleus Rhodnius domesticus
4b. Prophase with heterochromatic blocks dispersed inside the nucleus Tratoma sordida
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Karyotypes were the main tools used to differentiate species within each Brazilian state (Table 1). The karyosystematic of triatomines was initiated in 2012 when Alevi et al.17 proposed the exclusion of Triatoma melanocephala Neiva & Pinto, 1923, Triatoma vitticeps (Stål, 1859) Triatoma tibiamaculata (Pinto, 1926) of the Brasiliensis subcomplex because they had different chromosomes. In addition, Alevi et al.18 point out that this tool made it possible to differentiate Triatoma rubrofasciata (De Geer, 1773) from all other triatomine species.

In the case where the species had the same number of chromosomes, the constitutive heterochromatin pattern and the location of the 45S rDNA probes were important tools to differentiate the species in the identification keys (Table 1). The presence or absence of heterochromatin in chromatin or chromosomes is a very common tool for the taxonomy and systematics of triatomines. For example, the characterization of the last described triatomine species, Rhodnius taquarussuensis Rosa et al.,19 was supported by the presence of heterochromatin in the autosomes, because most Rhodnius, including the related species Rhodnius neglectus Lent, 1954, do not present heterochromatin in the autosomes. Furthermore, the position of the 45S rDNA probes has also been used to differentiate/group Triatominae species, because the chromosomal site of rDNA clusters is a species-specific character.20

Taking into account that the correct classification of triatomines helps directly the activity of vector control programs, we present, for the first time, an identification key based on cytogenetic data that allows differentiating the triatomine species present in five Brazilian states. Moreover, we emphasize the need for new cytogenetic studies, with emphasis on triatomine species that have never been analyzed, so that an identification key of all vectors of Chagas disease in Brazil can created.

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