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Mem. Inst. Oswaldo Cruz vol.95 s.1 Rio de Janeiro 2000
Sylvatic Population of Triatoma infestans from the Bolivian Chaco: from Field Collection to Characterization
Vol. 95, Suppl. I: 119-122, 2000
François Noireau/*/+, Brigitte Bastrenta**, Silvia Catalá***, Jean-Pierre Dujardin**, Francisco Panzera****, Magdalena Torres***, Ruben Perez****, Cleber Galvão*, José Jurberg*
Institut de Recherche pour le Développement (IRD), France, and *Laboratório Nacional e Internacional de Referência em Taxonomia de Triatomíneos, Departamento de Entomologia, Instituto Oswaldo Cruz, Av. Brasil 4365, 21045-900 Rio de Janeiro, RJ, Brasil **IRD, La Paz, Bolivia ***CRILAR, La Rioja, Argentina ****Instituto de Biologia, Seccion Genética Evolutiva, Facultad de Ciencias, Montevideo, Uruguay
A sylvatic Triatoma infestans DM (dark morph) population detected in the Bolivian Chaco was characterized and compared with various domestic ones. The degree of differentiation of DM was clearly within the T. infestans intra-specific level. Nevertheless marked chromatic and morphometric differences as well as differences in antennal pattern, chromosome banding and randomly amplified polymorphic DNA support the hypothesis of a distinct population. Continuous exchange of insects between wild and domestic habitats seems unlikely in the Chaco.
Key words: Triatoma infestans - sylvatic focus - characterization of population
Bolivia is the only country where true sylvatic foci of Triatoma infestans were reported from Andean valleys (Torrico 1946, Bermudez et al. 1993). These sylvatic populations occur always among rock-piles between 2,400 and 2,600 m altitude. Chromatic pattern, isozyme profiles and DNA sequence analysis are similar between domestic and wild Andean specimens but randomly amplifield polymorphic DNA (RAPD) and morphometrics allow them to be distinguished (Dujardin et al. 1987, 1997, Carlier et al. 1996, Monteiro et al. 1999).
Three years ago, another sylvatic T. infestans population was detected in the Bolivian Chaco (Noireau et al. 1997). Because of chromatic differences with domestic T. infestans, they were named dark morphs (DM). Among various natural habitats searched for the presence of T. infestans DM, hollow trees were frequently positive (Noireau et al. 2000).
A traditional canonical variate analysis (CVA) was performed using four variables (head measures). Results were illustrated by plotting the individuals on the two canonical variables (CV1 and CV2, Fig. 1) with polygons enclosing each group ("convex hulls"). The canonical factors resulting from their derived shape components clearly isolated the DM specimens from the domestic ones. Their regression on size indicated that these differences were not attributable to allometry, suggesting between DM and domestic T. infestans the existence of true size-independent divergence.
A multivariate analysis of the antennal pattern using six variables corresponding to the number of receptors located on the three distal antennal segments was performed. Two discriminant functions (P<0.001) allowed to classified accurately 100% of individuals inside their corresponding groups (Fig. 2). The Mahalonobis distances derived from the analysis were all significant (P<0.001). The DM population showed an greater similitude with domestic T. infestans from Paraguay than Bolivian domestic one. The sylvatic population from Cochabamba presented the more distant antennal pattern with regard to the T. infestans DM.
Crossing experiments were performed between DM and domestic T. infestans from the Chaco. The reciprocal crosses among pairwise combinations were all successful (reproductive compatibility). The chromosome analysis of these hybrids showed normal meiosis with a complete chromosome pairing between homeologous chromosomes.
The cytogenetic analysis confirmed that the DM individuals present the same meiotic process observed in others populations of T. infestans. Using the C-banding technique, we observed that only three autosomal pairs showed heterochromatic blocks in one or both chromosomal ends. The T. infestans specimens originating from the nearest domestic population (Izozog, located 80 km apart) showed four autosomal pairs with C-positive blocks. From the whole T. infestans previously analyzed, T. infestans DM contained low quantity of C-heterochromatin and, consequently, was more similar to insects from Brazil or Uruguay than from Bolivian highland.
Isoenzymatic analyses were applied to distinguish T. infestans DM with domestic specimens from the Chaco. The comparison between domestic population and DM did not show no significant allelic differences (23 loci tested). The genetic distance calculed between domestic T. infestans and DM was very low (0.037) confirming the pertaining of both populations to the same species.
DM specimens were analyzed by RAPD (3 primers used) and compared with domestic T. infestans from varied origin and sylvatic insects from Andean region. The phylogenetic relationships visualized by a Wagner network showed a geographical separation: Chaco and Andean origin. Inside each branch, sylvatic Andean and DM specimens were detached from domestic ones (Fig. 3).
Except for research on Andean populations of T. infestans in Bolivia (Torrico 1946, Dujardin et al. 1987, Bermudez et al. 1993), very few studies have considered wild foci. Despite some former reports in other countries (Argentina, Paraguay and Brazil), it was considered that T. infestans did not maintain wild foci in these areas because most specimens were found in ecotopes relatively close to human dwellings (Usinger et al. 1966). The inquiry of T. infestans wild populations must be again considered in non-Andean areas after the detection of a new true focus in the Bolivian Chaco.
In the Chaco, continuous exchange of insects between wild and domestic habitats seems unlikely because of the marked chromatic and morphometric differences as well as differences in antennal pattern, chromosome banding and RAPD. Nevertheless, as proved by all the techniques, the degree of differentiation of DM was clearly within the T. infestans intra-specific level where it would represent a distinct population. At last, the detection of wild T. infestans in the Bolivian Chaco questions about the ancestral population of this species (Carcavallo 1998) when it is classically considered that Andean population represented the original wild focus (Schofield 1988).
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Financial support from the IRD, the UNDP/World Bank/WHO Special Program for Research and Training in Tropical Diseases (grant 970182), the convention Fiocruz-CNPq/FNS and the ECLAT network.
+Corresponding author. Fax: +55-21-290.9339
Received 7 August 2000
Accepted 4 September 2000