Comparative morphometric and molecular genetic analyses of triatominae (Hemiptera: Reduviidae)

Abstract

Triatomine bugs (Hemiptera: Reduviidae: Triatominae) are the vectors of Chagas disease in South and Central America. Chagas disease predominantly affects poor rural communities with simply constructed housing susceptible to infestation by triatomines. Chagas disease is restricted to the Americas largely due to the limited distribution of triatomine bugs. The global diversity of triatomines is -130 species, of which only -10% are known to occur outside the Americas, one species (Triatoma rubrofasciata) is tropicopolitan, and the others are concentrated on the Indian subcontinent (Linshcosteus spp. ) and adjacent south east Asian island groups (Triatoma spp. ). The main objectives of this PhD programme were to: a) assess the facility of morphometric approaches (measurement and robust statistical analysis of morphological variation) in the study of population structure of vector species with proximal domestic and silvatic distributions to detect population structure and give information on the risk of reinvasion, b) study interspecific and higher taxonomic level relationships of New World and Old World triatomine bugs. To these ends geometric morphometric analyses were conducted in concert with molecular genetic analyses of mitochondrial and nuclear DNA sequences. The principal question being: Does the relatively low cost method of morphometrics reveal patterns consistent with population structure, as otherwise determined by more expensive molecular genotyping methods? Or are such patterns disrupted by environmental effects and intraspecific convergent/divergent morphological evolution? Combined morphometrics and molecular genetics were used to study vector populations in three of the countries that continue to be most affected by Chagas disease. In Venezuela and Ecuador Rhodnius species (R. prolixus and R. ecuadoriensis respectively) were studied, in areas where they occur in both domestic and silvatic environments, and in Paraguay T. infestans from a domestic and a putative silvatic focus. Head and wing morphometrics were compared to mitochondrial DNA sequence data to assess the population structure and disparity among domestic and silvatic samples in each case. The results presented suggest that head shape variation is subject to morphological plasticity and/or selective pressure and functional constraint and does not correlate well with the 11 Abstract phylogeny. However, in all examples, wing shape was found to be congruent with the phylogenetic patterns inferred from sequence analysis. Consequently, it is recommended that wing shape and not head shape be used in morphometric assessments of population dynamics. It is also asserted here that if population structure is suggested by morphometrics, it should be followed by robust population genetic analysis. As such, morphometrics could be used as a tool for broad surveillance to identify areas of concern. A further objective was to elucidate the broader phylogeny of Triatominae and their relationships with other reduviid subfamilies. To investigate the debated polyphyletic origin of the Triatominae molecular approaches were used. Combined head and wing morphometric and molecular genetic analyses of New World and Old World Triatominae have revealed patterns of convergent morphological evolution (among New World and Old World Triatoma) and striking examples of strongly divergent morphological evolution (between Old World Triatoma and Linshcosteus). Applying a molecular clock based on the rate of sequence divergence for a fragment of ribosomal DNA (D2-28S), calibrated to the fossil record and vicariant events (the divergence of ancestral lineages due to separation by topographical or ecological barriers) it has been possible to reconstruct a likely evolutionary history for the Triatominae and the Reduviidae as a whole. The weight of evidence presented supports a polyphylectic origin for blood-feeding for the Triatominae. The apparent independent development of blood feeding among the main lineages of the Triatominae represented by the genera Triatoma and Rhodnius highlights a fundamental biological difference among important vector species. This difference is likely to become evident in the eventual post genomic era in studies of vector/parasite interactions and it highlights the importance of sequencing genomes from different vector genera.