NOTA RESEARCH NOTE

Introduction

The subfamily Triatominae (Hemiptera: Reduviidae) is customarily organized into 5 tribes with 14 genera (Lent & Wygodzinsky, 1979; Schofield, 1994), although three additional genera have recently been proposed (Carcavallo et al., 1998; Lent et al., 1994; Jurberg & Galvão, 1997). At present, 130 species are recognized (Table 1), all characterized by an obligate blood-sucking habit and various associated adaptations including modifications of the mouthparts, saliva, and digestive functions. The subfamily is defined as a collection of hematophagous Reduviidae (Jeannel, 1919), although it is increasingly apparent that this assemblage is rather artificial and is probably unsustainable as a monophyletic grouping (Schofield, 1988). For example, the two main tribes ­ Triatomini and Rhodniini ­ seem to have little in common other than a basic reduviid form overlaid with convergent characters associated with a blood-sucking diet. Apart from their differences in morphology (Lent & Wygodzinsky, 1979) these two tribes show consistent differences in sensorial patterns (Catalá, 1997; Catalá & Schofield, 1994), salivary gland physiology (Ribeiro et al., 1998), and DNA analysis (Garcia et al., 1998; Stothard et al., 1998; Lyman et al., 1999) and have probably originated from quite different predatory forms. The Indian genus Linshcosteus seems to have evolved independently from the New World species of Triatominae (Gorla et al., 1997) and so probably merits independent tribal status.

Several lines of evidence support the idea that the Triatominae have evolved relatively recently from various of the predatory reduviids (Schofield, 1988; Schofield & Dolling, 1993). The evolution of these blood-sucking forms can be envisaged through a transitional phase of nest-dwelling forms initially feeding on the guilds of nest-dwelling invertebrates but progressively adapting to exploit the vertebrate nest-building host itself. Such a transition brings several advantages, especially in terms of protection from other predators and from climatic extremes, as well as the more reliable supply of protein-rich food. The improved food supply and protection from climatic extremes mean that reproduction becomes less dependent on seasonal changes, and population density can increase up to the limits of the available nutrition. However, the transition also requires several adaptations. Mouthparts of predators are generally robust and strongly chitinized (often curved), whereas those of blood-sucking forms are slim and slender, reflecting the physical differences of the host integument. Salivary function must also be modified. The requirement of predators is for a toxic saliva that can rapidly immobilize the invertebrate prey, whereas blood-suckers need a more benign saliva that will not stimulate adverse reactions from the vertebrate host (Schofield et al., 1986). Thus for humans the bite of a predatory reduviid is generally very painful, and amongst the Triatominae a painful bite ­ such as that of Triatoma rubrofasciata, whose bite can provoke anaphylactic shock ­ can be considered a primitive character (cf. Ryckman & Bentley 1979; Ribeiro et al., 1998).

At the level of digestion, a whole series of physiological adaptations is required for an obligate hematophage (see Lehane, 1991). Blood is a nutritionally rich resource, but it is highly alkaline, and much of the protein is locked in the blood cells. Consequently the Triatominae require both a hemolysin to open the blood cells and a system to acidify the blood meal before it can be digested. Reduviidae are derived from plant-sucking Hemiptera which have lost the ability to secrete trypsin, the usual digestive protease, because plant sap has virtually no protein, and plant seeds have potent anti-trypsins (Schofield, 1996). Thus the Reduviidae, including Triatominae, must make use of secreted cathepsins as proteases, which are generally active only at acid pH. Blood is also generally deficient in certain vitamins, particularly folate and B-vitamins, so that all obligate blood-suckers require symbionts to assist in producing these compounds. These symbionts are so important that all other obligate blood-suckers carefully conserve them either intracellularly or in a special organ known as the mycetome. But in Triatominae these symbionts are free in the gut lumen, which is taken as additional evidence that the blood-sucking habit is a relatively recent adaptation (Schofield & Dolling, 1993).

The process of speciation within the Triatominae is not well understood, but is subject to a major research effort because it encompasses the processes by which Triatominae can adapt from sylvatic to domestic environments ­ and so assume greater significance as vectors of Chagas disease to humans. By morphological, biochemical, and genetic criteria, the different genera and species of Triatominae seem to form a series of species groups, which generally show a discrete geographical distribution. This is additional evidence for their polyphyletic origins, which is now being supported by physiological comparisons and by genetic analysis based on isoenzymes and RAPD profiles (Garcia et al., 1998), as well as sequence analysis of ribosomal and mitochondrial DNA (Garcia & Powell, 1998; Stothard et al., 1998; Lyman et al., 1999). We can assume that amongst sylvatic species of Triatominae, natural selection may play an important role in speciation, so that many species and species groups have adapted to highly specific host and habitat associations. Well-known examples include Cavernicola pilosa which is invariably found in bat caves, and species of Psammolestes which are invariably associated with the woven-stick nests of furnariid weaver birds. In contrast, there are many groups of sylvatic species that remain relative generalists, such as the Triatoma sordida group which can be found in a very wide range of habitats. To a certain extent, species can be ranked in accordance with their degree of adaptation to more stable habitats, such as the domestic environment, and it may be that the more generalist species represent those which still have the potential for further adaptation to more specific habitats.

Studies of Triatoma infestans and Rhodnius prolixus, the two most widespread domestic species, reveal a further adaptive process which seems of major importance. In the case of T. infestans, true sylvatic populations are known only from the Cochabamba/Sucre region of central Bolivia, where bugs can be found amongst rock piles associated with wild guinea pigs. Elsewhere, this species is almost exclusively domestic, although peridomestic populations can also be found in regions with a climate similar to that of central Bolivia. Comparison of T. infestans populations from many different localities reveals that each has a generally low level of population variability (in fact, they have been likened to pseudo-clones), but there are detectable genetic and phenetic differences between panmictic populations. The overall population structure can be well explained by a process of genetic drift, using a model of "isolation by distance". The genetic differences between populations reflect their geographic distance apart (Dujardin et al., 1998a). It is also interesting to note a general reduction in physical size and in genome size in populations further from the putative source in central Bolivia. For example, populations from Uruguay are noticeably smaller than Bolivian populations, and have only about two-thirds of the total DNA content per cell (Panzera et al., 1998).

A similar pattern is shown by R. prolixus. This species is believed to derive from palm tree populations in the llanos of Venezuela and Colombia, but like T. infestans, has been spread in association with humans into other areas of these countries and into parts of Central America. The Central American populations are also smaller than their South American cousins, and show a highly restricted genetic repertoire as revealed by RAPD profiles (Dujardin et al., 1998b). In this case, the differences seem to reflect a series of genetic bottlenecks and subsequent genetic drift, and lend substantial support to the arguments favoring control of Chagas disease in Central America by eradicating all R. prolixus populations (Schofield & Dujardin, 1997). As the processes of speciation and adaptation in Triatominae become better understood, it may be possible to propose future measures to assist in monitoring this process in areas experiencing major land-use change, and also to consider interventions to prevent the eventual domestication of some species.

References

CARCAVALLO, R. U.; JURBERG, J. & LENT, H., 1998. Torrealbaia martinezi, gen. nov. sp. n, da tribo Cavernicolini (Hemiptera, Reduviidae, Triatominae): Uma abordagem filogenética. Entomología y Vectores, 5:143-150.         

CATALA, S., 1997. Antennal sensilla of Triatominae (Hemiptera, Reduviidae): A comparative study over five genera. International Journal of Insect Morphology and Embryology, 26:67-73.         

CATALA, S. & SCHOFIELD, C. J., 1994. The antennal sensilla of Rhodnius. Journal of Morphology, 219: 193-203.         

DUJARDIN, J. P.; SCHOFIELD, C. J. & TIBAYRENC, M., 1998a. Population structure of Andean Triatoma infestans: Allozyme frequencies and their epidemiological relevance. Medical & Veterinary Entomology, 12:20-29.         

DUJARDIN, J. P.; MUÑOZ, M.; CHAVEZ, T.; PONCE, C.; MORENO, J. & SCHOFIELD, C. J., 1998b. The origin of Rhodnius prolixus in Central America. Medical & Veterinary Entomology, 12:113-115.         

FRIAS, D.; HENRY, A. & GONZALEZ, C. R., 1998. Mepraia gajardoi: A new species of Triatominae (Hemiptera: Reduviidae) from Chile and its comparison with Mepraia spinolai. Revista Chilena de Historia Natural, 71:177-188.         

GARCIA, A. L.; CARRASCO, H. J.; SCHOFIELD, C. J.; VALENTE, S. A.; FRAME, I. A.; STOTHARD, R. & MILES, M. A., 1998. Random Amplification of Polymorphic DNA as a tool for taxonomic studies of Triatomine bugs (Hemiptera: Reduviidae). Journal of Medical Entomology, 35:38-45.         

GARCIA, B. A. & POWELL, J. R., 1998. Phylogeny of species of Triatoma (Hemiptera: Reduviidae) based on mitochondrial DNA sequences. Journal of Medical Entomology, 35:232-238.         

GORLA, D. E.; DUJARDIN, J. P. & SCHOFIELD, C. J., 1997. Biosystematics of Old World Triatominae. Acta Tropica, 63:127-140.         

JEANNEL, R., 1919. Henicocephalidae et Reduviidae. In: Voyage de Ch. Alluaud et R. Jeannel en Afrique Orientale (1911-1912). Résultats Scientifiques ­ Hemiptera. vol. 3. pp. 131-314, Paris: s.n.         

JURBERG, J. & GALVÃO, C., 1997. Hermanlentia n. gen. da tribo Triatomini, com um rol de especies de Triatominae (Hemiptera, Reduviidae). Memórias do Instituto Oswaldo Cruz, 92:181-185.         

LEHANE, M. J., 1991. Biology of Blood-sucking Insects. London: Harper Collins Academic.         

LENT, H. & WYGODZINSKY, P., 1979. Revision of the Triatominae (Hemiptera, Reduviidae), and their significance as vectors of Chagas disease. Bulletin of the American Museum of Natural History, 163: 123-520.         

LENT, H.; JURBERG, J. & GALVÃO, C., 1994. Revalidação do gênero Mepraia Mazza, Gajardo & Jorg, 1940 (Hemiptera, Reduviidae, Triatominae). Memórias do Instituto Oswaldo Cruz, 89:347-352.         

LYMAN, D. L.; MONTEIRO, F. A.; ESCALANTE, A. E.; CORDON­ROSALES, C.; WESSON, D. M.; DUJARDIN, J.-P. & BEARD, C. B., 1999. Mitochondrial DNA sequence variation among triatomine vectors of Chagas disease. American Journal of Tropical Medicine and Hygiene, 60:377-386.         

PANZERA, F.; SCVORTZOFF, E.; PEREZ, R.; PANZERA, Y.; HORNOS, S.; CESTAU, R.; NICOLINI, P.; DELGADO; V.; ALVAREZ, F.; MAZZELA, M.; COSSIO, B.; MARTINEZ, M. & SALVATELLA, R., 1998. Citogenética de Triatomíneos. In: Atlas de Triatomíneos da América (R. Carcavallo, ed.), pp. 621-664, Rio de Janeiro: Editora Fiocruz.         

RIBEIRO, J. M. C.; SCHNEIDER, M.; ISAIAS, T.; JURBERG, J.; GALVÃO, C. & GUIMARÃES, J. A., 1998. Role of salivary antihemostatic components in blood feeding by triatomine bugs (Heteroptera). Journal of Medical Entomology, 35:599-610.         

RYCKMAN, R. E. & BENTLEY, D. G., 1979. Host reactions to bug bites (Hemiptera, Homoptera): A literature review and annotated bibliography (parts I and II). California Vector Views, 26:1-49.         

SCHOFIELD, C. J., 1988. The biosystematics of Triatominae. In: Biosystematics of Haematophagous Insects (M. W. Service, ed.), pp. 284-312, special volume 37, Oxford: Systematics Association/ Clarendon Press.         

SCHOFIELD, C. J., 1994. Triatominae ­ Biology & Control. West Sussex: Eurocommunica Publications.         

SCHOFIELD, C. J., 1996. Overview ­ Biosystematics of the Reduviidae. In: Proceedings of the International Workshop on Population Genetics and Control of Triatominae, Santo Domingo de Los Colorados, Ecuador (C. J. Schofield, J. P. Dujardin & J. Jurberg, eds.), pp. 45-50, Mexico City: INDRE.         

SCHOFIELD, C. J. & DOLLING, W. R., 1993. Bedbugs and kissing-bugs (bloodsucking Hemiptera). In: Medical Insects and Arachnids (R. P. Lane & R. W. Crosskey, eds.), pp. 483-516, London: Chapman & Hall.         

SCHOFIELD, C. J. & DUJARDIN, J. P., 1997. Chagas disease vector control in Central America. Parasitology Today, 13:141-144.         

SCHOFIELD, C. J.; WILLIAMS, N. G. & MARSHALL, T. F. C., 1986. Density dependent perception of bites of triatomine bugs. Annals of Tropical Medicine and Parasitology, 80:351-358.         

STOTHARD, J. R.; YAMAMOTO, Y.; CHERCHI, A.; GARCIA, A. L.; VALENTE, S. A. S.; SCHOFIELD, C. J. & MILES, M. A., 1998. A preliminary survey of mitochondrial sequence variation in Triatominae (Hemiptera: Reduviidae) using polymerase chain reaction-based single strand conformational polymorphism (SSCP) analysis and direct sequencing. Bulletin of Entomological Research, 88:553-560.