Inter-specific and Developmental Differences on the Array of Antennal Chemoreceptors in Four Species of Triatominae (Hemiptera: Reduviidae)
Vol. 95(1): 67-74, Jan./Feb. 2000
M Gracco, S Catalá+
CRILAR, Mendoza y Entre Ríos, Anillaco, 5301 La Rioja, Argentina
The aim of the work was to investigate the pattern of chemoreceptor sensilla in adults and fifth stage nymphs of Rhodnius prolixus, R. neglectus, Triatoma infestans and T. sordida in order to study differences and similarities between genera and species. Three types of sensilla were analyzed by light microscopy: thin-walled trichoidea, thick-walled trichoidea and basiconica. The number of sensilla of each three types were counted. The length of the antennal segments were also used as a variable for the analysis. The statistical analysis showed that the number of these antennal chemoreceptors had significant differences between species and between adults and nymphs of each species. Discriminant analysis separates incompletely the fifth stage nymphs of the four species and showed similarity between them. Discriminant analysis performed with 12 variables of the antennae, allowed a complete separation of the adults of the four species.
Key words: sensilla - chemoreceptors - Triatominae - Rhodnius prolixus - Rhodnius neglectus - Triatoma infestans - Triatoma sordida
Triatominae are widely distributed in the Americas. All of them are obligate bloodfeeders and many are important as vectors of Trypanosoma cruzi _ the causative agent of Chagas disease or American trypanosomiasis (Schofield 1994). Their mechanism of host finding and orientation is not understood in detail, but involves detection of CO2, warmth (probably as radiant heat) and a series of odour stimuli including ammonia, and lactic, butyric and pyruvic acids (Bernard 1974, Lazzari 1990, Lorenzo Figueiras et al. 1994, Taneja & Guerin 1997). The antenna is the main organ devoted to the detection of such stimuli.
The Triatominae are equipped with a pair of four-segmented antennae bearing a variety of sensilla including several types of chemoreceptors and mechanoreceptors, as well as putative thermo-hygroreceptors (Wigglesworth an& Gillet 1934, Bernard 1974, Catalá 1994, 1997, Catalá & Schofield 1994, Catalá et al. 1998). A recent study of the antennal sensilla of five triatomine genera using scanning electron microscopy (Catalá 1997) demonstrated inter-specific differences in sensilla density and pattern of distribution. Developmental differences had been observed in Triatoma sordida and Rhodnius pictipes, but only sexual differences in T. sordida. Developmental and sexual changes are related to basiconica and trichoidea thin- and thick-walled sensilla, which are considered as chemoreceptors.
Complete morphometric data of the antennae and its sensilla are of paramount importance to the understanding of the specificity thresholds and transduction processes of an antennal sensory system (Zacharuk 1985). Former works about triatomine sensilla pattern (Catalá & Schofield 1994, Catalá 1996, 1997) used density of sensilla and were based on numerous photographs of scanning electron microscopy, a rather expensive approach. In the present paper, the use of light microscopy allowed the study of the sensilla pattern from the whole antennae and the estimation of the total population of chemoreceptors.
The multiporous thin-walled trichoidea sensilla were first described in T. infestans by Bernard (1974). Electrophysiological studies showed that they respond to direct breathing (Mayer 1968) a range of fatty acids, particularly pyruvic and lactic acids, and to amyl acetate (Bernard 1974). They are superficially similar to the E1, E2 and D sensilla of the bedbug, Cimex lectularius (Steinbrecht & Muller 1976). The basiconica sensilla, which are also multiporous but double walled, have recently been implicated in the reception of NH3 (Taneja & Guerin 1997) and fatty acid molecules. They also respond to changes in temperature and humidity (Type F, Bernard 1974) and are similar to the C receptors in C. lectularius responding to fatty acids (Steinbrecht & Muller 1976). In the case of the single-pored thick-walled trichoidea sensilla however, we neither have a clear indication of function nor the stimuli to which they respond in Triatominae. Morphologically, they are similar to the trichoidea sensilla described for other insects (Steinbrecht & Muller 1976, Schaller 1978). From the general description of these sensilla given by Zacharuck (1985) we can suppose them to be gustatory or contact chemoreceptors, primarily detecting molecules in liquid phase.
The aim of this work was to investigate the pattern of chemoreceptor sensilla in adults and fifth stage nymphs of R. prolixus, R. neglectus, T. infestans and T. sordida in order to study differences and similarities between genera and species. The four species of Triatominae selected for the study have primitive silvatic habitats, but all of them can invade domestic and peridomestic habitats. R. prolixus is considered the primary domestic vector of T. cruzi in Venezuela, Colombia and parts of Central America, while R. neglectus occurs mainly in the cerrado region of central Brazil and invades houses less frequently (Barret 1991). T. sordida is known from a wide range of silvatic habitats in Brazil, Paraguay, Bolivia and Argentina, where it frequently occupies peridomestic habitats and occasionally forms domestic colonies. T. infestans is believed to originate from silvatic populations amongst rockpiles in central Bolivia, but it has been transported as a domestic species over a wide area of southern Latin America where it is the primary vector of T. cruzi.
MATERIALS AND METHODS
The insects - We examined the antennae of 80 specimens: 10 males and 10 fifth stage nymphs of each four species of Triatominae: R. prolixus, R. neglectus, T. infestans and T. sordida. All had been reared at approximately 28°C and 60% RH in laboratory colonies at the Servicio Nacional de Chagas, Punilla (Cordoba), where they were fed fortnightly on chickens.
Microscopical preparations - Antennae from nymphs and adult bugs were cut from the head at the level of the antennal tubercle, and prepared for gross morphological studies. Antennae were diaphanised in 3% NaOH (Lazzari 1990) mounted in glycerine and examined under oil immersion using an ocular micrometer for measurements.
Sensilla analysis - Recognition of the different sensilla by light microscopy was made with reference to the classification of Catalá and Schofield (1994), Catalá (1997) and unpublished SEM photomicrographs from those works provided by S Catalá (cf. Fig. 1). The three main types of chemoreceptor (basiconica, thin and thick walled trichoidea) were counted from the whole antennal preparations, excluding the scape, as neither adults nor nymphs hold chemoreceptors on this proximal segment. The length of each of the three distal antennal segments was also measured.
Differences between means were compared by unifactorial ANOVA and Tuckey's test (Sokal & Rohlf 1979). Discriminant analysis allowed studying the multivariate relationships between species and Mahalanobis distances were used for measure the distance between two points in the space defined by two or more correlated variables.
To Dr CJ Schofield for criticisms and fruitful discussions as well as for the revision of the manuscript. To the Servicio Nacional de Chagas that provided specimens and to TL Victor Tomasi who kindly helped with the antenna preparation.
Fig. 1 | Fig. 2 | Fig. 3 | Fig. 4 | Table I | Table II | Table III
The project received financial support from CONICOR, British Council, RED ECLAT and SECYT Universidad Nacional de Córdoba.
+Corresponding author. Fax: +54 03827-494231. E-mail: scatala@crilar.com.ar
Received 15 June 1999
Accepted 5 August 1999
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