Characterization of melanic and non-melanic forms in domestic and peridomestic populations of Triatoma infestans (Hemiptera: Reduviidae)

Abstract

Background: Melanic (dark) morphs have been barely reported in peridomestic and sylvatic conditions for Triatoma infestans, the most important vector of Chagas disease in the Southern Cone of South America. Adults with dark and small yellow markings on the connexivum were collected after manual searches conducted by technical personnel in 62 domiciliary units in Cruz del Eje, Córdoba Province, Argentina. The last community-wide insecticide spraying campaign before the study had been conducted three years earlier. We investigated if there was a measurable color morph variation (melanic and non-melanic) in wings and connexivum; we determined infestation, distribution of melanic and non-melanic forms, and correspondence of colorimetric variation with variations in morphology (wing size and shape and body length), development (wing fluctuating asymmetry), physiology (nutritional status) or behaviour (flight initiation).

Results: Forty-nine females, 54 males and 217 nymphs were collected in 24 domiciliary units. House infestation and colonization were 53% and 47%, respectively. Most of the T. infestans individuals (83.2%) were collected in chicken coops; intradomicile infestation was recorded in only one case. The chromatic cluster analysis showed two well-defined groups: melanic and non-melanic. The melanic group included 17 (35%) females and 25 (46%) males. Peridomestic infestation was lower for melanic than for non-melanic adults. Melanic morphs were collected in houses from several localities. Sexual dimorphisms were confirmed by morphometric measurements. Body length was large in melanic adults (P < 0.01 only for males). Differences between groups were significant for wing size and shape, but not for weight or weight/body length ratio. Melanic females and males showed significantly higher fluctuating asymmetry (FA) indices than their non-melanic counterparts.

Conclusions: This is the second report of melanic forms of T. infestans in domestic and peridomestic habitats in the Dry Chaco region of Argentina. Although non-melanic adults exhibited a higher infestation rate, melanic adults were widespread in the area and were collected in the infested domicile and in most types of peridomestic annexes. Differences in morphometric variables between groups might be due to different ecological adaptations. The higher FA levels observed in melanic individuals suggest a higher developmental instability and a selective advantage of non-melanic individuals in domestic and peridomestic habitats.

Keywords: Colorimetric analysis; Developmental instability; Flight initiation; Melanic/non-melanic; Morphological differences; Selective advantage; Triatoma infestans.

Fig. 1

Maps of the study area. a Córdoba Province (light grey) and Chaco region (dark grey) in Argentina. b Cruz del Eje department, Córdoba Province, Argentina. c Individual collection localities

Fig. 2

Example of colorimetric analysis of wings and connexivum of T. infestans adults collected in Cruz del Eje, Córdoba Province, Argentina. a Example of a collected melanic (left) and non-melanic (right) females. b Portions of right wing and connexivum subjected to colorimetric analysis. c RGB histogram derived from portions of wing and connexivum for melanic and non-melanic forms

Fig. 3

K-means clustering analysis. a Plot of the curve of the total within-cluster sum of square for 10 K-values (elbow method). The optimal number of clusters suggested by this method (two) is indicated with a dashed line. b Biplot derived from the first two axes of a principal components analysis showing the spatial distribution of the two clusters derived from a K-means clustering analysis (K = 2)

Fig. 4

Sex-specific variations of T. infestans for total body length (a), total body area (b) and wing size (CS) (c) between melanic and non-melanic forms. Box plots show the median (solid line), first and third quartiles. Whiskers indicate the 90th and 10th percentiles. Dots outside whiskers are potential outliers. Key: Left box plots: females. Right box plot: males. Asterisks within graphs indicate significant differences in the ANOVA tests between melanic and non-melanic groups. *P < 0.05, **P < 0.01

Fig. 5

Fluctuating asymmetry index for wing size and wing shape for melanic and non-melanic females and males of T. infestans collected in Cruz del Eje, Córdoba Province, Argentina. FA indices represent the mean square (MS) of the side-individual interaction effect corrected for measurement error (MS/ME), as shown in the ANOVA output (Table 3). a Wing size FA index. b Wing shape FA index