Identifying Chagas disease vectors using elliptic Fourier descriptors of body contour: a case for the cryptic dimidiata complex

Abstract

Background: Triatoma dimidiata (Reduviidae: Triatominae) is an important vector of Chagas disease in various countries in the Americas. Phylogenetic studies have defined three lineages in Mexico and part of Central America. While there is a marked genetic differentiation, methods for identifying them using morphometric analyses with landmarks have not yet been fully resolutive. Elliptical Fourier descriptors (EFDs), which mathematically describe the shape of any closed two-dimensional contours, could be a potentially useful alternative method. Our objective was to validate the use of EFDs for the identification of three lineages of this species complex.

Method: A total of 84 dorsal view images of individuals of the three lineages were used. Body contours were described with EFDs using between 5 and 30 harmonics. The number of obtained coefficients was reduced by a principal components analysis and the first axis scores were used as shape variables. A linear discriminant function analysis and an ordination plot of the discriminant analysis were performed using the shape variables. A confusion matrix of the ordination plot of the discriminant analysis was obtained to estimate the classification errors, the first five PC scores were statistically compared, and a neural network were then performed using the shape variables.

Results: The first principal component explained 50% of the variability, regardless the number of harmonics used. The results of discriminant analysis get improved by increasing the number of harmonics and components considered. With 25 harmonics and 30 components, the identification of haplogroups was achieved with an overall efficiency greater than 97%. The ordering diagram showed the correct discrimination of haplogroups, with only one error of discrimination corroborated by the confusion matrix. When comparing the first five PC scores, significant differences were found among at least two haplogroups. The 30 multilayer perceptron neural networks were also efficient in identification, reaching 91% efficiency with the validation data.

Conclusions: The use of EFD is a simple and useful method for the identification of the main lineages of Triatoma dimidiata, with high values of correct identification.

Keywords: Contours; Fourier; Identification; Morphometric analysis; Triatomine.

Fig. 1

Image examples of the three haplogroups of Triatoma dimidiata (Hemiptera: Reduviidae) and of a body contour after image processing for the analysis of the elliptical Fourier descriptors. Sample sizes per haplogroup are shown in parentheses under each image. Copyright: Creative Commons Attribution 1.0 Universal (CC0 1.0) Public Domain Dedication license (https://creativecommons.org/licenses/by/1.0). Images modified from Gurgel-Gonçalves R, Komp E, Campbell LP, Khalighifar A, Mellenbruch J, Mendonça VJ, et al. Automated identification of insect vectors of Chagas disease in Brazil and Mexico: the virtual vector lab. PeerJ. 2017;5:e3040 [40]

Fig. 2

Digital reconstruction and variability of the contours in three haplogroups of Triatoma dimidiata (Hemiptera: Reduviidae). The contours were obtained from the first principal components obtained with the elliptical Fourier descriptors. Red contours represent the consensus of the three haplogroups. Abbreviations: H1, haplogroup 1; H2, haplogroup 3; H3, haplogroup 3

Fig. 3

Behaviour of the variance explained by the principal components as the number of harmonics used to characterize the shape of three haplogroups of Triatoma dimidiata (Hemiptera: Reduviidae) is increased

Fig. 4

Ordination plot of the discriminant analysis using shape variables 30 principal components resulting from the elliptical Fourier coefficients of 25 harmonics of three haplogroups of Triatoma dimidiata (Hemiptera: Reduviidae)

Fig. 5

Score differences in the first five principal components among three haplogroups of Triatoma dimidiata (Hemiptera: Reduviidae). Abbreviations: H1, haplogroup 1; H2, haplogroup 2; H3, haplogroup 3