Head shape variation in response to diet in Triatoma williami (Hemiptera, Reduviidae: Triatominae), a possible Chagas disease vector of legal Amazônia

The Body of Chagas Disease Vectors

Morphometry is an effort to describe or measure the morphology of the body, or parts of it. It also provides quantitative data on the interactions of living organisms with their environment, external or internal. As a discipline, morphometrics has undergone significant developments in the last decade, making its implementation more visual and less laborious. Chagas disease vectors, often referred to by the common name of "kissing bugs", belong to the subfamily Triatominae. Due to their apparent morphological plasticity, they have been the subject of numerous morphometric studies. Most of these have been applied taking into account the particularities of this group of vectors, such as domesticity (synanthropy), food preferences, dispersal ability, insecticide resistance, as well as some taxonomic issues. This brief review over nearly three decades is organized here according to the body organs considered by the authors.

Morphological Stasis in Time? A Triatoma brasiliensis brasiliensis Study Using Geometric Morphometrics in the Long Run

Simple Summary

Triatomines are vector insects capable of transmitting the protozoan that causes Chagas disease, thus representing a health risk in several countries, especially in Central and South America. Triatoma brasiliensis brasiliensis, the main triatomine vector in northeastern Brazil, needs frequent monitoring as it is able to colonize various natural and artificial ecotopes as well as to infest domiciles. This research uses geometric morphometrics as a tool to evaluate changes in the morphology and analyze a large temporal dataset of 102 years of collections of T. b. brasiliensis. Finding novelty results which indicate a morphological stasis in the morphology.

Abstract

Triatoma brasiliensis brasiliensis Neiva, 1911 is one of the most important vectors of Chagas disease in the Brazilian semiarid regions in the north-east. The risk imposed by T. b. brasiliensis to the human populations, due to frequent invasions and/or colonization of the domiciles, demands constant monitoring and control actions as well as an understanding of its evolutionary process. In this context, the following research studies the pattern of shape adaptation over time using a large dataset from 102 years of specimen collections in order to identify the morphological plasticity of this vector in Brazil. This dataset was analyzed using geometric morphometrics tools and the timescale was divided into eight different groups, containing specimens from 1912 to 2014. Geometric morphometrics analysis showed an interesting morphological stasis in the wing shape of T. b. brasiliensis, which allowed us to understand the high capacity of adaptation to changes in climate condition through time, and the invasive status which Triatoma species have around the world. Moreover, these results showed novel findings as an interesting phenotypic pattern, with no modifications in more than 100 years, leading us to understand the shape evolution in Triatominae as a vector species of diseases.

Body shape and fluctuating asymmetry following different feeding sources and feeding time in a triatomine, Triatoma pallidipennis (Stål, 1892)

Even when an animal has a generalist diet, different food sources can impact its body shape and fluctuating asymmetry (a stress indicator; FA). To test this, we varied the food source (mammalian, avian or defibrinated mammalian blood; and control animals - ad libitum feeding) and the time of feeding (every 8 days, 45 days and ad libitum) having the Chagas triatomine vector, Triatoma pallidipennis (Stål, 1892), as a study animal which has presumable generalist feeding habits. This factorial design was applied since first instar animals until adulthood. As response variables, we measured body shape and FA in adults of both sexes, using a two-dimensional geometric morphometrics protocol. The highest variance in body shape was explained by diet (17%), followed by sex nested within diet (12%). Males had less morphological differentiation than females: females with defibrinated blood provided every 45 days differentiated more, while those that fed on mammalian blood every 8 days differed less. Distances among the averages of the FA component related to shape indicated greater distances between avian blood provided every 45 days and mammalian blood provided every 8 days, as well as between the two groups fed on avian blood (feeding every 8 and 45 days), followed by avian and defibrinated blood, both fed every 8 days. These results indicate that blood source and feeding time have significant effects on the body shape, and FA in females and both sexes. Thus, despite general feeding habits, avian blood showed a greater impact on shape and FA in triatomines. This may select for triatomines to use mammal blood rather than avian blood if they have the chance to do so.

Triatoma williami in intradomiciliary environments of urban areas in Mato Grosso State, Brazil: domiciliation process of a wild species?

Background

Triatomines in Latin America are natural Chagas disease (ChD) vectors. Triatomine domiciliation is one of the main factors increasing the occurrence risk of this disease in humans. There are 66 triatomine species in Brazil, with three genera of significant epidemiological importance—Panstrongylus, Rhodnius, and Triatoma. Among the Triatoma species, Triatoma williami, a wild species, has been reported in Goiás, Mato Grosso, and Mato Grosso do Sul. In the Barra do Garças, Mato Grosso, the invasion by triatomines has been reported, with T. williami being the most common species. This study aimed to survey triatomine fauna and determine the Trypanosoma cruzi natural infection rates in triatomines in the urban area of Barra do Garças, Mato Grosso, Brazil.

Methods

Triatomine specimens were sampled by passive surveillance or active search by agents combating endemic diseases from 2019 to 2020. A parasitological feces diagnosis was performed to detect the presence of T. cruzi after the specimens were identified. Concerning T. cruzi identification, molecular diagnosis and genetic sequencing were performed to determine the strain, also called discrete typing units (DTUs).

Results

The 211 triatomines were collected, distributed in specimens of T. williami (84.4%), P. geniculatus (3.3%), P. diasi (1.4%), and R. neglectus (10.9%). Two colonies of T. williami were found through morphological analyses. These insects were sampled inside domiciles in an urban area neighboring Jardim Pitaluga (15° 51′57.7″ N, 052° 16′ 04.5 E). The records were sampled in September 2019 and January 2021. The rate of natural infection by T. cruzi was 39.4%. Two T. williami specimens from the sampled colonies were positive for the T. cruzi strain DTU IV.

Conclusions

This is the first time that T. williami has been confirmed in an urban area of Barra do Garças, Mato Grosso, Brazil. Further studies are needed for a clearer understanding of the ecology of this species for prevention and control mechanisms since its sampled specimens had a high rate of natural infection by T. cruzi.

Graphical Abstract

A review of the taxonomy and biology of Triatominae subspecies (Hemiptera: Reduviidae)

Due to its public health importance, as vectors of Chagas disease, the subfamily Triatominae, has received the attention of taxonomists. Knowing how to correctly identify these insects is of paramount importance for the surveillance and control of these vectors. Over many years, more than 40 triatomine subspecies were proposed. Infraspecific taxa are accepted as real entities in nature and recognized as biologically significant. Infraspecific diversity and population subdivision are frequently confused with the underlying mechanisms that lead to the formation of new species and species-level diversity. The distinction between infraspecific and interspecific divergence processes has received far less attention than species delimitation. In this work, new literature data on subspecies included in the subfamily Triatominae are presented. Data were compiled about Triatoma brasiliensis, Triatoma breyeri, Triatoma circummaculata, Triatoma dimidiata, Triatoma incrassata, Triatoma infestans, Triatoma phyllosoma, Triatoma protracta, Triatoma recurva, Triatoma rubida, Triatoma sanguisuga, Triatoma vitticeps, Panstrongylus megistus, Paratriatoma hirsuta, and Paratriatoma lecticularia subspecies. Although several of these subspecies have been synonymized, herein, we emphasize that some subspecies may be valid. Therefore, further studies are needed on the taxonomy, evolution, phylogeny, biogeography, ecology, physiology, and behavior of species to reinforce, or to invalidate the infraspecific status in the subfamily.

Molecular identification and morphological variations of Dermacentor albipictus collected from two deer species in northern Mexico

In total, 57 ticks were collected from six white-tailed deer (Odocoileus virginianus) and three mule deer (O. hemionus) in northern Mexico during the 2017, 2018 and 2019 hunting seasons. Morphological features of adult male and female ticks were observed and photographed using a stereo-microscope and scanning electron micrography. The ticks were identified as Dermacentor albipictus based on taxonomic keys. Molecular analysis using DNA amplification of the 16S rDNA and cytochrome oxidase 1 (COI) genes was employed to resolve the phylogenetic relationships from 18 strains of Dermacentor species. Bayesian phylogenetic analysis was performed in order to obtain a phylogenetic tree based on the concatenated sequence in the D. albipictus clade. The geometric morphometric analysis compared the body shape of ticks collected from specimens of two deer species by analyzing nine dorsal and ventral landmarks from both males and females. The results suggest that body shape variation in dorsal structures might be related to the host.

Morphology, Life Cycle, Environmental Factors and Fitness – a Machine Learning Analysis in Kissing Bugs (Hemiptera, Reduviidae, Triatominae)

Populations are permanently evolving and their evolution will influence their survival and reproduction, which will then alter demographic parameters. Several phenotypic, life history and environmental variables are known to be related to fitness measures. The goal of this article was to look into the possible types of those relationships in insects of the subfamily Triatominae, vectors of Trypanosoma cruzi, the causative agent of Chagas disease. After an exhaustive literature review of 7,207 records of publications referring exclusively to all possible features of the triatomines, using 15 keywords those records were reduced to 2,968 publications, that were analyzed individually; after deleting those publications that did not have the data in quantitative form as needed for the objective of this article, I found that 171 papers were adequate for the present analysis. From them I compiled a dataset of 11 variables and 90 cases from 36 triatomine species. Those variables included four environmental, two life cycle, and four morphological variables, and one demographic parameter: a fitness measure (the population intrinsic rate of natural increase, r0), used as dependent variable. However, the relationship between T. cruzi and its vector host was not included in this analysis despite triatomine-T. cruzi interactions constitute an important factor in the evolution of triatomine’s life history. I resorted to the Random Forest method as a machine learning approach for the analysis of this dataset, and found that –in addition to the triatomine species themselves– only the two life cycle variables (mean development time from egg to adult, and mean fecundity, expressed as the average number of female eggs laid per female per day) were statistically significant in determining fitness (r0). The machine learning approach used in the analysis provided a similar but deeper insight into these relationships than classical regression. Except for an analysis on senescence, this is the first study in triatomines addressing these questions. These results will be useful for other theoretical optimization approaches (frequency-dependence, density-dependence, evolutionary game theory, and adaptive dynamics), thus contributing to the theoretical framework for interpreting the succession of stages in insect adaptations, a framework yet to be constructed.

Unraveling the Morphological Variation of Triatoma infestans in the Peridomestic Habitats of Chuquisaca Bolivia: A Geometric Morphometric Approach

Morphometrics has been used on Triatomines, a well-known phenotypically variable insect, to understand the process of morphological plasticity and infer the changes of this phenomenon. The following research was carried out in two regions of the inter-Andean valleys and two Chaco regions of Chuquisaca-Bolivia. Triatoma infestans adults were collected from the peridomestic (pens and chicken coops) along a geographic gradient in order to evaluate the morphological differentiation between groups and their pattern of sexual shape dimorphism. Geometric morphometric methods were applied on the wings and heads of T. infestans. The main findings include that we proved sexual dimorphism in heads and wings, determined the impact of environmental factors on size and shape and validated the impact of nutrition on head shape variation. These results show that geometric morphometric procedures can be used to provide key insight into the biological adaptation of T. infestans on different biotic (nutrition) and abiotic (environment) conditions, which could serve in understanding and evaluating infestation processes and further vector control programs.

Traditional and Geometric Morphometry Analyses of Lutzomyia cruciata (Diptera: Psychodidae: Phlebotominae) Populations of Chiapas, Mexico

The presence of Lutzomyia (Tricholateralis) cruciata (Coquillett 1907) species complex has been suggested by morphological analysis of eggs and genetic studies of females. The present work aimed to compare the diversity in morphology of four populations of Lu. cruciata from the Coast of Chiapas, Mexico, using traditional (TM) and geometric (GM) methods. Several morphological characteristics that were analyzed provided consistency to differentiate at least, three populations of Lu. cruciata. Both methods were effective to detect morphological differences associated with the geographical sites of capture. In both sexes, three and four groups were detected by TM and GM, respectively. These results suggest marked morphological differences in both sexes of Lu. cruciata that make these methods potentially useful to identify the geographical origin of any specimen of this species captured in the study region. Although the results produced by both methods are coincident, geometric morphometrics turned out to be most advantageous with respect to traditional morphometry, since the latter requires more time and effort. The consistency of our results shows that the variability of environmental conditions on the coast of Chiapas determines a high degree of phenotypic plasticity in Lu. cruciata, with the possibility of prezygotic isolation and the formation of species complex.

Head shape variation in cerambycid saproxylic beetles as a function of host plant selection

Saproxylic insects depend on deadwood for larval development, and a certain degree of specialization may be involved in their choice of host plants and/or wood in a particular stage of degradation. The plant species chosen for oviposition in turn act as an environmental pressure on the head morphology of larvae and it is expected that head shape plasticity varies directly with the number of woody plant species used for larval development in each insect species. We analyzed head shape variation in saproxylic beetles with respect to host plant species, maximum time of larval emergence and season of the year when insects colonized branches. Generalist species in the use of host plants showed significant variation in head shape and size. Time of emergence and season did not appear to affect head shape, although season was a determinant factor of abundance and possibly head size variation.