The Importance Of Physiology On Insect Geographical Distribution: The Role Of Desiccation Resistance For The Geographical Distribution Of Chagas Disease Vectors

Why do we not find polar bears in the tropics? Why are fewer insect species at higher latitudes (e.g., closer to the poles) or altitudes than near the equator or at sea level? Finding an explanation for the distribution pattern of species and understanding the factors that limit their geographic range have long been central issues in ecology. This knowledge is vital for making critical predictions about climate change impacts on conservation, economics, and human health issues, for example, when studying disease vectors.

Vector-borne diseases account for more than 17% of all infectious diseases, causing more than 1 million deaths annually around the world. Many of these vectors are bloodsucking insects, being mosquitoes the best-known disease vectors (e.g., Malaria, Dengue, Yellow fever, Zika). Others include ticks, flies, sandflies, fleas and triatomine bugs, which are the Chagas disease vectors, one of the most important parasitemia in Americas.

Species distributions are constrained by their physiological tolerances and environmental abiotic factors being temperature and humidity the most important ones for terrestrial life. Most of the studies focus on temperature to explain species distribution limits. However, and particular for arid environments, small animals, such insects, are particularly prone to desiccation due to its high surface-area to volume ratio.

As a consequence, in order to survive, they have physiological and behavioral mechanisms to reduce water loss. Insects from arid environments would have higher desiccation tolerance, which could be reflected in their distribution range limited by areas with high relative values of vapor pressure deficit (e.g., hot and dry). Thus, our work examines the desiccation tolerance in seven vectors of Chagas disease chosen by their different degree of epidemiological relevance in America, geographical distribution, i.e., from northern Mexico to Patagonia Argentina, and phylogenetic relationship (two different genera and different complexes and clades) and links the most important abiotic limiting factors of their distribution with their desiccation tolerance.

This approach allows us to test the ecological hypothesis and provide a different standpoint for the geo-epidemiology of the disease. For doing this, we combined different methodological approaches. First, we modeled the ecological niche of each of the seven species of triatomines used, i.e., Rhodnius prolixus, Triatoma dimidiata, T. infestans, T. vitticeps, T. sordida, T. delpontei and T. patagonica using standard bioclimatic variables from the WorldClim dataset and generate a new bioclimatic variable, the maximum vapor pressure deficit from the driest month, which is a meaningful ecologically variable. Then, the desiccation tolerance limits for all species were analyzed and compared by measuring their total water content and amount of water that the insect can lose until moribund and calculate water loss rate, survival time and cuticular permeability.

The results show that the maximum vapor pressure deficit limits the western and southern distributions of T. vitticeps, T. delpontei, and T. patagonica. All species showed very high tolerance to desiccation with survival times at 35°C and ~ 15% relative humidity ranging from 24 to 38 days, except for T. dimidiata that survive 9 days, which can be explained by a higher water loss rate, due to a higher cuticular permeability along with low amount of water that the insect can lose until moribund compared with the other species. This approach indicates that most of these triatomine bugs could be exploiting the full dryness dimension of their habitat. Incorporating such species-specific traits in studies of distribution, range, and limits under scenarios of changing climate could enhance predictions of movement of disease-causing vectors into novel regions.

In a global warming scenario, an increase of temperature and frequency and severity of droughts is predicted. Therefore, it would be expected that future distributions of some of these vectors could be constrained by their desiccation tolerance. However, due to the buffering of macroclimate changes inside the microhabitats, this conclusion cannot be generalized to domiciliated species, which are usually the most important disease vectors.

These modeling and measurements have important implications for fields such as macrophysiology, physiological ecology, ecology, insect physiology and epidemiology of the Chagas disease, and could be used for other terrestrial organisms. It is very important to try to understand the underlying causes of species pattern distribution, and not only generate correlational models of species distribution.

An eco-physiological approach could help researchers to discard false projections of geographical distribution (e.g., reduction in the future distribution) of disease vectors by correlational models due to the effect of global climate change. These false projections used by policymakers could place them at high risk of making incorrect decisions with subsequent negative consequences on health issues.

This study, Using ecophysiological traits to understand the Realized Niche: the role of desiccation tolerance in Chagas disease vectors was recently published by Gerardo de la Vega and Pablo E. Schilman from the University of Buenos Aires and CONICET, Argentina in the journal Oecologia.

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