Growth changes in Rhodnius pallescens under simulated domestic and sylvatic conditions

Rhodnius pallescens Barber 1932 is a silvatic species of Triatominae living in palm trees in Colombia and part of Central America. In Colombia, the species did not adapt to domestic structures and is not considered as an important vector for humans. In Panama, Costa Rica and Nicaragua it is a recognized vector adapting to peridomestic and domestic structures. The main condition required for a Triatominae to be a significant vector of Chagas disease is its ability to colonize human dwellings. The mechanisms of this process are unknown. In this adaptation to domesticity, previous authors have reported a fairly constant reduction in general size and discussed its possible causes in terms of natural selection across various generations or of simple growth changes within one single generation. Some authors suggested that this size change could correspond to the relaxation of a selective pressure in the wild, where larger phenotypes are possibly selected due to their greater capacity to resist temporary food shortages. Others suggested that growth patterns could change quickly in domestic conditions because of faster development time or higher population density. Up to now, these hypotheses have not been explored experimentally. We built a laboratory protocol measuring the effects of population density and feeding patterns on the growth of R. pallescens. A total of 320 first instar nymphs from a 10 generations old laboratory colony were randomly subdivided into four regimes combining density and feeding frequency variation. Using geometric morphometric techniques applied to the emerging adults, we evaluated size and shape variation of the heads and the wings across the four experimental designs. The regimes tentatively paralleled changes occurring for a Triatominae in the transition from silvatic (low population density, low feeding frequency) to domestic (higher density, higher feeding frequency) habitats. Density and feeding frequency combined their effect within the span of one single generation to produce similar size changes for wings and heads, but significant only for the wings. No significant variation could be detected for shape variation, neither for the wing nor for the head. Our data suggest that selection is not needed to account for observed changes between sylvatic and domestic ecotopes, and they did not agree with an effect of development time on size. They highlighted the importance of the interaction between population density and feeding frequency to produce specific and significant variation in the insect dimensions.