5.2 Local adaptation and capacity to adapt in the future
There is growing interest in the factors driving adaptation of mosquitoes to local environmental conditions for providing insights into the long-term responses of mosquito species to future warming. Mosquito species are composed of an array of locally adapted populations across their respective ranges. Substantial genetic variation exists in mosquito species (Holt et al. 2002; Fouet et al. 2017; Maffey et al. 2020; Pless et al. 2020; Yurchenko et al. 2020; Kang et al. 2021) and at fine-spatial scales (Gutiérrez et al. 2010; Jasper et al. 2019; Matowoet al. 2019; Ayala et al. 2020; Carvajal et al.2020), with significant consequences for transmission potential (Azaret al. 2017; Palmer et al. 2018; Vega-Rúa et al.2020). This genetic variation can interact with local environmental conditions to impact the capacity of mosquito vectors to transmit human pathogens (e.g., dengue; Gloria-Soria et al. (2017) and chikungunya; Zouache et al. (2014)). Yet, we still do not have a clear understanding of what environmental factors are driving this differentiation.
The work that has been done in this area to date has largely focused on the effects of temperature variation in driving local adaptation of current mosquito populations (Sternberg & Thomas 2014; Couper et al. 2021). However, research from the broader field of ectotherms [e.g., reviewed in Rozen-Rechels et al. (2019), vertebrates; Chown et al. (2011), insects] suggests that selection on thermal response curves are constrained by other metabolic stressors, like desiccation stress, as temperatures warm. For example, a study on 94 Drosophila species from diverse climates found substantial variation in the upper thermal limits among species. Further, the species specific CTmax correlated positively with increasing temperature in dry environments, with species from hot and dry environments exhibited higher heat tolerance. However, this relationship completely disappeared for species inhabiting wet environments suggesting temperature as a selective force is less important when humidity is high (Kellermann et al. 2012). A similar study in ectothermic vertebrates (400 lizards), found the thermal optimum to be more strongly related to ambient precipitation than to average temperature (Clusella-Trullas et al. 2011). Environmental mean temperature was only found to be predictive of the lower thermal limit (CTmin ) (Clusella-Trullaset al. 2011).
Both common garden and experimental evolution studies, two standard approaches to measure local adaptation and evolutionary potential of a particular species, could be incorrectly attributing observed phenotypic responses to temperature selection when they could be responding to a combination of energetic effects and moisture stress. This impacts our ability to accurately characterize thermal response curves of mosquitoes, as well as their capacity to adapt to future environmental change. From our conceptual framework outlined above (Fig. 5), we would predict that the current approach to studying local adaptation, steeped in metabolic theory of ecology, will be most predictive of mosquito population responses to future warming in regions of the world that currently exist below the species specific thermal optima (Topt ). However, for mosquito populations that inhabit environments above their thermal optima, humidity will be an important determinant of their capacity to respond to future environmental change. For example, mosquito populations in warm and wet, humid environments may have less capacity to adapt to future climate change in a warming and drying environment than what would be predicted from evolutionary models that consider the effects of temperature alone. Conversely, mosquito populations that currently live in warm and dry environments may have a greater capacity to adapt to warming conditions if they exhibit higher heat tolerance than their counterparts inhabiting wetter areas of the geographic distribution.