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.