Both phenotypic plasticity and genotypic specialization can contribute to differences in physiological performance between populations that are locally adapted to different environments, but their relative contributions are predicted to vary according to the spatial grain of environmental variation (Levins 1968; Scheiner 1993, 1998; de Jong and Behera 2002; Sultan and Spencer 2002; Baythavong 2011). In animal species that are distributed across steep elevational gradients, physiological challenges associated with hypoxia and cold exposure increase as a positive function of altitude, and dramatic changes in these environmental stressors can occur over relatively small spatial scales. This fine-grained environmental variation across elevational gradients should be especially conducive to the evolution of phenotypic plasticity because an increased acclimatization capacity enables organisms to track changes in local trait optima (Storz et al. 2010b). Plasticity in organismal phenotypes is often mediated by transcriptional plasticity in underlying regulatory networks (Sambandan et al. 2008; Ayroles et al. 2009; Edwards et al. 2009; Harbison et al. 2009; Zhou et al. 2009, 2012), whereas genotypic specialization may result from a canalized transcriptional program. Assessing the degree of regulatory plasticity and regulatory canalization in species with broad elevation ranges can therefore provide fundamental insights into mechanisms of physiological adaptation to changing environmental conditions.
For small, winter-active endotherms that inhabit alpine or subalpine environments, a sustained capacity for metabolic heat production is critical for survival during prolonged periods of cold exposure. At high altitude, the energetic challenges of maintaining a constant body temperature are especially acute because environmental hypoxia imposes constraints on maximal rates of aerobic thermogenesis (Hayes and Chappell 1986; Ward et al. 1995; Chappell and Hammond 2004). Consistent with these expected effects on fitness, survivorship studies of high-altitude deer mice (Peromyscus maniculatus) have demonstrated that naturally occurring variation in thermogenic capacity is subject to strong directional selection in the wild, with higher capacities being associated with greater survival probabilities (Hayes and O'Connor 1999).
Deer mice are distributed from sea level to elevations of >4300 m in western North America, which makes it possible to examine evolved physiological differences between conspecific populations that are native to different elevational zones (Snyder 1981, 1982, 1985; Snyder et al. 1982; Chappell and Snyder 1984; Chappell et al. 1988; Storz 2007; Storz et al. 2007, 2009, 2010a; Cheviron et al. 2012, 2013; Tufts et al. 2013).
High-altitude deer mice in the Rocky Mountains have significantly higher thermogenic capacities under hypoxia than their lowland conspecifics (Cheviron et al. 2012, 2013), and these population differences in whole-organism performance are associated with an increased capacity to oxidize lipids as a primary fuel source during aerobic thermogenesis (Cheviron et al. 2012). These whole-organism differences in lipid catabolic capacities are in turn associated with differences in the activities of enzymes that influence flux through fatty-acid oxidation and oxidative phosphorylation pathways, and with concerted changes in the expression of genes in these same pathways (Cheviron et al. 2012). However, the extent to which these fitness-related transcriptomic differences stem from regulatory plasticity is largely unknown. Because thermogenic capacity has empirically verified effects on Darwinian fitness in high-altitude deer mice (Hayes and O'Connor 1999), and because mice that are native to different elevations exhibit pronounced differences in this fitness-related measure of physiological performance (Cheviron et al. 2012, 2013), integrative studies of performance-associated transcriptional variation can be expected to yield important insights into the role of regulatory plasticity in physiological acclimatization and adaptation to high-altitude environments.
Here we report the results of common-garden experiments that were designed to elucidate the role of regulatory plasticity in physiological acclimatization and adaptation to hypoxic cold-stress in deer mice. We combine new data on genomic transcriptional profiles and metabolic enzyme activities in skeletal muscle with previously published data on thermogenic performance under hypoxia in the same experimental animals (Cheviron et al. 2013). To assess how physiological plasticity and developmental plasticity in gene expression contribute to thermogenic performance differences, we measured highland and lowland mice from three experimental treatment groups (Table 1): (1) wild-caught mice that were acclimatized to the prevailing conditions of their native habitats; (2) wild-caught/lab-reared mice that were deacclimated to low-elevation conditions in a common-garden lab environment; and (3) the F1 progeny of the wild-caught mice that were born and reared in the common garden. Genomic transcriptional profiles and measures of metabolic enzyme activities enabled us to identify regulatory changes in specific genes and pathways that are associated with population differences in thermogenesis under hypoxia. The combined results revealed that the enhanced thermogenic performance of high-altitude deer mice is associated with changes in the expression of genes involved in angiogenesis, muscle growth, metabolic fuel use, and mitochondrial oxidative capacity. These results suggest that coping with the twin stressors of hypoxia and cold-exposure involves regulatory changes in several intersecting physiological pathways. Consistent with previous studies of altitudinal variation in gene expression (Cheviron et al. 2008), most of these performance-related transcriptomic changes occurred over physiological and developmental timescales, suggesting that regulatory plasticity makes important contributions to fitness-related physiological performance in highland deer mice.
|Highland (4350 m),||Lowland (430 m),|
|Treatment Groups||Description||n (males/females)||n (males/females)|
|In situ||Sampled at native elevations within 1–2 days of capture.||10 (6/4)||10 (6/4)|
|6-week deacclimation||Sampled after 6 weeks of deacclimation to low-elevation (360 m a.s.l.) common-garden conditions.||10 (6/4)||10 (6/4)|
|F1||Progeny of highland and lowland mice born and reared under low-elevation (360 m a.s.l.) common-garden conditions.||10 (5/5)||10 (5/5)|