Powerhouses in the cold: mitochondrial function during thermal acclimation in montane mayflies

Abstract

Mitochondria provide the vast majority of cellular energy available to eukaryotes. Therefore, adjustments in mitochondrial function through genetic changes in mitochondrial or nuclear-encoded genes might underlie environmental adaptation. Environmentally induced plasticity in mitochondrial function is also common, especially in response to thermal acclimation in aquatic systems. Here, we examined mitochondrial function in mayfly larvae (Baetis and Drunella spp.) from high and low elevation mountain streams during thermal acclimation to ecologically relevant temperatures. A multi-substrate titration protocol was used to evaluate different respiratory states in isolated mitochondria, along with cytochrome oxidase and citrate synthase activities. In general, maximal mitochondrial respiratory capacity and oxidative phosphorylation coupling efficiency decreased during acclimation to higher temperatures, suggesting montane insects may be especially vulnerable to rapid climate change. Consistent with predictions of the climate variability hypothesis, mitochondria from Baetis collected at a low elevation site with highly variable daily and seasonal temperatures exhibited greater thermal tolerance than Baetis from a high elevation site with comparatively stable temperatures. However, mitochondrial phenotypes were more resilient than whole-organism phenotypes in the face of thermal stress. These results highlight the complex relationships between mitochondrial and organismal genotypes, phenotypes and environmental adaptation. This article is part of the theme issue 'Linking the mitochondrial genotype to phenotype: a complex endeavour'.

Keywords: G × E effects; Oroboros Oxygraph 2k; climate variability hypothesis; flux control factor; mitochondrial respiration; thermal tolerance.

Figure 1.

Thermal variation at high elevation (Killpecker) and low elevation (Elkhorn) streams in Colorado, USA. (a) Temperature measured every 30 min at each site over a year. (b) The difference between maximum and minimum temperatures recorded during a 24 h time period at each site averaged for July and August 2014. Error bars show standard deviation. Asterisk indicates a significant difference between high and low elevation (p < 0.001, Student's t-test). (Online version in colour.)

Figure 2.

The protocol used here for quantifying mitochondrial respiration from aquatic insects. In both (a) Drunella and (b) Baetis, a representative set of data from a single experiment are shown. Lower case letters and arrows represent adding the following: (a) mitochondrial isolate, (b) malate, pyruvate and glutamate, (c) ADP and (d) succinate. For Baetis, (e) and (f) indicate when the temperature of the respiration chamber was decreased to 16°C and 6°C, respectively. Grey boxes represent stable stretches of data used to calculate oxygen consumption for each respiratory state. (Online version in colour.)

Figure 3.

Mitochondrial respiration in Drunella (a–d) and Baetis (e–h) mayfly larvae acclimated to either cold (6°C) or warm (16°C) temperatures. LEAK (a,e) and maximal respiration (i.e. CI + CII) is presented (b,f) along with two flux control factors that were analysed: OXPHOS coupling efficiency (c,g) and CII flux control (d,h). Elevation and test temperature effects are presented for Baetis in (f). Error bars show ±s.e.m. @, T, E and I indicate significant main effects for acclimation temperature, test temperature, elevation or their interactions in the 2 × 2 ANOVAs, respectively. Asterisks indicate significant effects of acclimation temperature for individual comparisons based on Student's t-tests. For Drunella, n = 10 for 6°C acclimation, n = 9 for 16°C acclimation tested at 16°C and n = 6 for 16°C acclimation tested at 6°C. For Baetis, n = 10 for all treatments. (Online version in colour.)

Figure 4.

Cytochrome c oxidase (a,c) and citrate synthase (b,d) activities in Drunella (a,b) and Baetis (c,d) mayfly larvae acclimated to either cold (6°C) or warm (16°C) temperatures. Elevation effects are presented for Baetis (c,d). Error bars show ±s.e.m. For Drunella, n = 5–9. For Baetis, n = 9–11. No significant effects were found. (Online version in colour.)