Mitochondrial genotype influences the response to cold stress in the European green crab, Carcinus maenas

Disproportionate role of nuclear-encoded proteins in organismal and mitochondrial thermal performance in a copepod

Determining the mechanisms by which organisms evolve thermal tolerance is crucial to predicting how populations may respond to changes in local temperature regimes. Although evidence of relationships between mitochondrial background and thermal adaptation have been found, the presence of both nuclear-encoded and mitochondrial DNA (mtDNA)-encoded proteins warrants experiments aimed at parsing out the relative role of each genome in thermal adaptation. We investigated the relative role of mtDNA-encoded products in thermal tolerance between two divergent populations of Tigriopus californicus using first-generation (F1) hybrids that vary in maternally inherited mtDNA but are heterozygous for population-specific alleles across nuclear loci. We tested two measures of thermal tolerance, (1) survivorship to acute thermal stress and (2) thermal stability of mitochondrial performance in Complex I-fueled ATP synthesis, both across a range of increasing temperatures. We found that the southern population (San Diego, CA, USA) outperformed the northern population (Strawberry Hill, OR, USA) in survivorship, and that both reciprocal F1 hybrid crosses had intermediate survival. Mitochondria from the San Diego population displayed greater stability in ATP synthesis with increasing temperatures compared with those from Strawberry Hill. Interestingly, hybrids from both cross directions had synthesis profiles that were very similar to that of Strawberry Hill. Taken together, these results suggest that the relative role of the mtDNA in these phenotypes is negligible compared with that of elements encoded by nuclear DNA in this system.

Latitudinal variation and plasticity in response to temperature in Geukensia demissa

As global temperatures warm, species must adapt to a changing climate or transition to a different location suitable for their survival. Understanding the extent to which species are able to do so, particularly keystone species, is imperative to ensuring the survival of key ecosystems. The ribbed mussel Geukensia demissa is an integral part of salt marshes along the Atlantic coast of North America. Spatial patterns of genomic and phenotypic divergence have been previously documented, although their link with coastal environmental variation is unknown. Here, we study how populations of G. demissa in the northern (Massachusetts) and southern (Georgia) portions of the species range respond to changes in temperature. We combine assays of variation in oxygen consumption and RNA transcriptomic data with genomic divergence analyses to identify how separate populations of G. demissa may vary in distinct thermal environments. Our results show differences in constitutive oxygen consumption between mussels from Georgia and Massachusetts, as well as shared and disparate patterns of gene expression across temperature profiles. We also find that metabolic genes seem to be a strong component of divergence between these two populations. Our analysis highlights the importance of studying integrative patterns of genomic and phenotypic variation in species that are key for particular ecosystems, and how they might respond to further changes in climate.

Rapid Adaptation to Temperature via a Potential Genomic Island of Divergence in the Invasive Green Crab, Carcinus maenas

Widespread species often adapt easily to novel conditions – both those found in new habitats and those generated by climate change. However, rapid adaptation may be hindered in the marine realm, where long-distance dispersal and consequently high gene flow are predicted to limit potential for local adaptation. Here, we use a highly dispersive invasive marine crab to test the nature and speed of adaptation to temperature in the sea. Using single nucleotide polymorphisms (SNPs) generated from cardiac transcriptome sequencing, we characterized six populations of the European green crab (Carcinus maenas) located across parallel thermal gradients in their native and invasive ranges. We compared SNP frequencies with local temperatures and previously generated data on cardiac heat and cold tolerance to identify candidate markers associated with population-level differences in thermal physiology. Of 10,790 SNPs, 104 were identified as frequency outliers, a signal that was strongly driven by association with temperature and/or cold tolerance. Seventy-two of these outlier markers, representing 28 different genes, were in a cluster of SNPs identified as a potential inversion polymorphism using linkage disequilibrium network analysis. This SNP cluster was unique in the data set, which was otherwise characterized by low levels of linkage disequilibrium, and markers in this cluster showed a significant enrichment of coding substitutions relative to the full SNP set. These 72 outlier SNPs appear to be transmitted as a unit, and represent a putative genomic island of divergence which varied in frequency with organismal cold tolerance. This relationship was strikingly similar across both native and invasive populations, all of which showed a very strong correlation with cold tolerance (R2 = 0.96 over all six populations). Notably, three of these populations have diverged recently (<100 years) and show little to no neutral divergence, suggesting that this genomic region may be responding to temperature on a relatively short time scale. This relationship indicates adaptation to temperature based on the action of a putative genomic island of divergence, perhaps partially explaining the extraordinary invasive ability of this species.

Mitochondria and the thermal limits of ectotherms

Temperature is a critical abiotic factor shaping the distribution and abundance of species, but the mechanisms that underpin organismal thermal limits remain poorly understood. One possible mechanism underlying these limits is the failure of mitochondrial processes, as mitochondria play a crucial role in animals as the primary site of ATP production. Conventional measures of mitochondrial performance suggest that these organelles can function at temperatures much higher than those that limit whole-organism function, suggesting that they are unlikely to set organismal thermal limits. However, this conclusion is challenged by recent data connecting sequence variation in mitochondrial genes to whole-organism thermal tolerance. Here, we review the current state of knowledge of mitochondrial responses to thermal extremes and ask whether they are consistent with a role for mitochondrial function in shaping whole-organism thermal limits. The available data are fragmentary, but it is possible to draw some conclusions. There is little evidence that failure of maximal mitochondrial oxidative capacity as assessed in vitro sets thermal limits, but there is some evidence to suggest that temperature effects on ATP synthetic capacity may be important. Several studies suggest that loss of mitochondrial coupling is associated with the thermal limits for organismal growth, although this needs to be rigorously tested. Most studies have utilized isolated mitochondrial preparations to assess the effects of temperature on these organelles, and there remain many untapped opportunities to address these questions using preparations that retain more of their biological context to better connect these subcellular processes with whole-organism thermal limits.