Mitochondria are present in all cells of all complex life. They are unusual because they carry a complement of their own DNA, separate from the nucleus, which is inherited maternally. Decades of phylogeographic studies, which have revealed strong inter-population divergence in mitochondrial DNA, have assumed that variation is evolutionarily neutral. In stark contrast, Hill (2015 Mol.Biol.Evol) has suggested that mitochondrial variation may be fundamental for adaptation to environmental change, given that mitochondria contain perhaps the most critically important machinery of complex life (Lane 2016 The Vital Question), which converts nutrients into available energy. However, we know almost nothing about the functional consequences of mitochondrial variation in wild organisms, or its ecological and evolutionary significance (Greenway et al. 2020 PNAS).

Three-spined stickleback are an outstanding model in which to explore mitochondrial variation. They are highly experimentally amenable and exhibit substantial geographic variation in mitochondrial sequence across their range, with good evidence for differences in selection between eight major clades (evolutionary groupings)(MacColl unpublished). Many environmental transitions (e.g. sea to freshwater and lake to stream) are accompanied by transitions from one major clade to another. Two of these clades, the 'European' and 'Atlantic', with especially distinct differences in energy processing gene sequence, come into contact on the Scottish island of North Uist (Dean et al. 2019 Mol.Biol.Evol). Importantly, ~50% of Uist marine stickleback have Atlantic mitochondria and the other 50% European, with no obvious morphological differences, likely as a result of Holocene hybridisation. This provides an extremely powerful and highly unusual opportunity to quantify the functional consequences of major mitochondrial variation within a population, while controlling for differences in nuclear DNA. In this project the student will use a diversity of approaches to quantify putative differences in energy demand, swimming performance, oxygen demand and temperature tolerance between stickleback with European versus Atlantic mitochondria.