Exploring the evolution of fishes at high latitudes

Abstract

Fish species found in high latitude waters are especially vulnerable to climatic changes due to their inability to regulate body temperature and long evolution in cold, oxygen-rich aquatic environments. Antarctic notothenioid fishes have evolved to thrive in an extremely stable, cold, oxygen-rich environment. Likewise, lake trout live in a cold, oxygen-rich environment, but commonly experience a wider range of temperatures than notothenioids. Millions of years of evolution in the cold have shaped the genetics of these fishes, but the effects on their biology remain largely unexplored. This dissertation seeks to learn from the past evolution of these two evolutionarily distant types of high latitude fishes to enhance predictions for how these animals can cope with future environmental changes. Specifically, this dissertation examines the genetics of fishes with limited thermal tolerance from the population level down to a single gene. These studies relied on DNA sequence evidence produced with two different technologies and analyzed under functional, phylogenetic, and population genetic frameworks. In the first study, mitochondrial DNA (mtDNA) variation is examined to determine ancestral affinities and geographic distribution of mtDNA variants in lake trout across Alaska. Lake trout in Alaska descend from two distinct mtDNA lineages. One mtDNA lineage is restricted to Arctic Alaska, north of the Brooks Range, while the other lineage is found across Alaska. Lake trout likely dispersed from glacial refugia in western Canada to recolonize Alaska and the movement patterns from recolonization assist in determining how lake trout are likely to move across the landscape in the future. In the second study, genome wide genetic diversity of lake trout in seven Alaskan lakes is explored to determine ancestral affinities and colonization pathways. Despite past movement, the lake trout population currently found in each of the sampled lakes is genetically distinct from all other sampled populations and no migration currently seems to be occurring, even between lakes less than 20 km from each other. This research shows lake trout in Alaska are genetically diverse, but with little gene flow, genetic rescue and transfer of genetic variation between populations is unlikely to occur. In the third component of this dissertation, the evolution of the critical hypoxia transcription factor is examined in Antarctic notothenioids. The hypoxia-inducible factor-1alpha (HIF-1alpha) of Antarctic notothenioids contains a polyglutamine/glutamic acid insert that may impact the function of this key transcription factor. Thus, Antarctic notothenioids may have difficulties in responding to climate change induced hypoxia. Overall, the adaptive consequences of evolution in high latitude aquatic environments may be detrimental to fishes as they face climate changes. Other high latitude freshwater fishes like lake trout may have limited gene flow among populations, reducing potential for adaptation and genetic rescue in response to climate change. More research into the evolution and functional implications of different natural genetic variants is needed to protect these unique high latitude species.