Genetic structure, demographic change and extinction dynamics in the collared lemming and woolly mammoth

Abstract: In light of current climate change it is critical to understand how different species have been affected in the past by well-known climatic events. In this thesis, DNA was retrieved from ancient material to study the population dynamics of two cold-adapted taxa that capture the extremes of body size range in mammals: the collared lemming (Dicrostonyx sp.) and the woolly mammoth (Mammuthus primigenius). The aim was to reconstruct their histories to investigate possible associations between past population events and changes in climate. Mitochondrial DNA sequences from collared lemming fossil remains in western Europe suggested that the history of this small mammal was characterized by repeated population extinctions followed by recolonizations, and that these were related to millennial-scale climatic fluctuations. Further genetic sampling of more than 300 mitochondrial DNA sequences, covering a large part of the collared lemming’s historical distribution, showed that these population turnovers were not site-specific but widespread, occurring across Europe and western Russia. Extant populations were found to harbor only a small fraction of the historical genetic diversity demonstrating an extensive loss of genetic variation in this small mammal during the last 50,000 years. For the woolly mammoth, a comprehensive dataset of novel and publicly available mitochondrial DNA sequences was compiled, covering a broad geographical and temporal range. In addition, complete genome sequencing was performed on two mammoth specimens, the first representing one of the last surviving individuals from Wrangel Island and the second representing the ancestral Late Pleistocene Siberian population. Genome-wide as well as mitochondrial DNA data revealed that climatic changes have played a major role in shaping the demographic history of the woolly mammoth. For example, two severe population reductions were identified, with the first one encompassing the last warm interglacial period (~130,000 – 116,000 years ago) and the second coinciding with the end of the last Ice Age (~11,000 years ago). Moreover, climate-driven sea level changes appear to have had considerable impact by enabling increased gene flow across the Bering land bridge, as well as the isolation of mammoths on Wrangel Island. When comparing the two complete genomes, the one from Wrangel Island displayed 20% lower genome-wide diversity and a markedly higher fraction of runs of homozygosity. Consequently, loss of genetic variation and inbreeding may have contributed to the extinction of the woolly mammoth. Overall, the findings presented in this thesis illustrate the power of ancient DNA in providing unique insights into past evolutionary processes.