Phylogeography and mitochondrial DNA diversity in arctic lemmings
Abstract: The aim of this thesis is to evaluate the effects of Quaternary environmental fluctuations on the present patterns of mitochondrial DNA variation in the wood lemming, Myopus schisticolor, and the two genera of Arctic lemmings: collared lemmings, Dicrostonyx, and true lemmings, Lemmus.The phylogeographic pattern and the limited mtDNA diversity in the Scandinavian populations of the wood lemming reflect recent colonization by a limited number of founders during the Holocene boreal forest expansion.The mtDNA phylogeny in the two genera of Arctic lemmings gives no support for the existence of a single Beringian refungium since the mid Pleistocene. Isolation by intermittent inundation of the Bering Strait during the interglacials was most probably important for speciation in both genera, Dicrostonyx and Lemmus.Comparative phylogeography of the two genera of Arctic lemmings, Lemmus and Dicrostonyx gives evidence for the vicariant separation by the glacial barriers in the Eurasian Arctic. There is genetic support for the importance of the Asian Beringia as a refugial area for the tundra specialist, D. torquatus, during warming periods in the interglacials.There is no evidence for the direct effect of the last glaciation on the present level of mtDNA diversity and population structure in Arctic lemmings. Intrapopulation and intraregion mtDNA diversity estimates in each genus reflect the demographic events which were likely a result of environmental changes in the Holocene. Comparisons of mtDNA diversity estimates across the two genera of Arctic lemmings suggest that historically and presently codistributed Lemmus and Dicrostonyx responded differently to environmental changes resulting from the Holocene warming events in the Eurasian Arctic.The mtDNA phylogeny supports the importance of vicariant events generated by the glacial-interglacial periods for allopatric speciation in chromosomally variable collared lemmings, Dicrostonyx, in the North America Arctic. In the Eurasian Arctic, the congruence between the phylogeographic structure in mtDNA variation and geographic distribution of chromosome races indicates that the fragmentation by the glacial barriers during the late Pleistocene and bottleneck events due to warming events in the Holocene were important for the origin of intraspecific chromosome races in the collared lemming, D. torquatus
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