Using whole genome sequencing and cutting-edge analyses researchers at Stockholm University have uncovered the surprising evolutionary history of the Norwegian lemming (Lemmus lemmus), revealing it to be one of the most recently evolved mammal species. The results published in Proceedings of the National Academy of Sciences (PNAS), reveal that the Norwegian lemming is a distinct species that split from its closest relative, the Western Siberian lemming, approximately 35,000 years ago — just before the peak of the last Ice Age.
"The Norwegian lemming is a key ecological species in the Fennoscandian tundra. Among other things, it serves as primary food for many predator species, including some threatened ones such as the Arctic fox. However, it is also a very interesting species from an evolutionary perspective, which so far has not been studied using genomics. Our study starts to fill that gap," says David Díez del Molino, Researcher at the Centre for Palaeogenetics and the Department of Zoology at Stockholm University, senior author of the study.
The study, that compared genomes from nine modern and two ancient lemming specimens, not only confirms that the Norwegian and Siberian lemmings are separate evolutionary lineages, but also finds no evidence of interbreeding — a surprising result given how recently they diverged and that their distributions nearly overlap. This lack of gene flow stands in contrast to many other mammalian species, where recent splits are often accompanied by hybridization.
"Our findings indicate that the Norwegian lemming is among the most recently evolved mammals, diverging from its sister taxon, the Western Siberian lemming, at a remarkably shallow time depth. After this, these species seem to have remained isolated, as we don't find any indication of interbreeding between them", says Edana Lord, Postdoctoral Researcher at the Centre for Palaeogenetics and the Department of Zoology at Stockholm University lead author of the study.
The researchers also identified hundreds of mutations unique to the Norwegian lemming, particularly in genes related to coat colour, fat metabolism, and possibly even behaviour. These genomic differences likely contribute to its iconic black-yellow fur, as well as helping the lemmings stay active during winter, traits thought to be adaptations to the harsh Fennoscandian tundra and to predator pressure.
In resolving the phylogeny of the Lemmus genus, the study also supports the classification of the Eastern Siberian lemming as a separate species — Lemmus paulus — and clarifies the taxonomy of a group long muddled by uncertain evolutionary relationships.
"This work represents a big step in our understanding of lemming speciation and evolution. It paves the way for exciting future research, particularly in exploring ancient gene flow and precisely dating the emergence of the unique genetic adaptations we see in the Norwegian lemming.", says Love Dalén, Professor in Evolutionary Genomics at the Centre for Palaeogenetics and the Department of Zoology at Stockholm University, co-author of the study.
This work highlights the powerful insights genomic tools can bring to longstanding evolutionary questions and shows how even recent climatic changes can drive rapid species formation and isolation.
