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Abstract:
Background: Populations and subspecies of the house mouse Mus musculus were able to invade new regions
worldwide in the wake of human expansion. Here we investigate the origin and colonization history of the house
mouse inhabiting the small island of Heligoland on the German Bight - Mus musculus helgolandicus. It was first
described by Zimmermann in 1953, based on morphological descriptions which were considered to be a mosaic
between the subspecies M. m. domesticus and M. m. musculus. Since mice on islands are excellent evolutionary
model systems, we have focused here on a molecular characterization and an extended phenotype analysis.
Results: The molecular data show that the mice from Heligoland are derived from M. m. domesticus based on
mitochondrial D-loop sequences as well as on four nuclear diagnostic markers, including one each from the
sex-chromosomes. STRUCTURE analysis based on 21 microsatellite markers assigns Heligoland mice to a distinct
population and D-loop network analysis suggests that they are derived from a single colonization event. In spite of
mice from the mainland arriving by ships, they are apparently genetically refractory against further immigration.
Mutation frequencies in complete mitochondrial genome sequences date the colonization age to approximately
400 years ago. Complete genome sequences from three animals revealed a genomic admixture with M. m.
musculus genomic regions with at least 6.5 % of the genome affected. Geometric morphometric analysis of
mandible shapes including skull samples from two time points during the last century suggest specific adaptations
to a more carnivorous diet.
Conclusions: The molecular and morphological analyses confirm that M. m. helgolandicus consists of a distinct
evolutionary lineage with specific adaptations. It shows a remarkable resilience against genetic mixture with
mainland populations of M. m. domesticus despite major disturbances in the past century and a high ship traffic.
The genomic admixture with M. m. musculus genetic material may have contributed to the genomic distinction of
the Heligoland mice. In spite of its young age, M. m. helgolandicus may thus be considered as a true subspecies of
Mus, whose evolution was triggered through fast divergence on a small island.
