Ice sheets and genetics: Insights into the phylogeography of Scottish Atlantic salmon, Salmo salar L

Aim
We constructed an independent phylogeographical hypothesis (IPH) for Atlantic salmon (Salmo salar L.) in Scotland and adjacent northern England and tested if the distribution of contemporary microsatellite variation accorded with the IPH.

Location
NW Europe.

Methods
Knowledge of post-Last Glacial Maximum (LGM) landscape development and salmon biology were used to define a temporal sequence of late Quaternary ecological niche models and, combined with the central-marginal (CM) model of diversity evolution, to formulate an IPH. Observed microsatellite locus variation for salmon from 102 rivers in Scotland and northern England was tested for agreement with the IPH.

Results
The IPH postulated (1) initial colonization (c. 17 ka bp) of multiple isolated deglaciating coastal regions by long-distance dispersal, predominantly from SW Europe via the Atlantic, (2) intra-region colonization dominated by local range expansion, (3) differentiation by founder effects/genetic drift, in line with CM, until deglaciation complete and regions merged (c. 10 ka bp). IPH regions were weakly but significantly differentiated, showed broad agreement with genetically defined regions, and genetic diversity within them positively correlated with time post-deglaciation, conforming to the CM model. The greatest differentiation was seen for the most recently diverged populations, with evidence that accumulating homoplastic mutations muted older regional phylogeographical signatures.

Main conclusions
Microsatellite differentiation in Scottish Atlantic salmon is significantly but weakly conditioned by post-LGM deglaciation, with a mosaic of regional groups and a temporal cascade of within-region CM effects caused by local range expansion as patches became deglaciated. Colonization was predominantly or exclusively from the south-western European LGM refugium. Accumulating homoplastic mutations have increasingly reduced regional differentiation.