Original Article

Heredity (2007) 99, 224–232; doi:10.1038/sj.hdy.6800987; published online 9 May 2007

Adaptive gradients and isolation-by-distance with postglacial migration in Picea sitchensis

M Mimura1 and S N Aitken1

1Forest Sciences and Centre for Forest Gene Conservation, University of British Columbia, Vancouver, Canada

Correspondence: Professor SN Aitken, Forest Sciences and Centre for Forest Gene Conservation, University of British Columbia, 3041-2424 Main Mall, Vancouver, BC, Canada V6T 1Z4. E-mail: sally.aitken@ubc.ca

Received 15 May 2006; Revised 27 October 2006; Accepted 16 February 2007; Published online 9 May 2007.

Top

Abstract

Fossil pollen records suggest rapid migration of tree species in response to Quaternary climate warming. Long-distance dispersal and high gene flow would facilitate rapid migration, but would initially homogenize variation among populations. However, contemporary clinal variation in adaptive traits along environmental gradients shown in many tree species suggests that local adaptation can occur during rapid migration over just a few generations in interglacial periods. In this study, we compared growth performance and pollen genetic structure among populations to investigate how populations of Sitka spruce (Picea sitchensis) have responded to local selection along the historical migration route. The results suggest strong adaptive divergence among populations (average QST=0.61), corresponding to climatic gradients. The population genetic structure, determined by microsatellite markers (RST=0.09; FST=0.11), was higher than previous estimates from less polymorphic genetic markers. The significant correlation between geographic and pollen haplotype genetic (RST) distances (r=0.73, P<0.01) indicates that the current genetic structure has been shaped by isolation-by-distance, and has developed in relatively few generations. This suggests relatively limited gene flow among populations on a recent timescale. Gene flow from neighboring populations may have provided genetic diversity to founder populations during rapid migration in the early stages of range expansion. Increased genetic diversity subsequently enhanced the efficiency of local selection, limiting gene flow primarily to among similar environments and facilitating the evolution of adaptive clinal variation along environmental gradients.

Keywords:

range shift, adaptation, gene flow, population size, adaptive cline, conifers