Population differences in levels of linkage disequilibrium in the wild

Information about the levels of linkage disequilibrium (LD) in wild animal populations is still limited, and this is true particularly with respect to possible interpopulation variation in the levels of LD. We compared the levels and extent of LD at the genome-wide scale in three Siberian jay (Perisoreus infaustus) populations, two of which (Kuusamo and Ylläs) represented outbred populations within the main distribution area of the species, whereas the third (Suupohja) was a semi-isolated, partially inbred population at the margin of the species' distribution area. Although extensive long-range LD (>20 cM) was observed in all three populations, LD generally decayed to background levels at a distance of 1-5 cM or c. 200-600 kb. The degree and extent of LD differed markedly between populations but aligned closely with both observed levels of within-population genetic variation and expectations based on population history. The levels of LD were highest in the most inbred population with strong population substructure (Suupohja), compared with the two outbred populations. Furthermore, the decay of LD with increasing distance was slower in Suupohja, compared with the other two populations. By demonstrating that levels of LD can vary greatly over relatively short geographical distances within a species, these results suggest that prospects for association mapping differ from population to population. In this example, the prospects are best in the Suupohja population, given that minimized marker genotyping and a minimum marker spacing of 1-5 cM (c. 200-600 kb) would be sufficient for a whole genome scan for detecting QTL.