Search for nucleotide diversity patterns of local adaptation in dehydrins and other cold-related candidate genes in Scots pine (Pinus sylvestris L.)

Abstract

Nucleotide variation at several cold candidate genes including seven members of the dehydrin gene family was surveyed in haplotypes of Scots pine (Pinus sylvestris) sampled in populations showing divergence for cold tolerance in Europe. Patterns of nucleotide diversity, linkage disequilibrium, and frequency spectrum of alleles were compared between north and south populations to search for signs of directional selection potentially underlying adaptation to cold. Significant differentiation between populations in allelic frequency or haplotype structure was detected at dhn1, dhn3, and abaH loci. Allelic dimorphism with no evidence of haplotype clustering by geographical distribution was found at dhn9. An excess of fixed non-synonymous mutations as compared to the outgroup P. pinaster pine species was found at dhn1. Differences in nucleotide polymorphisms were found between the members of the Kn class of dehydrin upregulated during cold acclimation (average πsil = 0.004) as compared to the SKn class (average πsil = 0.024). The multilocus nucleotide diversity at silent sites (θW = 0.009) was moderate compared to other conifer species, but higher than previous estimates for Scots pine. There was an excess of rare and high frequency derived variants as revealed by significantly negative multilocus value of Tajima’s D (D = −0.72, P < 0.01) and negative mean value of Fay and Wu H statistics (H = −0.50). The level of linkage disequilibrium decayed rapidly with an average expected r2 of 0.2 at about 200 bp. Overall, there was a positive correlation between polymorphism and divergence at ten loci when outgroup sequence was available. The discovered polymorphism will be used for further evaluation of the adaptive role of genes through association mapping studies.