A dehydrin gene was isolated from wood-forming tissue in Pinus radiata, and nucleotide diversity at this locus was initially studied in 150, 25 and 12 chromosomes from P. radiata, P. pinaster and P. taeda respectively. Sequence variation was high in P. radiata, with 12 sequence-based haplotypes. Phylogenetic analysis of the haplotypes revealed two distinct branches, B1 and B2. Both branches were present in P. radiata. Non-synonymous substitutions were 6.4 times less frequent than synonymous SNPs between B1 and B2 haplotypes. Z-tests for neutrality of nucleotide polymorphism were negative and statistically significant suggesting purifying selection. Several lines of evidence suggested strong balancing selection in P. radiata. Firstly, there were deep branch lengths (15 fixed differences) between B1 and B2 haplotypes. Secondly, each branch was present at intermediate frequency in two New Zealand, and four American populations (1,976 trees in total). Thirdly, two tests for natural selection (Tajima¿s D and Fu and Li¿s F tests) were statistically significant and positive. There was no evidence for balancing selection between P. pinaster and P. taeda sequences. Support for a heterozygous effect was obtained by examining wood density. Trees with one copy of an intermediate frequency B2-haplotype had significantly higher wood densities than either homozygote in both of the New Zealand populations. However these trees had significantly lower radial growth rates when compared to homozygous trees from the oldest phylogenetic branch (B1). Based on the putative function of dehydrin proteins, we suggest that the target of balancing selection at this locus was increased tolerance to drought or cold stress. (Texte intégral)