NUCLEOTIDE DIVERSITY, LINKAGE DISEQUILIBRIUM AND ADAPTIVE VARIATION IN NATURAL POPULATIONS OF LOBLOLLY PINE

¹ 2Center of Forest Research, INIA, PO Box 8111 28080 Madrid, Spain

² Environmental Horticulture, University of California, Davis CA 95616, US

³ Weyerhaeuser Company, Weyerhaeuser Technical Center, Tacoma WA 98477, US

⁴ Institute of Forest Genetics, USDA Forest Service, Davis CA 95616, US

Quantitative genetics has been widely applied to the analysis of adaptive traits in long-term common garden experiments. However, the molecular pattern underlying adaptation to changing environmental conditions is fairly unknown. With this objective in view, a candidate gene-based strategy was developed to detect natural adaptive variation for drought tolerance in loblolly pine (Pinus taeda L.). DNA samples from 32 megagametophytes (haploid tissue) were sequenced for ~20 drought tolerance related genes, including some, such as plp3-1 and dhn-1, where expression studies had been previously conducted. Based on this sample, collected mainly from the south-easternmost part of loblolly pine’s distribution, nucleotide diversity estimates were computed. Silent nucleotide diversity was high (q = 0.0074) but nonsynonymous substitutions were relatively rare (q = 0.0021). Haplotypes were identified and a set of SNPs (Single Nucleotide Polymorphisms) distinguishing the most common haplotypes and also those rare haplotypes containing nonsynonymous substitutions was selected. Screening of SNPs in a full-range sample (435 trees) provided relevant information about differential selection and adaptive processes related to drought tolerance in this economically important and widespread conifer species.