Differences in heat tolerance, water use efficiency and growth among Douglas-fir families and varieties evidenced by GWAS and common garden studies

Abstract

Severe and frequent heat and drought events challenge the survival and development of long-generation trees. In this study, we investigated the genomic basis of heat tolerance, water use efficiency, and growth by performing genome-wide association studies in coastal Douglas-fir (Pseudotsuga menziesii) and intervarietal (menziesii x glauca) hybrid seedlings. GWAS results identified 32 candidate genes involved in primary and secondary metabolism, abiotic stress and signaling, among other functions. Water use efficiency (inferred from carbon isotope discrimination), photosynthetic capacity (inferred from %N), height, and heat tolerance (inferred from electrolyte leakage in a heat stress experiment) were significantly different among Douglas-fir families and varieties. High elevation seed sources had increased water use efficiency, which could be a result of higher photosynthetic capacity. Similarly, families with greater heat tolerance also had higher water use efficiency and slower growth, suggesting a conservative growth strategy. Intervarietal hybrids showed increased heat tolerance (lower electrolyte leakage at 50 oC and 55 oC) and higher water use efficiency compared to coastal families, suggesting hybridization might be a source of pre-adapted alleles to warming climates and should be considered for large-scale reforestation projects under increasingly arid conditions.

Weak local adaptation to drought in seedlings of a widespread conifer

“…glauca populations into var. menziesii populations in dry locations, or the use of inter‐varietal hybrids could be an alternative approach (Compton et al ., 2023), but the large difference in growth potential between varieties and the potential for maladaptation to other biotic and abiotic factors (Pickles et al ., 2015) still require further testing.…”

Spatially heterogeneous selection and inter-varietal differentiation maintain population structure and local adaptation in a widespread conifer

“…Conifers, being wind pollinated species, may be susceptible to elevational differences between populations [51]. Douglas-fir grows from sea level to over 3000 m of elevation with northern interior populations growing at the highest altitude [52,8]. Hybrids grow at intermediate elevations between northern coastal and northern interior populations which may explain the higher ancestry of the interior north variety as gene flow from the coastal populations is limited due to the elevational gradient.…”

“…A similar pattern may exist in our study system where asymmetric introgression from interior douglas-fir confers a fitness advantage in natural populations of hybrids. Previous work in douglas-fir suggests that adaptive introgression from the interior variety may have resulted in natural populations of hybrids with increased water-use efficiency (WUE) and heat tolerance [8].…”

“…Samples were genotyped using a custom-designed gene-based Illumina Infinium SNP array containing 16,146 Douglas-fir single nucleotide polymorphisms (SNPs) at the University of California-Davis Genome Center. This array was designed to represent genome-wide variation in the species by using 10X whole-genome re-sequencing data from individuals across the species' geographic range as input for array construction [7,8]. Genotypes were called using Illumina's Genome Studio Genotyping Module v 2.0.5 (Illumina, 2016).…”

“…It grows under a wide range of climatic conditions and has been part of the landscape of western North America since the Pleistocene. With warming environmental conditions, research on Douglas-fir has focused on testing the ability of the species to grow while withstanding changes in temperature (heat stress) and low soil water content (drought stress) [4,5,6,7,8]. Two varieties with adaptive variation are formally recognized: the coastal variety along the Pacific coast ( P. menziesii var.…”