Genotypic-Phenotypic Variation and Marker-Based Heritability Estimates of a Shrub Willow (Salix purpurea) Association Population

Salix spp. and hybrids (shrub willow) have been bred as dedicated bioenergy crops, however there is still untapped potential for genetic improvement.  One way to accelerate and improve upon the breeding and selection is to utilize the large amount of genetic variation present within the genus through the collection of phenotypic datasets, high-density genotyping-by-sequencing and genetic mapping studies. Salix purpurea is a reference species for breeding shrub willow bioenergy crops in North America. We assembled a genetically diverse germplasm collection of 112 accessions and obtained high-quality data for 21 biomass, morphological, phenological, physiological and wood compositional traits from three replicated field trials across two years. With a filtered set of 25,566 GBS markers, marker-based estimations of narrow sense heritability of all traits were calculated and ranged between 0.24-0.89 while estimates based on genotypic means ranged from 0.29-0.87. Genotypic and phenotypic correlations as well as multiple linear regression using step-wise selection of all traits were highly correlated with final biomass yield at harvest with the highest correlation of r=0.78. The North American collection of S. purpurea was also expanded through the addition of 165 accessions from Europe which revealed population stratification and subdivisions using GBS SNP markers. Using STRUCTURE and DAPC to characterize population genetic structure, results showed five to six subpopulations, respectively. The expansion of this analysis with accessions from the native range of S. purpurea provides an opportunity for enhanced studies of phenotype-genotype associations and the genetic bottleneck that occurred through naturalization of Salix purpurea in North America.