Genetic Diversity and Population Structure of Collard Landraces Collected in the Southeast

Collard (Brassica oleracea L. covar. Acephala var. viridis) is a common vegetable grown throughout the southeastern United States.   This non-heading, leafy-green cole crop is mostly grown during cooler seasons (e.g., fall through spring), but in some areas it is effectively grown year round.  In the past, the home collard patch and even commercial plantings made use of local varieties that were saved by individual gardeners or farmers. Evidence indicates that many of these local varieties were true heirlooms saved for many decades and passed down from one generation to the next.  Through much of the 20^th Century, the regional diversity of collard was probably very significant; however, genetic erosion of the germplasm pool for this vegetable occurred as commercial hybrids became popular and widely grown by vegetable producers as well as home gardeners.  With the support of the USDA Plant Exploration Office, we traveled the Southeast from 2003 to 2006 in search of collard landraces, obtaining close to 90 samples from various seed-savers, a majority of whom were aging and not apt to pass on seed of their particular saved variety. Using the Illumina 60K Brassica SNP BeadChip array of 52,157 SNPs, we assessed the genetic diversity of 75 of the collard landraces and clarified the relationship of collard to the main B. oleracea crop varieties. The collard landraces had twice the polymorphic markers (11,322 SNPs) and ten times the variety-specific alleles (521 alleles) of the remaining varieties. Although other relationships varied, the previous placement of collard with cabbage and Portuguese tronchuda cabbage was confirmed with STRUCTURE, PCoA, phylogenetic analyses using 15,951 unlinked markers.