Exploring Wild Barley Germplasm in an Advanced Backcross-Nested Association Mapping (AB-NAM) Population

To increase the accessibility of wild barley germplasm to barley breeders and geneticists, we developed a wild barley advanced backcross-nested association mapping (AB-NAM) population. This population incorporates a large proportion of the genetic diversity present in wild barley, into a 6-row malting barley background. A total of 25 wild barley accessions were selected from the 318 accession Wild Barley Diversity Collection (WBDC). Using over 4,000 SNP, DArT, and SSR markers, these 25 accessions were selected to maximize the allelic diversity of the chosen set. Each selected accession was backcrossed twice to the cultivar Rasmusson. The resulting 796 lines were genotyped with a custom 384-SNP assay, and the parents were genotyped with the 9K iSelect SNP assay and exome-capture sequenced. To increase the mapping resolution, we imputed 4,022 SNP markers using the Munoz et al. 2014 consensus map and 438,757 sequence variants using the Mascher et al. 2013 POPSEQ Morex x Barke sequence contig map. Several traits, including yield, height, heading date, productive tiller count, wax production, and grain protein were scored and mapped using genotyped and imputed marker sets. Furthermore, to compare the dynamics of wild barley-derived mapping populations, genomewide linkage disequilibrium (LD) was calculated for the AB-NAM, the WBDC, and two biparental wild x cultivated barley mapping populations: the Harrington x OUH-602 advanced backcross (AB) and recombinant inbred line (RIL) populations. While LD decay (r² = 0.2) is lowest in the WBDC (0.36 cM), the AB-NAM (9.1 cM) exhibits quicker LD decay than the AB (28.6 cM) and RIL (32.7 cM) populations.  This illustrates a higher potential mapping resolution in the AB-NAM compared to the AB and RIL populations. The combination of low LD, negligible population structure, and high diversity in an adapted background make the AB-NAM an important tool for exploring wild barley germplasm.