Genotyping by sequencing for SNP marker development in onion

Onion (Allium cepa) is not highly tractable for development of molecular markers due to its large (16 gigabases per 1C) nuclear genome. Single nucleotide polymorphisms (SNPs) are useful for genetic characterization and marker-aided selection of onion because of codominance and common occurrence in elite germplasm. We completed genotyping by sequencing (GBS) to identify SNPs in onion using 46 F₂ plants, parents of the F₂ plants (Ailsa Craig 43 and Brigham Yellow Globe 15-23), two doubled haploid (DH) lines (DH2107 and DH2110), and plants from 94 accessions in the USDA National Plant Germplasm System (NPGS). SNPs were called using the TASSEL 3.0 Universal Network Enabled Analysis (UNEAK) bioinformatics pipeline. Sequences from the F₂ and DH plants were used to construct a pseudo-reference genome against which genotypes from all accessions were scored. Quality filters were used to identify a set of 284 high quality SNPs, which were placed onto an existing genetic map for the F₂ family. Accessions showed a moderate level of diversity (mean H_e = 0.341) and evidence of inbreeding (mean F = 0.592). GBS is promising for SNP discovery in onion, although lack of a reference genome required extensive custom scripts for bioinformatics analyses to identify high quality markers.