title: Genetic analyses of bolting in bulb onion (Allium cepa L.).
abstract: We present the first evidence for a QTL conditioning an adaptive trait in bulb onion, and the first linkage and population genetics analyses of candidate genes involved in photoperiod and vernalization physiology. Economic production of bulb onion (Allium cepa L.) requires adaptation to photoperiod and temperature such that a bulb is formed in the first year and a flowering umbel in the second. 'Bolting', or premature flowering before bulb maturation, is an undesirable trait strongly selected against by breeders during adaptation of germplasm. To identify genome regions associated with adaptive traits we conducted linkage mapping and population genetic analyses of candidate genes, and QTL analysis of bolting using a low-density linkage map. We performed tagged amplicon sequencing of ten candidate genes, including the FT-like gene family, in eight diverse populations to identify polymorphisms and seek evidence of differentiation. Low nucleotide diversity and negative estimates of Tajima's D were observed for most genes, consistent with purifying selection. Significant population differentiation was observed only in AcFT2 and AcSOC1. Selective genotyping in a large 'Nasik Red × CUDH2150' F2 family revealed genome regions on chromosomes 1, 3 and 6 associated (LOD > 3) with bolting. Validation genotyping of two F2 families grown in two environments confirmed that a QTL on chromosome 1, which we designate AcBlt1, consistently conditions bolting susceptibility in this cross. The chromosome 3 region, which coincides with a functionally characterised acid invertase, was not associated with bolting in other environments, but showed significant association with bulb sucrose content in this and other mapping pedigrees. These putative QTL and candidate genes were placed on the onion map, enabling future comparative studies of adaptive traits.
authors.affiliation: New Zealand Institute for Plant and Food Research, Private Bag, 4704, Christchurch, New Zealand.
authors.forename: Samantha
authors.initials: S
authors.lastname: Baldwin
authors.forename: Roopashree
authors.initials: R
authors.lastname: Revanna
authors.forename: Meeghan
authors.initials: M
authors.lastname: Pither-Joyce
authors.forename: Martin
authors.initials: M
authors.lastname: Shaw
authors.forename: Kathryn
authors.initials: K
authors.lastname: Wright
authors.forename: Susan
authors.initials: S
authors.lastname: Thomson
authors.forename: Leire
authors.initials: L
authors.lastname: Moya
authors.forename: Robyn
authors.initials: R
authors.lastname: Lee
authors.forename: Richard
authors.initials: R
authors.lastname: Macknight
authors.forename: John
authors.initials: J
authors.lastname: McCallum
mesh_terms: D002874:Chromosome Mapping; D018744:DNA, Plant / Q000235:genetics; D017343:Genes, Plant; D008040:Genetic Linkage; D005838:Genotype; D019697:Onions / Q000235:genetics; D010641:Phenotype; D018517:Plant Roots / Q000235:genetics*; D040641:Quantitative Trait Loci
chemical_list: D018744:DNA, Plant