A novel strategy to map a locus associated with flowering time in canola (Brassica napus L.)

Flowering time is an important agronomic trait for canola breeders, as it provides growers with options for minimizing exposure to heat stress during flowering and to more effectively utilize soil moisture. Plants have evolved various systems to control seasonal rhythms in reproductive phenology including an internal circadian clock that responds to environmental signals. In this study, we used canola cultivar 'Westar' as a recurrent parent and canola cultivar 'Surpass 400' as the donor parent to generate a chromosome segment substitution line (CSSL) and to map a flowering time locus on chromosome A10 using molecular marker-assisted selection. This CSSL contains an introgressed 4.6 mega-bases (Mb) segment (between 13 and 17.6 Mb) of Surpass 400, which substantially delayed flowering compared with Westar. To map flowering time gene(s) within this locus, eight introgression lines (ILs) were developed carrying a series of different lengths of introgressed chromosome A10 segments using five co-dominant polymorphic markers located at 13.5, 14.0, 14.5, 15.0, 15.5, and 16.0 Mb. Eight ILs were crossed with Westar reciprocally and flowering time of resultant 16 F₁ hybrids and parents were evaluated in a greenhouse (2021 and 2022). Four ILs (IL005, IL017, IL035, and IL013) showed delayed flowering compared to Westar (P < 0.0001), and their reciprocal crosses displayed a phenotype intermediate in flowering time of both homozygote parents. These results indicated that flowering time is partial or incomplete dominance, and the flowering time locus mapped within a 1 Mb region between two co-dominant polymorphic markers at 14.5-15.5 Mb on chromosome A10. The flowering time locus was delineated to be between 14.60 and 15.5 Mb based on genotypic data at the crossover site, and candidate genes within this region are associated with flowering time in canola and/or Arabidopsis. The co-dominant markers identified on chromosome A10 should be useful for marker assisted selection in breeding programs but will need to be validated to other breeding populations or germplasm accessions of canola.