Non-vernalization requirement in Chinese kale caused by loss of BoFLC and low expressions of its paralogs.

We identified the loss of BoFLC gene as the cause of non-vernalization requirement in B. oleracea. Our developed codominant marker of BoFLC gene can be used for breeding program of B. oleracea crops. Many species of the Brassicaceae family, including some Brassica crops, require vernalization to avoid pre-winter flowering. Vernalization is an unfavorable trait for Chinese kale (Brassica oleracea var. chinensis Lei), a stem vegetable, and therefore it has been lost during its domestication/breeding process. To reveal the genetics of vernalization variation, we constructed an F population through crossing a Chinese kale (a non-vernalization crop) with a kale (a vernalization crop). Using bulked segregant analysis (BSA) and RNA-seq, we identified one major quantitative trait locus (QTL) controlling vernalization and fine-mapped it to a region spanning 80 kb. Synteny analysis and PCR-based sequencing results revealed that compared to that of the kale parent, the candidate region of the Chinese kale parent lost a 9,325-bp fragment containing FLC homolog (BoFLC). In addition to the BoFLC gene, there are four other FLC homologs in the genome of B. oleracea, including Bo3g005470, Bo3g024250, Bo9g173370, and Bo9g173400. The qPCR analysis showed that the BoFLC had the highest expression among the five members of the FLC family. Considering the low expression levels of the four paralogs of BoFLC, we speculate that its paralogs cannot compensate the function of the lost BoFLC, therefore the presence/absence (PA) polymorphism of BoFLC determines the vernalization variation. Based on the PA polymorphism of BoFLC, we designed a codominant marker for the vernalization trait, which can be used for breeding programs of B. oleracea crops.