Validation of the two-gene epistatic model for vernalization response in a winter x spring barley cross

A two gene epistatic model in which a dominant 'winter growth habit' allele at Vrn-H2 encodes a repressor with a corresponding binding site in a recessive vrn-H1 allele explains the vernalization response phenotypes in an array of barley germplasm. In order to validate the model genetically, we developed an F sub(2) population (and F sub(2)-derived F sub(3) families) from the cross of Hardy (winter) x Jubilant (spring). Using gene-specific primers, we determined the Vrn-H1 and Vrn-H2 allele architecture of each F sub(2) plant and we measured the growth habit phenotype of each F sub(2) plant via phenotyping of its F sub(3) progeny under controlled environment conditions. We used a set of treatments involving plus/minus vernalization under long photoperiod and vernalization under short photoperiod. Alleles at the two loci showed expected patterns of segregation and independent assortment. Under long day conditions, the two Vrn genes were the primary determinants of heading date, regardless of the vernalization treatment. Under short photoperiod, the effects of these loci were not significant. There was incomplete dominance at Vrn-H1: heterozygotes were significantly later to head than Vrn-H1Vrn-H1 genotypes. Vrn-H2 genotypes were also significantly later to head, even when plants were vernalized. These results validate the two-gene epistatic model for vernalization response under long-day conditions. The results under short photoperiod, and the variance in flowering with vernalization, confirm that while the two Vrn genes are the primary determinants of vernalization response, they are part of a larger interactome that determines the timing of the vegetative to reproductive transition.