Diverse VRN-A1 Proteins in Winter Wheat and Spring Wheat Cultivars
The two PCR markers, the SNPs in exon 4 and exon 7 between the Jagger and 2174 alleles, were used to determine the genotypes of wheat germplasm (Table 13.1). Among 73 cultivars of hexaploid wheat, 18 cultivars carry the vrn-A1a allele, 50 cultivars carry the vrn-A1b allele, and 5 cultivars carry the vrn-A1c allele. The higher frequency of the vrn-A1b allele in contemporary wheat cultivars indicated that breeders in the southern Great Plains have inadvertently selected this allele, contributing to delay stem elongation and extend the vegetative phase of the dual purpose wheat to produce more biomass for cattle.
Five cultivars carrying the vrn-A1c allele, Jaypee (VA), Pio 26R61 (NY), OK Bullet, IL369, and Nongda 3338, do not have the same pedigree, indicating that the vrn-A1c allele in these cultivars has evolved independently. The possibility that these cultivars have the same tetraploid wheat donor carrying the vrn-A1c allele cannot be excluded, though none of the tetraploid wheat accessions tested in this study carried the vrn-A1c allele.
When vrn-A1 acts as a protein form, it should appear as a VRN-A1-TaHOX1 protein complex because of a direct binding between them. VRN-A1 protein and TaHOX1 protein should also have direct interaction in spring wheat cultivars. It was found that both vrn-A1a and vrn-A1b existed in spring wheat cultivars. Nine spring wheat cultivars in the CAP group carry the vrn-A1a allele, including UC1110 (CA), ID0556 and Zak (ID), McNeal and Thatcher (MT), Louise and Panawawa (WA), GRN*5/ND614-A (MN), Jupeteco (TX); whereas four spring wheat cultivars carry the vrn-A1b allele, including CIMMYT-2 (PI610750) (CA), OR9900553 (OR), NY18/ Clark's Cream 40–1 (MN), and Weebill (TX).
Allelic variation in the dominant Vrn-A1 locus also indicated pleiotropic genetic effects in spring wheat cultivars (Blake et al. 2009; Santra et al. 2009; Zhang et al. 2008), supporting that the VRN-A1 gene has different mechanisms in controlling wheat development. There were three CAP populations, UC1110 and CIMMYT-2 (PI610750) (CA), GRN*5/ND614-A and NY18/ Clark's Cream 40–1 (MN), and Jupeteco and Weebill (TX) that have different VRN-A1a and VRN-A1b forms. It would be interesting to investigate how allelic variation in VRN-A1 is associated with variation in development in spring wheat populations. Similarly, Jaypee (VA) and Pio 26R61 (NY), which have a VRN-A1c form, can be used to investigate effects of a single point substitution Ala180/Val180 on development in wheat.
Application of Multiple Molecular Markers for VRN-A1
Molecular breeding is a new strategy that uses contemporary methods of molecular genetics and genomic sequencing to assist in all steps throughout the procedure of conventional breeding. Most of the molecular markers currently being applied in breeding are SSR (simple sequence repeat) markers due to their relative simplicity for breeding applications. However, the 'repeat' feature of a SSR marker results in multiple and inconsistent locations of the same marker among divergent wheat cultivars. SNP markers would provide more powerful tools to assist selection in breeding (Akhunov et al. 2009). In most cases, however, a SNP marker is derived from the sequence of an EST that is randomly distributed among genomes; thus a SNP marker may not associate with a given trait. A gene marker (perfect marker) that is developed for the specific regulatory site of a functional gene can provide the ultimate resolution to select for a given trait.
Four markers have been developed for allelic variation in VRN-A1. It is not yet known if the allelic variation in exon 4 is associated with any trait, though the point mutation occurred in the K-box, a critical domain for protein-protein interaction. Two markers for allelic variation in the promoter (Yan et al. 2004) and intron one (Fu et al. 2005) should be used to distinguish between spring wheat and winter wheat, whereas the marker for the SNP intron 7 should be used to distinguish semiwinter and strong winter wheat cultivars.