Ppd-B1 and Ppd-D1 and their effects in southern Australian wheat

Abstract

Photoperiod and vernalisation genes are important for the adaptation of wheat to variable environments. Previously, using diagnostic markers and a large, unbalanced dataset from southern Australia, we estimated the effects on days to heading of frequent alleles of Vrn-A1, Vrn-B1, and Vrn-D1, and also two allelic classes of Ppd-D1. These genes accounted for ~45% of the genotypic variance for that trait. We now extend these analyses to further alleles of Ppd-D1, and four alleles of Ppd-B1 associated with copy number. Variation in copy number of Ppd-B1 occurred in our population, with one to four linked copies present. Additionally, in rare instances, the Ppd-B1 gene was absent (a null allele). The one-copy allele, which we labelled Ppd-B1b, and the three-copy allele, which we labelled Ppd-B1a, occurred through a century of wheat breeding, and are still frequent. With several distinct progenitors, the one-copy allele might not be homogenous. The two-copy allele, which we labelled Ppd-B1d, was generally introduced from WW15 (syn. Anza), and the four-copy allele, which we labelled Ppd-B1c, came from Chinese Spring. In paired comparisons, Ppd-B1a and Ppd-B1c reduced days to heading, but Ppd-B1d increased days to heading. Ppd-D1a, with a promoter deletion, Ppd-D1d, with a deletion in Exon 7, and Ppd-D1b, the intact allele, were frequent in modern Australian germplasm. Differences between Ppd-D1a and Ppd-D1d for days to heading under our field conditions depended on alleles of the vernalisation genes, confirming our previous report of large epistatic interactions between these classes of genes. The Ppd-D1b allele conferred a photoperiod response that might be useful for developing cultivars with closer to optimal heading dates from variable sowing dates. Inclusion of Ppd-B1 genotypes, and more precise resolution of Ppd-D1, increased the proportion of the genotypic variance attributed to these vernalisation and photoperiod genes to ~53%.