Genetic mapping of the wheat leaf rust resistance gene Lr2a and its importance in Canadian wheat cultivars

Leaf rust resistance gene Lr2a was located to chromosome arm 2DS in three mapping populations, which will facilitate map-based cloning and marker-assisted selection of Lr2a in wheat breeding programs. Incorporating effective leaf rust resistance (Lr) genes into high-yielding wheat cultivars has been an efficient method of disease control. One of the most widely used genes in Canada is the multi-allelic resistance gene Lr2, with alleles Lr2a, Lr2b, Lr2c, and Lr2d. The Lr2a allele confers complete resistance to a large portion of the Puccinia triticina (Pt) population in Canada. In this study, Lr2a was genetically mapped in two doubled haploid populations developed from the crosses Superb/BW278 and Superb/86ISMN 2137, and an F₂ population developed from the cross Chinese Spring/RL6016. Seedlings were tested with the Lr2a avirulent Pt races 74-2 MGBJ (Superb/BW278) and 12-3 MBDS (Superb/86ISMN 2137 and Chinese Spring/RL6016) in greenhouse assays and were genotyped with 90K wheat Infinium SNP and kompetitive allele-specific PCR (KASP) markers. Lr2a was mapped to a collinear position on chromosome arm 2DS in all three populations, within a 1.00 cM genetic interval between KASP markers kwm1620 and kwm1623. This corresponded to a 305 kb genomic region of chromosome 2D in Chinese Spring RefSeq v2.1. The KASP marker kwh740 was predictive of Lr2a in all mapping populations. A panel of 260 wheats were tested with three Pt isolates, which revealed that Lr2a is common in Canadian wheats. The KASP markers kwh740 and kwm1584 were highly associated with resistance at the Lr2 locus, while kwm1622 was slightly less correlated. Genetic mapping of the leaf rust resistance gene Lr2a and DNA markers developed here will facilitate its use in wheat breeding programs.