Molecular breeding for the BaMMV/BaYMV resistance gene ym4 in winter barley

The aim of this investigation was to develop a procedure for the largescale molecular breeding for ym4, allowing resistance to BaMMV/BaYMV to be fixed in early breeding generations of winter barley. A codominant STS marker derived from the restriction fragment length polymorphism marker MWG838 for the ym4 resistance gene was combined with a new and easy procedure for preparing leaf samples for polymerase chain reaction (PCR), theoretically allowing one person to extract DNA from 5000 samples in a single day. In the procedure for molecular breeding for ym4, all steps, including leaf sampling, DNA extraction, PCR amplification and digestion with restriction enzyme were assembled in microtitre plates allowing multipipetting throughout the procedure, including the loading of gels. The method is amenable to further automation with the aid of a robot arm. Double haploid (DH) lines, as well as F₂ and F₄ breeding lines were analysed and, based on markers, homozygous and heterozygous BaMMV/BaYMV resistant plants were identified for further breeding. The winter barley breeding programmes were modified to include marker-based selection for BaMMV/BaYMV resistance on DH or on F₂ individuals, which had been preselected for mildew and leaf rust resistance.