High-Resolution Genetic Mapping and Physical Map Construction for Rfm1, a Fertility Restoration Locus for Cytoplasmic Male Sterility in Barley

Cytoplasmic male sterility (CMS) is a maternally inherited trait that causes sterilities for plants because of the defect of male reproductive organs.  CMS is caused by lesion or rearrangement of mitochondrial genome, but nucleus-encoded restorer of fertility (Rf) genes can restore it (Hanson and Bentolila, 2004).  In barley, Rfm1 restores the fertility of msm1 CMS strain and is located within a 3.2 cM region delimited by 2 markers (NAS2090 and NAS1080) on the short arm of chromosome 6H (Matsui et al., 2001).  In this study, we developed novel 3 markers for the Rfm1 region, and then performed a fine mapping of Rfm1.  Considering the recombination rate calculated by previous researches, a large-scale F₂ population was required for the fine mapping of Rfm1.  Therefore, using 3,638 F₂ plants derived from the cross between the CMS strain and the Rf strain, we screened 175 F₂ plants with the recombination between NAS2090 and NAS1080 and then genotyped them with all markers in the Rfm1 region.  As a result, the markers completely linked to Rfm1 in the previous study recombined and we constructed a finer map for the Rfm1 region.  Rfm1 was located within the 0.14 cM region delimited by 2 markers (NAS9113 and NAS9200).  Then we screened barley BAC libraries containing the sequences of NAS9130, NAS9133 or NAS9200 and found 11 BAC clones with either marker.  After the finger printing, we constructed the physical map, which delimited the Rfm1 region in ca. 200kb window with 3 MTP clones.