An integrated physical, genetic and cytogenetic map around the sunn locus of Medicago truncatula

The sunn mutation of Medicago truncatula is a single-gene mutation that confers a novel supernodulation phenotype in response to inoculation with Sinorhizobium meliloti. We took advantage of the publicly available codominant PCR markers, the high-density genetic map, and a linked cytogenetic map to define the physical and genetic region containing sunn. We determined that sunn is located at the bottom of linkage group 4, where a fine-structure genetic map was used to place the locus within a similar to400-kb contig of bacterial artificial chromosome (BAC) clones. Genetic analyses of the sunn contig, as well as of a second, closely linked BAC contig designated NUM1, indicate that the physical to genetic distance within this chromosome region is in the range of 1000 -1100 kb.cM(-1). The ratio of genetic to cytogenetic distance determined across the entire region is 0.3 cM-mum(-1). These estimates are in good agreement with the empirically determined value of similar to300 kb-mum(-1) measured for the NUM1 contig. The assignment of sunn to a defined physical interval should provide a basis for sequencing and ultimately cloning the responsible gene.