Complexities of chromosome landing in a highly polyploid, aneuploid, interspecific genome: Towards map-based cloning of a resistance gene (Bru1) in sugarcane (2n=Ca 115)

Modern sugarcane cultivars have a particularly complex genome, highly polyploid (100-130 chromosomes), aneuploid and of interspecific origin. A major gene conferring resistance to brown rust has been identified in the cultivar R570. It is the first major resistance gene identified in sugarcane. We have been working for several years to characterize this locus and to clone this gene following a map-based approach. Exploiting the good syntenic relations between sugarcane, sorghum and rice, we have built a dense genetic map around the gene and have initiated a physical map of the target region. Probes derived from the physical map have been used to complete the genetic map. The target haplotype map encompasses 15 markers that cosegregate with Bru1 and markers at 0,14 and 0.28 cM on each side of the gene. The physical map of the region encompasses 16 BAC clones that could be assigned to 7 different homologous haplotypes. Two BAC corresponding to two homologous haplotypes cover completely the target area. However, the contig corresponding to the target haplotype comprised 7 BAC clones that only partially overlap, with two gaps still not covered. Moreover several probes derived from the BAC contig of the target haplotype do not hybridize with the homologous contig revealing the presence of an insertion in the target haplotype. Several BAC clones representing the different homologous haplotypes are currently being sequenced. Their sequences will be used to continue the characterization of this resistance locus and the complete map-based cloning of the gene. (Texte intégral)