Saccharum spp.

A major gene conferring resistance to brown rust (Bru1) in the cultivar R570 was identified. It is the first major resistance gene identified in sugarcane. The team has been working for several years, with the support of the International Consortium for Sugarcane Biotechnology, to characterize this locus and to clone this gene following a map-based approach. Exploiting the good syntenic relations between sugarcane, sorghum and rice, a dense genetic map around the gene has been built and a physical map of the target region exploiting a BAC library of 130 000 clones which cover 1.3 time the total genome and subsequent expansions to increase coverage has been initiated.

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 hom(oe)ologous haplotypes. Two BAC clones corresponding to two hom(oe)ologous haplotypes cover completely the target area. However, the contig corresponding to the target haplotype comprised seven BAC clones that only partially overlap, with two gaps still not covered. The team discovered that this in due to the presence of an insertion in the target haplotype as compared to the other haplotypes.

Ten sugarcane BAC clones representing the different hom(oe)ologous haplotypes are currently being sequenced. Their sequences will be used to continue the characterization of this resistance locus and to complete map-based cloning of the gene.

In addition these sequences will be used to compare the organization of the various hom(oe)ologous haplotypes and thus to investigate genome organization and dynamics in this highly polyploid context. Indeed polyploidization has often been reported as followed by extensive and rapid genomic alterations with massive silencing and elimination of duplicated genes. However the first results on another locus suggested that it might not be the case in sugarcane.

Three sorghum BAC clones, orthologous to the sugarcane target region, are also being sequenced to analyze the microsynteny between these two crops that diverged 8 Mya.

Contacts : Sylvie Samain (Genoscope) - Angélique D’Hont (CIRAD)

Bibliography :

Grivet L, Arruda P (2001). Sugarcane genomics : depicting the complex genome of an important tropical crop. Current opinion in Plant Biology. 5 : 122-127.

Asnaghi C., D’Hont A., Glaszmann J.C. and. Rott P. (2001). Resistance of sugarcane cultivar R570 to Puccinia melanocephala isolates from different geographic locations. Plant Disease Vol 85, 32 : 82-286.

Asnaghi C, Roques D, Ruffel S, Kaye C, Hoarau J-Y, Télismart H, Girard JC, Raboin LM, Risterucci AM, Grivet L, D’Hont A. (2004). Targeted mapping of a sugarcane rust resistance gene (Bru1) using bulked segregant analysis and AFLP markers. Theor. Appl. Genet. 108 : 759-764.

D’Hont Angélique. Unravelling the genome structure of polyploids using FISH and GISH ; examples of sugarcane and banana (2005). Cytogenet Genome Res. 109(1-3) : 27-33.

Jannoo N, Grivet L, Chantret N, Garsmeur O, Glaszmann J C, Arruda P and D’Hont A. (2007). Orthologous comparison in a gene-rich region among grasses reveals stability in the sugarcane polyploid genome. Plant Journal 50 (4) : 574–585.