Comparative genetics and disease resistance in wheat

The hexaploid wheat genome is too complex for direct map-basedcloning and model genomes have to be used to isolate genes from wheat.Comparative genomic analysis at the genetic map level has shown extensiveconservation of the gene order between the different grass genomes inmany chromosomal regions. However, little is known about the geneorganization in grass genomes at the microlevel. We have investigated themicrocollinearity at Lrk gene loci in the genomes of four grass species:wheat, barley, maize and rice. The Lrk genes, which encodereceptor-like kinases, were found to be consistently associated with anothertype of receptor-like kinase (Tak) on chromosome groups 1 and 3 inTriticeae and on chromosomes homoeologous to Triticeae group 3 in theother grass genomes. On Triticeae chromosome group 1, Tak and Lrk together with genes putatively encoding NBS/LRR proteins form acluster of genes. Comparison of the gene composition at orthologous Lrk loci in wheat, barley and rice revealed a maximal gene density of onegene per 5 kb. We conclude that small and large grass genomes containregions which are highly enriched in genes. Microrearrangements betweendifferent grass genomes have been found and therefore, the choice of agood model genome is critical. We have recently started to work on theT. monococcum model genome and confirmed its usefulness foranalysis of the Lr10 leaf rust disease resistance locus in wheat.