Comparative Genetics and Disease Resistance in Wheat

Abstract

The hexaploid wheat genome is too complex for direct map-based cloning and model genomes have to be used to isolate genes from wheat. Comparative genomic analysis at the genetic map level has shown extensive conservation of the gene order between the different grass genomes in many chromosomal regions. However, little is known about the gene organization in grass genomes at the microlevel. We have investigated the microcollinearity at Lrk gene loci in the genomes of four grass species: wheat, barley, maize and rice. The Lrk genes, which encode receptor-like kinases, were found to be consistently associated with another type of receptor-like kinase (Tak) on chromosome groups 1 and 3 in Triticeae and on chromosomes homoeologous to Triticeae group 3 in the other grass genomes. On Triticeae chromosome group 1, Tak and Lrk together with genes putatively encoding NBS/LRR proteins form a cluster of genes. Comparison of the gene composition at orthologous Lrk loci in wheat, barley and rice revealed a maximal gene density of one gene per 5 kb. We conclude that small and large grass genomes contain regions which are highly enriched in genes. Microrearrangements between different grass genomes have been found and therefore, the choice of a good model genome is critical. We have recently started to work on the T monococcum model genome and confirmed its usefulness for analysis of the Lr10 leaf rust disease resistance locus in wheat.