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. Comparison of gene coding regions between maize, rice and sorghum showed that the distance between the genes is correlated with the genome size. We have investigated the microcolinearity 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 genomes in rice and maize. On Triticeae chromosome group 1, Tak and Lrk together with genes putatively encoding NBS/LRR proteins form a cluster of genes possibly involved in signal transduction. Comparison of the gene composition at orthologous Lrk loci in wheat, barley and rice revealed a maximal gene density of one gene per 4～5 kb, very similar to the gene density in Arabidopsis thaliana . We conclude that small and large grass genomes contain regions which are highly enriched in genes with very little or no repetitive DNA. The comparison of the gene organization suggested various genome rearrangements during the evolution of the different grass species, including a duplication of the Lrk region specific for the Triticeae on group 1 chromosomes. We are now analyzing the gene organization in the Lrk regions using BAC clones of the A genome (from T. monococcum ) and the D genome (from Ae.tauschii ). In addition, we are investigating the A, B and D genome in hexaploid wheat using a cosmid library. The accumulation of sequence information around the Lrk loci in several species (orthologs) and in the same species (paralogous genes) has allowed comparisons of genome relationships in the investigated regions.
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