HOMOEOLOGOUS RELATIONSHIPS OF RICE, WHEAT AND MAIZE CHROMOSOMES

A set of cDNA clones, which had previously been mapped onto wheat chro mosomes, was genetically mapped onto the chromosomes of rice. The resu lting comparative maps make it possible to estimate the degree of link age conservation between these two species. A number of chromosomal re arrangements, some of which must have involved interchromosomal transl ocations, differentiate the rice and wheat genomes. However, synteny o f a large proportion of the loci appears to be conserved between the t wo species. The results of this study, combined with those from a rece ntly published comparative map of the rice and maize genomes, suggest that rice, wheat and maize share extensive homoeologies in a number of regions in their genomes. Some chromosomes (e.g. chromosome 4 in rice , chromosomes 2 and 2S in wheat and maize, respectively) may have esca ped major rearrangement since the divergence of these species from the ir last common ancestor. Comparative maps for rice, wheat and maize sh ould make it possible to begin uniting the genetics of these species a nd allow for transfer of mapping information (including centromere pos itions) and molecular marker resources (e.g. RFLP probes) between spec ies. In addition, such maps should shed light on the nature of c;chrom osome evolution that accompanied the radiation of grasses in the early stages of plant diversification.