CHROMOSOME ASSIGNMENT OF 4 PHOTOSYNTHESIS-RELATED GENES AND THEIR VARIABILITY IN WHEAT SPECIES

Copy numbers of four photosynthesis-related genes, PhyA, Ppc, RbcS and Lhcb11,in wheat genomes were estimated by slot-blot analysis, and th ese genes were assigned to the chromosome arms of common wheat by Sout hern hybridization of DNA from an aneuploid series of the cultivar Chi nese Spring. The copy number of PhyA was estimated to be one locus per haploid genome, and this gene was assigned to chromosomes 4AL, 4BS an d 4DS. The Ppc gene showed a low copy number of small multigenes, and was located on the short arm of homoeologous group 3 chromosomes and t he long arm of chromosomes of homoeologous group 7. RbcS consisted of a multigene family, with approximately 100 copies in the common wheat genome, and was located on the short arm of group 2 chromosomes and th e long arm of group 5 chromosomes. Lhcb11 also consisted of a multige ne family with about 50 copies in common wheat. Only a limited number of restriction fragments (approximately 15%) were used to determine th e locations of members of this family on the long arm of group 1 chrom osomes owing to the multiplicity of DNA bands. The variability of hybr idized bands with the four genes was less in polyploids, but was more in the case of multigene families. RFLP analysis of polyploid wheats a nd their presumed ancestors was carried out with probes of the oat Phy A gene, the maize Ppc gene, the wheat RbcS gene and the wheat Lhcb11 gene. The RFLP patterns of common wheat most closely resembled those o f T. dicoccum (Emmer wheat), T. urartu (A genome), Ae. speltoides (S g enome) and Ae. squarossa (D genome). Diversification of genes in the w heat complex appear to have occurred mainly at the diploid level. Base d on RFLP patterns, B and S genomes were clustered into two major grou ps. The fragment numbers per genome were reduced in proportion to the increase of ploidy level for all four genes, suggesting that some mech anism(s) might operate to restrict, and so keep to a minimum, the gene numbers in the polyploid genomes. However, the RbcS genes, located on 2BS, were more conserved (double dosage), indicating that the above m echanism(s) does not operate equally on individual genes.