The nucleolus organizer regions (Nor loci) of hexaploid wheat cultivars

Abstract

We have used the techniques of molecular genetics to analyse the nucleolus organizer regions (Nor loci) of a broad range of Australian, Chinese, and other hexaploid wheats (Triticum aestivum L. em Thell.). Genomic DNA was extracted from the leaves of 260 wheat cultivars, 94 different F1 hybrids, and from more than 800 F2 plants. Samples of the DNA were digested with the restriction enzyme TaqI and electrophoretically separated in agarose gels. The DNA fragments were then transferred to DNA filters by Southern blotting and assayed with the probe pTa250.4. This probe specifically detects the intergenic spacer DNA sequences which alternate with the ribosomal-RNA (rRNA) genes in highly repeated tandem arrays within the Nor loci. Analyses of the spacer DNA restriction fragments present in P, F1 and F2 plants show that allelic variants of the Nor loci can be distinguished by the existence of different lengths of spacer DNA fragments, different combinations of different length fragments, or differing amounts of the same length fragments. Eight alleles of the Nor-B1 locus on chromosome 1B (Nor-Bla to Nor-B1h) and 20 variants of the Nor-B2 locus on chromosome 6B (Nor-B2a to Nor-B2t) have been identified. The DNA fragments present in the different alleles are expressed in a co-dominant fashion to give phenotypic F2 ratios of 1 : 2 : 1 and 1 : 2 : 1 : 2 : 4 : 2 : 1 : 2 : 1, indicating segregation at one or two loci, respectively. With two exceptions, individual alleles segregated in a 1 : 1 ratio. On the basis of the presence of different combinations of Nor-B1 and Nor-B2 alleles, the wheat cultivars investigated were divided into 53 phenotypic groups. These groups clearly reflect the germplasm used in wheat breeding programs in different countries. Hybrid wheats could also be recognized and plants with new combinations of alleles were produced.