Physical Mapping of the 5S rRNA Multigene Family in Common Wheat Y. Mukai, T. R. Endo, and B. S. Gill In situ hybridization in conjunction with deletion mapping was used to physically map the 5S rRNA multigene family in Triticum aestivum L. cv. 'Chinese Spring.' Twelve 5S rRNA loci were mapped on chromosomes of homoeologous group 1 (arms 1AS, 1 BS, and 1 DS) and group 5 (arms 5AS, 5BS, and 5DS). The 5S rRNA loci were mapped (fraction of the distance from the centromere) at positions 0.77, 0.96, 0.76, 0.63, and 0.64 on arms 1AS, 1DS, 5AS, 5BS, and 5DS, respectively. The 5S rRNA locus on BS was mapped at position ca. 0.5 of the satellite length. Deletions were used to further map the 5S rRNA loci to chromosome bands 1AS12, 1BS32, 1DS22, 5AS22, 5BS22, and 5DS22. The 5S rRNA loci were not associated with major C-bands on arms 1 BS, 1DS, and 5BS. The mapped 5S rRNA loci on the chromosomes of homoeologous group 1 are designated 5S-Rma-A 1, 5S-Rrna-B 1, and 5S-Rrna-D 1, and the homoeolo- gous group 5 set are designated 5S-Rma-A2, 5S-Rrna-B2, and 5S-Rrna-D2. The 5S rRNA locus in 1 AS is a new finding In Chinese Spring wheat. From the Department of Plant Pathology, Kansas State University, Manhattan. Dr. Mukai is now at Osaka Kyoiku University, Osaka, Japan, and Dr. Endo is now at Nara University, Nara, Japan. Research supported in part by USDA-CRGO grant 86-CRCR-1-2222 and USDA-CSRS special research grant "Wheat Genetics Resource Cen- ter at Kansas State University." Dr. Endo was sup- ported by a grant from the Japan Society for the Pro- motion of Science under the U.S.-Japan Cooperative Science Program. The work was completed while Dr. Endo and Dr. Mukai were visiting scientists at Kansas State University under the auspices of the Wheat Ge- netics Resource Centerand the U.S.-Japan Cooperative Science Program. Contribution No. 90-175-J from the Wheat Genetics Resource Center and Kansas Agricul- tural Experiment Station, Kansas State University, Manhattan. Address reprint requests to Dr. B. S. Gill, Department of Plant Pathology, Throckmorton Hall, Kansas State University, Manhattan, KS 66506-5502. Journal of Heredity 1990,81:290-295; 0022-1503/90/$2.00 The structure, organization, and evolution of the 5S rRNA (ribosomal ribose nucleic acid) multigene family have been studied in detail in the Triticeae.'T 7 's The 5S rRNA multigenes exist in two sizes: the long unit (ca. 500 bp), consisting of a 120-bp coding and 380-bp spacer region, and a short unit (ca. 400 bp), consisting of a coding region of the same length and a smaller spacer region (ca. 280 bp). 9 L8 In most cases, both short and long units are found in homo- geneous tandem arrays of repeating units in homoeologous groups 1 and 5, respec- tively. In wheat 5S rRNA loci have been assigned to chromosomes B, ID, 5A, 5B, and tentatively 5D. 5. ' 8 There is limited information on the physical location of the 5S rRNA loci. Ap- pels et al., 2 using in situ hybridization (ISH), mapped 5S rRNA loci distal to the sec- ondary constrictions in chromosomes 1B of wheat and IR of rye and in a nonnu- cleolar chromosome in barley. Because of technical limitations, the physical location of 5S loci on other chromosomes of wheat and rye was not determined. More re- cently, Reddy and Appels' 7 mapped an ad- ditional 5S rRNA locus in chromosome 5R of rye. Using a different approach, Kota and Dvorak" took advantage of a spontaneous deletion of chromosome 5BS and mapped a 5S rRNA locus close to the prominent interstitial band. From this data and those of Appels et al.,2 Dvorak et al. 5 speculated that 5S multigene families may be located in the intercalary heterochromatin bands on the short arms of chromosomes 5A, 5B, 1B (in the satellite), and D. In these studies, ISH or deletion stocks were used in combination with Southern blotting. Both methods are individually in- sufficient for the detailed physical local- ization of a gene on a chromosome, es- pecially with reference to band positions in a karyotype. When the two approaches are combined, more accurate location of a gene in a specific chromosome band should be possible. Rayburn and Gill' 6 reported a biotin-la- beling technique for the mapping of DNA sequences in plant chromosomes by means of ISH. This technique has several advan- tages, including rapid detection and ac- curate localization of hybridization sites on chromosomes. The biotin-labeling technique also permits the location of sin- gle-copy DNA sequences in plant chro- mosomes, suggesting that the technique may be as sensitive as the isotope-labeling technique for the detection of hybridiza- tion sites.' In addition to the sensitive ISH method, a number of deletion stocks have become available since the discovery by Endo 7 of a genetic system that allows the isolation of a virtually unlimited array of deletion stocks in common wheat. In this system, 290