Theor Appl Genet (1995) 90:1007-1011 9 Springer-Verlag 1995 L. Mickelson-Young 9 T. R. Endo 9 B. S. Gill A cytogenetic ladder-map of the wheat homoeologousgroup.4 chromosomes Received: 4 October 1994 / Accepted: 22 November 1994 Abstract We report the results of chromosome maps of wheat homoeologous chromosomes 4A, 4B, and 4D using 40 RFLP markers and 39 homozygous deletion lines. De- letion breakpoints divide the chromosomes into 45 sub- arm intervals with 32 intervals distinguished by molecular markers. The chromosome maps confirm the homoeology of arms 4AS to 4BL and 4DL, and 4AL to 4BS and 4DS. The chromosome map of 4A reveals novel information concerning the 4AL-5AL-7BS cyclical translocation. The presence of homoeologous group-4 long-arm markers, Xksu GIO and Xpsr 1051, intervening between the trans- located 5AL and 7BS chromosome segments in 4AL sug- gests that the translocation events are more complex than was earlier believed. Chromosome maps confirm a peri- centric inversion in Chinese Spring chromosome 4B. The consensus chromosome map is compared to the genetic map of wheat to construct a cytogenetic ladder-map (CLM). The CLM reveals an unequal distribution of re- combination along the length of the chromosome arms. Re- combination is highest in the distal half, and low in the proximal half, of the chromosome arms. Key words Genetic linkage-map 9 Chromosome map 9 Cereal 9 Deletion mapping Introduction Cytologically based physical maps, or chromosome maps, are idiograms of wheat chromosomes depicting the posi- tions of C-bands, deletion breakpoints, and the distribution Communicated by G. Wenzel L. Mickelson-Young. B. S. Gill ([]) Department of Plant Pathology, Throckmorton Hall, Kansas State University, Manhattan, KS 66506-5502, USA T. R. Endo Laboratory of Genetics, Faculty of Agriculture, Kyoto University, Kyoto 606, Japan of markers (Werner et al. 1992; Gill 1994). Chromosome maps are aligned to corresponding genetic maps by draw- ing lines between mutual loci, resulting in cytogenetic lad- der-maps (CLMs) (Gill and Gill 1994). CLMs have been constructed for wheat chromosome 1B (Kota et al. 1993), as well as for homoeologous groups 2 and 3 (Delaney et al. 1994a, b), 6 (Gill et al. 1993), and 7 (Werner et al. 1992; Hohmann et al. 1994). The CLMs reveal a non-random dis- tribution of RFLP loci and recombination along the length of the chromosome arms, with the distal region correlat- ing with the highest densities of both molecular markers and recombination. Saturation mapping of specific gene- rich regions will expedite map-based cloning of genes in wheat. In this report we present the CLM of the group-4 chromosomes of wheat. Deletion stocks and RFLP markers Thirty-nine homozygous deletion lines were used in the mapping of RFLP markers (Endo and Gill 1994). Break- points for each deletion are calculated from at least five C-banded chromosomes and are expressed as a fraction- length of the distance from the centromere. Because of their indirect calculation, fraction-lengths are not accurate and are regarded as only approximations of the dele- tion breakpoints. Forty low-copy probes were mapped and are listed in Table 1. The markers include seven barley cDNA probes (Xbcd, Cornell University, Ithaca, N.Y.), 12 oat cDNA probes (Xcdo, Cornell University, Ithaca, N.Y.), two Triticum tauschii (Coss) Schmal. genomic probes (Xksu, Kansas State University, Manhattan, Kan.), 13 wheat cDNA probes (Xpsr, Plant Science Research, Cam- bridge England), and six wheat genomic probes (Xwg, Cor- nell University, Ithaca, N.Y.). Of the 40 RFLP markers used, 17 are common among the A, B, and D genomes; three map only to the A and B genomes, one to the A and D genomes, seven to the B and D genomes, eight to the A genome, one to the B genome, and three to the D genome (Table 1, Fig. 1).