An ordered draft sequence of the 17-gigabase hexaploid bread wheat (Triticum aestivum) genome has been produced by sequencing isolated chromosome arms. We have annotated 124,201 gene loci distributed nearly evenly across the homeologous chromosomes and subgenomes. Comparative gene analysis of wheat subgenomes and extant diploid and tetraploid wheat relatives showed that high sequence similarity and structural conservation are retained, with limited gene loss, after polyploidization. However, across the genomes there was evidence of dynamic gene gain, loss, and duplication since the divergence of the wheat lineages. A high degree of transcriptional autonomy and no global dominance was found for the subgenomes. These insights into the genome biology of a polyploid crop provide a springboard for faster gene isolation, rapid genetic marker development, and precise breeding to meet the needs of increasing food demand worldwide.

ski, 1990; Graybosch, 1998). This translocation was developed to introduce into wheat resistance to greenbug Centric translocations of the short arm of rye (Secale cereale L.) (Schizaphis graminum Rond.) from rye (Sebesta and chromosome 1R are useful in wheat (Triticum aestivum L.) breeding because they confer resistance to several pests and diseases and im- Wood, 1978). It also appears to improve yield (Villareal prove yield. Their major disadvantage is in reduced bread making et al., 1996) and adversely affects bread making quality quality. To remedy this defect, rye chromosome arm 1RS in transloca- (Graybosch et al., 1993). tions 1RS.1BL and 1RS.1DL was induced by the ph1b mutation to Over the years, several attempts have been made to recombine with the short arms of wheat group-1 chromosomes. remedy the quality defect of the 1RS translocations in Among 20 234 progeny screened, 139 primary recombinant chromowheat by induced homoeologous recombination (Koebsomes were recovered including 103 with 1BS, 22 with 1AS and 14 ner and Shepherd, 1986; Koebner et al., 1986), by irradiwith 1DS. The Gli-1/Glu-3 loci of wheat were non-homoeoallelic to ation (Millet and Feldman, 1993), or by centric misdivithe Sec-1 locus of rye and were separated by about a 13-cM-long sion-fusion (Lukaszewski, 1993, 1997). None of these segment, which on the rye chromosome contained disease resistance attempts seem to have addressed the root of the probloci Pm8, Lr26, Sr31, and Yr9. Pairs of primary recombinants 1RS-1BS with breakpoints flanking the storage protein loci were inter-lem. The rye 1RS arm translocated into wheat replaces crossed and two types of secondary recombinant chromosomes 1RS one of the three short arms of the group-1 chromosomes were produced: a group of over 30 chromosomes where the Sec-1 of wheat. These arms carry several loci encoding the locus was replaced by segments of 1BS of various lengths, and two gluten fraction of the storage proteins (Payne, 1987); chromosomes where 1.4-and 3.2-cM segments of 1BS introduced the the 1RS arm carries locus Sec-1, encoding rye storage Gli-1/Glu-3 loci. Selected chromosomes from each class were allowed proteins (secalins) which do not belong to the gluten to recombine within the shared segments of 1RS separating the interfraction (Shewry et al., 1985). Regardless of the possible calary wheat segments and two tertiary recombinant chromosomes negative effects of secalins on bread making quality in were recovered. Cytologically, these chromosomes appear as normal wheat, the negative effect of the translocation may be 1RS arms but each has two intercalary segments of 1BS: one introducprimarily in the reduction of the number of the glutening the Gli-1/Glu-3 loci and the second one removing the Sec-1 locus. Because the protein composition of the resulting translocation lines encoding loci and the resulting lower amount of gluten is identical to that of normal wheat, it is believed that these manipu-in the translocation-carrying wheats. Depending on the lations could eliminate the qua...

Several studies have indicated a noncorrespondence between genetic and physical distances in wheat chromosomes. To study the physical distribution of recombination, polymorphism for C-banding patterns was used to monitor recombination in 67 segments in 11 B-genome chromosome arms of Triticum turgidum. Recombination was absent in proximal regions of all chromosome arms; its frequency increased exponentially with distance from the centromere. A significant difference was observed between the distribution of recombination in physically short and physically long arms. In physically short arms, recombination was almost exclusively concentrated in distal segments and only those regions were represented in their genetic maps. In physically long arms, while a majority of the genetic distance was again based upon recombination in distal chromosome segments, some interstitial recombination was observed. Consequently, these regions also contributed to the genetic maps. Such a pattern of recombination, skewed toward terminal segments of chromosomes, is probably a result of telomeric pairing initiation and strong positive chiasma interference. Interference averaged 0.81 in 35 pairs of adjacent segments and 0.57 across the entire recombining portions of chromosome arms. The total genetic map lengths of the arms corresponded closely to those expected on the basis of their metaphase-I chiasma frequencies. As a consequence of this uneven distribution of recombination there can be a 153-fold difference (or more) in the number of DNA base pairs per unit (centiMorgan) of genetic length.

Several generations of four triticale × wheat populations were cytologically analyzed on a plant-by-plant basis using C-banding. Among 785 karyotyped plants, 195 wheat/rye and 64 rye/rye translocated chromosomes were found, as well as 15 rye chromosomes that were modified by deletion or amplification of telomeric heterochromatin. Most of the translocations involved complete chromosome arms; only a few involved smaller segments of chromosomes. Out of 39 identified wheat/rye translocations, 10 occurred between homoeologous and 29 between non-homoelogous chromosomes, five involved A-genome chromosomes, six B-genome chromosomes and the remaining 28 involved D-genome chromosomes. The study indicated that wheat/rye translocations can be produced in sufficient numbers to allow the use of this method for the introduction of alien variation into wheat research programs. Changes in the C-banding technique used are discussed in detail.