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.

Wild relatives of common wheat, Triticum aestivum, and related species are an important source of disease and pest resistance and several useful traits have been transferred from these species to wheat. C-banding and in situ hybridization analyses are powerful cytological techniques allowing the detection of alien chromatin in wheat. Cbanding permits identification of the wheat and alien chromosomes involved in wheat-alien translocations, whereas genomic in situ hybridization analysis allows determination of their size and breakpoint positions. The present review summarizes the available data on wheat-alien transfers conferring resistance to diseases and pests. Ten of the 57 spontaneous and induced wheat-alien translocations were identified as whole arm translocations with the breakpoints within the centromeric regions. The majority of transfers (45) were identified as terminal translocations with distal alien segments translocated to wheat chromosome arms. Only two intercalary wheat-alien transloctions were identified, one induced by radiation treatment with a small segment of rye chromosome 6RL (H25) inserted into the long arm of wheat chromosome 4A, and the other probably induced by homoeologous recombination with a segment derived from the long arm of a group 7 Agropyron elongatum chromosome with Lrl9 inserted into the long arm of 7D. The presented information should be useful for further directed chromosome engineering aimed at producing superior germplasm.

A standard karyotype based on N-banding, C-banding, and modified C-banding has been constructed for Triticum aestivum L. 'Chinese Spring'. An idiogram and a nomenclature system have been developed for the description of individual bands. Nomenclatural rules have been proposed for the description of chromosomal structural aberrations and polymorphic bands in other wheat cultivars. As a rule each short arm (S) and a long arm (L) consists of a series of dark bands (C-bands) and light bands (mainly euchromatic) and by definition there are no interbands. In some cases, each arm has been subdivided into two or more regions. The description of a band requires designation of a chromosome number, arm (S or L), region, and band. The region number is separated from the band number by a decimal point. Except for arms 1AS, 3AL, 4AS, and 6AS, all wheat chromosome arms have one or more intercalary C-bands and are divisible into three or more bands. It is hoped that the proposed karyotype and nomenclature system will be widely adopted and lay the foundation of definitive chromosome analysis in wheat.Key words: C-banding, N-banding, common wheat, heterochromatin, idiogram.

The recent advances in alien gene transfer from distantly-related species into wheat are reviewed in the present paper . The main achievements during the last ten years include the great expansion of the range of wide hybridization and development of new techniques for production and characterization of wheat-alien chromosome translocations . Updated results of wide hybridization since 1983 and comprehensive characterization of wheat-alien translocation lines in our laboratory are compiled . The future outlook for alien gene transfer in wheat is also discussed .

SignificanceGlyphosate is a nonselective herbicide used around the globe for weed control in glyphosate-resistant (GR) and noncrop situations. The extensive and exclusive use of glyphosate has led to the evolution of herbicide resistance in many crop weeds. The molecular target of glyphosate, the 5-enolpyruvlyshikimate-3-phosphate synthase (EPSPS) gene, confers resistance upon amplification and was first documented in GR Amaranthus palmeri. We now report that amplified EPSPS copies in GR A. palmeri are present in the form of extrachromosomal circular DNA molecules (eccDNAs) with various conformations. We discovered that eccDNAs are transmitted to the next generation by tethering to mitotic and meiotic chromosomes. These results represent a report of extrachromosomal structures that drive rapid adaptive evolution in higher organisms.