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To map important ESTs to specific chromosomes and (or) chromosomal regions is difficult in hexaploid wheat because of its large genome size and serious interference of homoeologous sequences. Large-scale EST sequencing and subsequent chromosome localization are both laborious and time-consuming. The wheat alien addition line TAi-27 contains a pair of chromosomes of Thinopyrum intermedium (Host) Barkworth & D.R. Dewey that carry the resistance gene against barley yellow dwarf virus. In this research, we developed a modified technique based on chromosome microdissection and hybridization-specific amplification to isolate expressed sequences from the alien chromosome of TAi-27 by hybridization between the DNA of the microdissected alien chromosome and cDNA of Th. intermedium infected by barley yellow dwarf virus. Twelve clones were selected, sequenced, and analyzed. Three of them were unknown genes without any hit in the GenBank database and the other nine were highly homologous with ESTs of wheat, barley, and (or) other plants in Gramineae induced by abiotic or biotic stress. The method used in this research to isolate expressed sequences from a specific chromosome has the following advantages: (i) the obtained expressed sequences are larger in size and have 3' end information and (ii) the operation is less complicated. It would be an efficient improved method for genomics and functional genomics research of polyploid plants, especially for EST development and mapping. The obtained expressed sequence data are also informative in understanding the resistance genes on the alien chromosome of TAi-27. Key words: TAi-27, Thinopyrum intermedium, chromosome microdissection, hybridization-specific amplification (HSA), expressed sequences, barley yellow dwarf virus (BYDV). Il est difficile de situer des EST importants sur des chromosomes ou regions chromosomiques specifiques chez le ble hexaploide en raison de la grande taille de son genome et de serieuses interferences causees par des sequences homeologues. Le sequencage d'EST a grande echelle et leur localisation chromosomique sont laborieux et prennent beaucoup de temps. La lignee d'addition chromosomique exotique TAi-27 contient une paire de chromosomes du Thinopyrum intermedium (Host) Barkworth & D.R. Dewey qui porte le gene de resistance contre le virus de la jaunisse nanisante de l'orge. Dans ce travail, les auteurs ont developpe une technique modifiee basee sur la microdissection chromosomique et l'amplification specifique des sequences hybridees (<< hybridization specific amplification >>) pour isoler des sequences exprimees a partir du chromosome exotique dans TAi-27. Un tel isolement est realise par hybridation entre l'ADN du chromosome exotique, obtenu par microdissection, et l'ADNc du T. intermedium infecte par le virus de la jaunisse nanisante de l'orge. Douze clones ont ete selectionnes, sequences et analyses. Trois de ceux-ci correspondaient a des genes inconnus (sans homologue dans GenBank) et les neuf autres presentaient une forte homologie avec des EST induits par des stress abiotiques ou biotiques chez le ble, l'orge ou d'autres especes de graminees. La methode employee dans ce travail pour isoler des sequences exprimees a partir d'un chromosome specifique presente les avantages suivants : (i) les sequences exprimees obtenues sont de grande taille et fournissent de l'information sur l'extremite 3' et (ii) l'operation est moins compliquee. Il s'agirait d'une methode amelioree efficace pour des travaux en genomique ou en genomique fonctionnelle chez les plantes polyploides, particulierement pour le developpement d'EST et la cartographie. Les donnees obtenues sur les sequences ex primees sont egalement utiles pour la comprehension des genes de resistance situes sur le chromosome exotique present chez TAi-27. Mots-cles : TAi-27, Thinopyrum intermedium, microdissection chromosomique, amplification specifique de sequences hybridees (HSA), sequences exprimees, virus de la jaunisse nanisante de l'orge (BYDV)., Introduction Expressed sequence tag (EST) analysis is one of the ways to clone genes controlling desirable traits in plant (Rounsley and Linx 1998; Sasaki 1998). At present, most ESTs registered [...]

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