Cytological dissection of barley genome by the gametocidal system

Barley is one of the major cereals in the world. The analysis of genomes is important in modern breeding, but the barley genome is too huge and complicated that it is still difficult to arrange sequenced pieces of the genome in order. Each of the barley chromosomes or chromosome arms has been added to common wheat and such barley chromosome addition lines have been used in determining the chromosomal regions of genes and DNA markers. It is undoubtedly desirable to have the genome divided into smaller pieces in separate common wheat lines. There is a unique genetic system in common wheat that induces frequent chromosomal structural rearrangements. This system is called the gametocidal (Gc) system involving alien chromosomes called Gc chromosomes, which were introduced into common wheat from certain wild species belonging to the genus Aegilops. When the Gc chromosome exists in a common wheat plant in monosomic condition, the plant produces two types of gamete, one with the Gc chromosome and the other without the Gc chromosome;chromosomal rearrangements occur only in the latter one. Such Gc-induced chromosomal rearrangements are either lethal to gametes or semi-lethal, and in the latter case the gametes are fertilized to develop into viable zygotes carrying rearranged chromosomes. The Gc system proved to be effective in inducing structural rearrangements in barley chromosomes added to common wheat, as well as in common wheat chromosomes. Thus-induced rearranged chromosomes include deletions of barley chromosomes and translocations between the barley and wheat chromosomes. The present author termed common wheat lines carrying rearranged barley chromosomes 'dissection lines' of a barley chromosome. So far dissection lines for three barley chromosomes have been produced and used in the cytological mapping of the barley chromosomes. In this article the progress in the cytological dissection of the barley genome is described.