SELECTING THE OPTIMUM GENETIC BACKGROUND FOR TRANSGENIC VARIETIES, WITH EXAMPLES FROM BRASSICA

The performance of transgenic varieties depends not only upon the stab le and correctly-regulated expression of specific transgenes but also upon the agronomic potential of the background genotype. Ideally, tran sgenes should be introduced into agronomically-superior cultivars and transgenic varieties will become out-classed if their agronomic proper ties are not continually improved. It will often prove desirable to us e conventional breeding techniques, as opposed to new cycles of transf ormation, to carry out this process of varietal improvement. Continuin g advances in marker-assisted selection have made possible the selecti on and manipulation of an entire genetic background. This means that t ransgenes can be transferred to new and often 'untransformable' variet ies with relative ease. To carry out this process efficiently requires the correct choice of male and female parents, the use of appropriate marker-systems and the concentration of selection on the most appropr iate generations. Efficient, phenotypically-neutral marker-systems hav e revolutionised the identification and manipulation of quantitative t rait loci (QTLs). The loci which modify the expression of transgenes a re a form of QTL. Desirable alleles at modifier QTLs can be transferre d to new varieties along with the transgenes themselves, using marker- assisted breeding. The strategies for marker-assisted selection are be coming ever more sophisticated. A range of complementary marker system s allows the selection of desirable genotypes. In addition, the meioti c reassortment and recombination of chromosomes which produces new gen otypes is becoming better understood. The most efficient plant breedin g methods and the most powerful genetics will make optimal use of both markers and meiosis.