Breeding strategies for developing transgenic white clover cultivars

Abstract

The introduction of foreign genes into elite white clover (Trifolium repens L.) genotypes has resulted in the first field test of genetically modified white clover. We now require breeding strategies which efficiently deploy transgenes into commercial cultivars. Current white clover cultivars are synthetics, produced by randomly intermating selected parents or seed lines for up to 6 generations. Unfortunately, the uncontrolled nature of transgene insertion means each transformant has a different site of insertion and frequently has a variable number of inserts. This complicates breeding strategies because populations produced by intercrossing these transformants will contain multiple insertion sites, variable dosage at each insertion site, and be highly heterogeneous for expression level. A modified synthetic breeding method is proposed for crosspollinated crops such as white clover which overcomes these difficulties by identifying F2 progeny homozygous for a defined number of transgenes. These homozygotes are used as the parents to develop a synthetic with a specific genetic composition and high expression levels of the transgene in all progeny. The advantages of backcrossing into existing cultivars or directly transforming inbreds are also discussed. The initiation of new crossing programmes in successive years can be utilised to offset the initial lagphase until strategies which reduce the number of generations required to obtain commercial quantities are developed. Keywords: backcrossing, breeding methods, cross-pollinated, hybrids, inbreeding, plant breeding, transgenic plants, Trifolium repens L.