Partial genome transfer through micronuclei in plants

The introduction of gene manipulation techniques has greatly extended the possibilities for plant breeding. New techniques are still emerging, and many are being used, for the introduction of foreign genes, chromosomes and organelles in order to improve the existing cultivars or to develop new varieties, and to localize genes on the chromosomes. The techniques presently available range from the transfer of single, cloned genes via DNA transformation (reviews inNegrutiueta/. 1987; Gasser & Fraley 1989; Potrykus 1990), to the addition of a complete genome of a donor species by somatic hybridization (Negrutiu et al. 1989). Until now, only identified and cloned genes can be transferred through DNA transformation. Traits which are polygenically determined, or with unknown biochemical and molecular background, e.g. many disease resistances, yield, etc. are not yet amenable to this technique. In this regard, somatic hybridization may be a suitable approach, but it involves the fusion of whole protoplasts from two different parental species or genotypes, and thus adds two complete nuclear genomes and all cytoplasmic genomes from the chloroplasts and the mitochondria. This results in the production of highly complex somatic hybrids, with many unwanted additional genes. Moreover, when the genomes of the two parents are incompatible at the somatic level, random loss of chromosomes and organelle segregation or recombination will occur, leading to the formation of chimeric tissues. Several techniques have been used in recent years to eliminate or inactivate the unwanted chromosomes by treatment of the donor protoplasts prior to fusion. Commonly gamma or X-irradiation is used to obtain asymmetric hybrids. Although a large number of experiments have been carried out to transfer a limited number of chromosomes or chromosome fragments for integration into the recipient genome, so far only little success has been obtained through this technique. One of the major problems confronted by using irradiation is the stability of the introduced chromosomes or chromosome fragments, both at the cellular and the plant levels (Famelaer et al. 1989; Wijbrandi et al. 1990). The occurrence of chromosome breaks, deletions and rearrangements after irradiation makes this approach less suitable for transfer of large pieces of DNA with syntenic genes or intact chromosomes. The general application of gamma fusion in plant breeding is also hampered by the low frequency of plant regeneration as w