NOVEL TECHNIQUES OF GENE TRANSFER AND PLANT IMPROVEMENT: AN APPRAISAL OF TRANSFORMATION IN EUKARYOTES

Abstract

SUMMARYCertain recent theoretical and technological advances make possible the development of new techniques for combining genes from diverse sources without normal sexual hybridization. These include: (1) DNA‐ and phage‐induced transformation; (2) protoplast fusion and organelle transfer; (3) gametic transformation; and (4) graft hybridization. The essential feature of these novel techniques is ‘transformation’, broadly defined here as a phenomenon in which a part of the genetic information of one organism is combined with the entire genome of another. The present article reviews these topics briefly with reference to their potential value in plant improvement, and their contribution to a better understanding of induced transformation in flowering plants.A critical consideration of differences between prokaryotic and eukaryotic DNA leads to a better appreciation of possible natural constraints operating against gene transfer across this major evolutionary line. Since transformation is essentially a natural mechanism in prokaryotes, and occurs in place of true sexuality and recombination, the occurrence of artificial transformation in eukaryotes may be considered to indicate an inherent capacity for recombination by transformation which has been superceded by the evolutionary development of the sexual mechanisms, meiosis and syngamy, but which may still be expressed if appropriate conditions can be generated. Future major developments may lie in discovering factors and techniques which would make eukaryotic organisms ‘competent’ as donors and recipients in transformation. It is suggested that the egg, zygote, and early embryonic cells may be better targets for transformation than differentiated somatic cells. The rapid successive divisions following fertilization of a eukaryotic egg cell presumably place heavy demands on DNA metabolism, and this may allow exogenous fragments to compete more favourably for integration into new DNA strands.It is pointed out that in nature, as far as is known, none of the beneficial, healthy symbiotic or associative systems between eukaryotic and prokaryotic organisms involve gene transfers. The only case where gene transfer is known to occur is that of crown‐gall tumour disease–an unhealthy development.It is concluded that although simple genes comprising short nucleotide sequences, or certain major genes for which genetic homology may exist between the two partners, may be transferred and expressed across the prokaryotic‐eukaryotic boundary, major advances may be achieved by altering the structure or expression of genes within their respective boundary limits. It is suggested that the most promising form of genetic engineering involving prokaryotic and eukaryotic genomes may be the fostering of a new, or improved, symbiosis or association of two genetically engineered partners, without actual transfers of genes between them.