Somatic versus sexual hybridization: features, facts and future

The main goal of plant breeding is the construction of new genotypes. This can be achieved by the introduction (sexual and somatic hybridization and genetic transformation) and manipulation (recombination and selection) of genetic variation. The introduction of foreign genes through the generative cycle can be successfully achieved, for example in cereals, by means of chromosome addition and radiationinduced chromosome engineering. Reference is made to reviews of Sybenga (1983) and Brar & Khush (1986). The principle of hybridization has been extended to the somatic cycle by protoplast fusion with hybrids and cybrids being produced at different levels of ploidy and taxonomic distance (Harms 1983a,b; Gleba & Sytnik 1984; Glimelius 1986; Hinnisdaels el al. 1988). The objective of this paper is to present, in parallel, the respective achievements, merits, and limitations of sexual and somatic hybridization. Exploitation of the generative cycle has resulted in important contributions to plant breeding, but too often such discoveries had empirical bases. It was considered that plant tissue culture in general, and somatic hybridization in particular, have the intrinsic potential to fill many of the gaps in plant biology. More specifically, it was expected that somatic hybridization would allow one to bypass most of the incompatibility barriers built by nature to preserve genomic identity. However, this is not yet the case, though the evolution in techniques on sexual and somatic hybridization is revealing interesting convergence and complementarity. The discussion on the practical implications and applications of the hybridization-recombination-segregation process will be limited (they are extensively covered by most cited works); instead, emphasis will be put on analytical aspects, so important at a time when plant biotechnology is looking for more rational, less empirical principles that govern plant metabolism and development.