Somatic Hybridization Between Pyrus × Prunus Species

Protoplast fusion and somatic hybridization provide the opportunity for bypassing reproductive isolation barriers and, therefore, facilitating gene flow. For top-fruit trees, self-incompatibility and/or interspecific incompatibility is commonplace, thereby restricting the exploitation of novel germplasms by conventional crossing. Therefore, the greatest impact that protoplast technology will have on the breeding of fruit tree species is clearly in the field of somatic hybridization. There are many targets for somatic hybridization in top-fruit trees, including both rootstock/scion hybridization and the production of novel hybrid rootstocks. The generation of new rootstocks is very relevant, since lack (or a reduction) of fertility, as observed in many somatic hybrids, in the asexually propagated rootstocks would be of little consequence. Moreover, it has recently been shown for various Citrus species combinations that the somatic hybrids obtained were indeed fertile (Grosser 1993). Protoplast fusion technology is also used for the transfer of limited genetic material from an alien species into the chosen crop species (Gunn and Day 1986), leading to novel recombinations and reassortments of cytoplasmic and mitochondrial genomes. In fruit trees, little is known at this level. Therefore, the production of somatic hybrids could contribute towards the understanding of basic phenomena such as, for example, the rootstock effect (Lockard and Schneider 1981), by providing new plant materials that could serve as a basis for a more detailed study of the relationships, at the cell level, between scions and rootstocks. Also, with current advances in molelcular biology and the synthesis of new marker genes coupled with isolation of some agronomically useful ones (Jamea and Dandekar 1991), these are now available for future cell selection strategies, following fusion of protoplasts that carry those genetic markers with control (nontransformed) protoplasts.