Cybrids in genetics and breeding

In most higher plants, biparental transmission of organelle genomes can be achieved only through protoplast fusion. In such experimental systems the fates of these genomes in resulting regenerated plants are strikingly different. Chloroplast genomes segregate more or less rapidly during divisions of hybrid-derived cells, allowing the rapid and permanent exchange of chloroplasts between two cytoplasms and consequently between varieties or species. In contrast, mitochondrial genomes are generally involved in recombinations leading to new genomes. This phenomenon, in particular, is exploited to analyse cytoplasmic male sterility. In plant breeding, “genetic laws” concerning cybrids can be exploited in: (1) the transfer of chloroplastic traits such as herbicide resistance or better photosynthetic yield; and (2) the creation of new kinds of cytoplasmic male sterility or eventually the improvement of the cytoplasmically male sterile restoration system. Experiments with the genera Nicotiana and Brassica are described to illustrate these possibilities.