The Transfer of Cytoplasmic and Nuclear Genomes by Somatic Hybridisation

Abstract

For somatic hybridisation between two species to be successful, specific regenerability, compatibility and selection criteria must be met. The development of new methodologies has reduced the reliance on auxotrophic and albino mutants in selection strategies. Somatic hybridisation allows the transfer of chloroplast, mitochondrial or nuclear genomes in a single-step procedure and can extend the transfer boundaries defined by sexual hybridisation. Cytoplasmic genomes can be transferred over wider genetic distances than nuclear genomes. We consider the transfer of the three genomes but with particular emphasis on chloroplasts. Strategies for chloroplast transfer are reviewed. When no selection strategies for a particular chloroplast are utilised the regenerated plant will have chloroplasts from either one of the parents, but not a mixture of both. The rapid sorting out can be explained by the changes in plastid numbers that occur in the first few divisions. Biased segregation can frequently be related to an unequal input of plastids but, in some cases, plastid-mitochondrial-nuclear incompatibilities are presumably involved and more studies of the callus stage are required to identify these phenomena. Chloroplast DNA recombination is rare, consistent with known inheritance patterns and the relative conservation of the chloroplast genome. Stable heteroplasmy of chloroplasts as a result of somatic hybridisation has not been reported, although chloroplast DNA heteroplasmy occurs naturally in some species. Mitochondria can be transferred in a similar fashion to chloroplasts but, as their numbers are higher, their segregation is less likely to be complete in the regenerated plant. There are many reports of mitochondrial DNA recombination and cloned fragments containing sites of intergenomic recombination have been obtained. It appears that fusion between these organelles is common. Interspecific somatic hybridisation within a genus can produce nuclear hybrid plants that can be part of a breeding program to provide novel sources of germplasm. Aneuploids rather than amphiploids may be produced as a result of chromosome loss during the culture phase. Nuclear hybrids from wider crosses are usually infertile. However asymmetric somatic hybridisation offers the possibility of incorporating nuclear genetic material from more diverse sources. Somatic hybridisation has an important rc le in complementing conventional breeding in providing the gross genetic structure of chloroplast, mitochondrial and nuclear genomes. Specific gene transfer technologies can fine-tune the genomes thus provided.