Somatic Incompatibility in the Development of Higher Plant Somatic Hybrids

The fusion of somatic cells offers unique prospects for combining plant species that cannot be crossed sexually. This review investigates the involvement and consequences for intergeneric somatic hybridization of various types of somatic incompatibility as a phenomenon analogous to incompatibility and incongruity in sexual crosses. From an extensive survey of the literature on plant as well as vertebrate somatic cell fusion, various intracellular processes have been identified and are discussed as operational sites for somatic incompatibility reactions that interfere with the regular development of somatic hybrid cells and plants. The structural and dynamic instability of chromosomes can explain why many intergeneric fusion products have failed to proliferate continuously. The great complexity of the processes involved in the morphological differentiation of a hybrid plant makes heavy demands on the timing and sychronization of regulatory signals, and thus provides sites of action for somatic incompatibility mechanisms. The enforced coesixtence, in a hybrid cell, of the genomes and the cytoplasmic genophores from the two parents suggests that somatic incompatibility also results from genome-plasmon interactions. Functional plant regeneration from hybrid cells unimpaired by somatic incompatibility is required for agricultural uses of somatic hybridization. In contrast, somatic-cell genetical studies on the complementation, expression, and regulation of cellular traits are possible with non-morphogenic lines, and investigations of the chromosomal assignment, linkage, and segregation of relevant genes actually depend upon somatic incompatibility operating in somatic hybrid cells. Various experimental constraints of previous studies of discussed, and areas have been suggested where future investigations are needed for a further analysis and a better understanding of the mechanisms underlying somatic incompatibility.