Somatic fusion hybrids

Sometimes levels of resistance are not as high in somatic fusion hybrids as in the donor clones, presumably due to a dilution effect in the polyploid hybrids (Cooper-Bland, 1994 Rasmussen et al., 1998 Carputo et al, 2000b McGrath, 2002 Gavrilenko, 2003). On the other hand, somatic hybrids produced from fusions of cultivated potato with the wild species S. nigrum were often more resistant than the resistant wild parent, perhaps due to complementation of resistance genes (Zimnoch-Guzowska, 2003). 

Somatic cell hybrid The cell resulting from the fusion of animal cells derived from somatic cells that differ genetically. 

The use of protoplasts in studies of stress physiology and biochemistry expands the advantages of cell culture systems discussed in the preceding sections. Additional applications are related to the fusion of protoplasts. Intraspecifie and interspecific protoplast fusion greatly enhance genetic variability of the fused protoplasts (Kumar Cocking, 1987). The resulting somatic hybrids provide cells which can be used for selection of specific traits (e.g. environmental stress tolerance) provided by one or both donor cells and for basic studies on cytoplasmic and nuclear inheritance of desired characteristics. 

Combination of atrazine resistance and cytoplasmic male sterility in rapeseed. Transfer of atrazine resistant chloroplasts from bird s rape (B. oapestris) into oilseed rape has been accomplished by back-crossTng (s this volume). The purpose achieved by somatic cell fusion was to combine the atrazine-resistant chloroplasts carried by a B. napus line with cytoplasmic male sterility (CMS), a mitoohondrlal tralt (l9). The CMS trait was carried by a B. napus line with the cytoplasm of radish (Raphanus sativus). Since both traits, atrazine resistance and CMS, are maternally inherited in sexual crosses, the only way to combine the traits was by protoplast fusion. The desired atrazine-resistant cybrids were identified in a randomly regenerated sample of plants by the presence of the CMS flower morphology and their darker green color at low temperature. Somatic hybrids rather than cybrids were obtained in each clone since no irradiation had been used to facilitate the elimination of one of the parental nuclei. 

Chloroplast transfer into potato. Transfer of atrazine-resistant planum nigrum chloroplasts into S. tuberosum has been attempted by protoplast fusion (20,21 ). In the absence of irradiating the chloroplast donor S. nigrum, only somatic hybrids were obtained... 

Figure 2.5 Trichomes of mint plants (a) and resulting phenotype of a somatic hybridization (b, hybrid) between mint varieties (b, E33, E87 leaves of parent lines used for protoplast fusion, unpublished results E. Baumann, Phytowelt GreenTechnologies GmbH).

In the production of somatic hybrids by chloroplast fusion, both nucleus and cytoplasm are combined. Moreover, the occurrence of somaclonal variation during culture and assortment of the cytoplasmic organelles can result in a wide range of hybrid types regarding their nuclear and cytoplasmic constitutions. 

TABLE 1. Means and standard deviations for net photosynthesis, Rubisco activity and specific leaf weight of the fusion parents, isogenic parental lines and the somatic hybrids. 

Established methodologies for genetic analysis by means of conventional sexual crossing [1,6, 9] and somatic hybridization following protoplast fusion induced either chemically [6, 17, 18, 19] or electrically [6, 28]. 

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