Asymmetric Somatic Hybrids in Tomato.

Fusing somatic cell protoplasts from different species produces novel genotypes which often cannot be produced via sexual hybridization. After fusion, chromosome elimination is often observed and frequently regeneration of plants from fusions is dependent on partial elimination of chromosomes from one species. Fragmentation of DNA by ionizing radiation may result in chromosome elimination as well as deletions and exchanges. A practical application of directed chromosome elimination has been the use of radiation to generate asymmetric somatic hybrids and cybrids. Most asymmetric somatic hybrids are the result of chromosome elimination not exchanges. However, possible chromosome exchanges have been observed in Atropa belladonna and Nicotiana plumbaginifolia fusion products.

We are attempting to create asymmetric somatic hybrids in the genus Lycopersicon using gamma-radiation to induce chromosome elimination, fragmentation and exchange. We have used a wide range of radiation doses in an attempt to correlate dose with chromosome elimination and/or exchange. The well developed linkage map of tomato and availability of probes for RFLP analysis makes this study feasible.

Asymmetric somatic hybrids have been identified in plants regeneranted following fusion of L. pennellii protoplasts exposed to 15, 25 and 100 kRad with unirradiated protoplasts isolated from L. esculentum cultivar UC82. Calli from fusions involving 150 kRad treated L. pennellii protoplasts have also been shown to be asymmetric. No asymmetric somatic hybrids have been recovered following fusion of L. pennellii protoplasts exposed to doses of 5 and 10 kRad, only full somatic hybrids. With increasing radiation dose, regenerants displaying hybrid isozyme patterns often show stoichiometry shifts with the L. pennellii bands being reduced in intensity. Morphological analysis of regenerated plants also indicates a reduction in L. pennellii associated characteristics with increasing radiation dose administered to L. pennellii protoplasts (Table 1). Currently we are analyzing regenerants using RFLP's to determine the extent and nature of the asymmetry.

Table 1. Mean stigma exsertion and leaf ratio values for all regenerants from individual radiation doses. Lp, F1 and Le are L. pennellii, the sexual F1 between L. pennellii and L. esculentum and L. esculentum, respectively.

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Untreated      Radiation Dose  Stigma Exsertion   Leaf Ratio 
Lines             (kRad)             (mm)            (L/W)
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Lp + Le*              5               1.06           0.99
                     10               1.41           0.98
                     15               0.10           1.14
                     25               0.00           1.19
                    100               0.30           1.67
Lp                    -               2.50           0.99
F1                    -               1.50           1.35
Le                    -               0.00           2.10
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