Somaclonal variation versus chemically induced mutagenesis in tomato (Lycopersicon esculentum L.).

A comparison was made of the type and frequency of mutational events found in the progeny of tomato plants regenerated after one passage in vitro with those induced by chemical mutagenesis with ethyl methane sulphonate. Several mutants were recovered in the progeny of regenerated and mutagenized plants of two cultivars of tomato. They can be grouped into the following categories: seedling lethality, male sterility, resistance to Verticillium, short stature, change in number of lateral shoots or in leaf shape. The results indicate that the two sources of variability differ in their effect, changing the spectrum and frequency of the mutants as well as, at least in some cases, their pattern of segregation.

Nuclear DNA amplification in cultured cells of Oryza sativa L.

Highly repeated nuclear DNA sequences from suspension cultured cells of Oryza sativa L. cv. 'Roncarolo' have been cloned in pBR322. Ten clones with specific digestion patterns have been randomly selected. Nine sequences appear to be organized in a clustered tandem array while one is interpersed in the rice genome. The clones have been used to gather information on: (a) their modulation in cultured cells as compared to whole plant and (b) their distribution in different rice cultivars belonging to the Japonica or Indica subspecies of Oryza sativa L. Hybridization with nuclear DNA isolated either from suspension or from seedlings of the 'Roncarolo' cultivar revealed extensive quantitative variations, with most cloned sequences showing amplification (up to 75-fold) in cultured cells. Hybridization with nuclear DNA isolated from seedlings or suspension cultured cells from different cultivars belonging to the Japonica or to the Indica sub-species of O. sativa have shown that (a) amplification also occurs in a similar pattern in the case of DNA from the other tested suspension cultured cell types but not in the case of DNA from seedlings; (b) in some cases the tested sequences show minor but significant variations in different rice accessions.

Selection and Nuclear DNA Analysis of Cell Hybrids between Daucus carota and Oryza sativa.

In order to study the fate of the parental genomes in somatic cell hybrids between distantly related species, protoplasts from cultured cells of Daucus carota and Oryza sativa were fused. Selective conditions resulted, exclusively, in the growth of hybrid colonies which combined the capacity to multiply of carrot cells with the natural resistance to A2CA of rice cells. A methodology for measuring the relative contribution of the parental cells to the hybrid nuclear genome has been worked out. This is based both on hybridization of nuclear DNA bound to nitrocellulose filters (dot hybridization) with radioactively labelled nuclear DNA from one of the parents and on agarose gel fractionation of nuclear DNA digested with restriction endonucleases. The dot hybridization analysis, performed on one of the D. carota x O. sativa cell hybrids, showed that the major portion of the nuclear genome is homologous with the carrot partner, with rice contributing only a minor fraction, along with the selected resistance gene(s). The homology was confirmed after agarose gel fractionation of restriction endonuclease BamHI-digested nuclear DNA. Furthermore, strong homology at the level of gene expression between hybrid and carrot cells was shown by polyacrilamide gel electrophoresis of total soluble proteins.