Genetic nature of a nontumour mutant isolated from tumour-prone amphidiploid Nicotiana glauca-langsdorffii (GGLL): A critical assessment

The genetic nature of a nontumour mutant, isolated by Izard (1957) following irradiation of germinating seeds of the tumour-prone amphidiploid Nicotiana glauca-langsdorffii (GGLL-T), is re-examined in the light of previous and current studies on tumour formation in Nicotiana hybrids. As there are lingering questions regarding the dominant nature of the nontumour trait, I shall present experimental evidence and theoretical arguments that suggest that the nontumour condition does not necessarily involve a single dominant gene mutation, but rather mutations affecting tumour expression. In the framework of a genetic model of tumour formation in Nicotiana (Ahuja, 1968), the mutations are probably in those ee loci of N. glauca affecting tumour expression, and not in the I element of N. langsdorffii, as previously suggested by Smith (1972) and Sekine er al. (1993). As tumour-prone tissues are highly sensitive to environmental influences, including irradiation, it would be very difficult to obtain a mutation from tumour to a completely nontumour state, as in all likelihood, the irradiation treatments would accelerate tumour formation, or induce variation in the tumour expression. Instead, I propose a different approach for isolating a nontumour mutant, which involves irradiation of haploid cells (pollen) from N. langsdorffii (I species) and then pollinating N. glauca (ee species) with the irradiated pollen. As the cross N. glauca x N. langsdorffii normally yields a 100 per cent tumorous progeny, any deviation from this norm, i.e. a nontumour mutant, would be easily detected in the proposed cross. The reciprocal cross involving irradiated pollen from the ee parent N. glauca may also be attempted to check the validity of the hypothesis.