Reversion frequency of waxy pollen type in normal and hypoploid maize plants

Abstract

Maize pollen grains of constitution Yg I Sh Bz Wx were used to fertilize a stock possessing the corresponding recessive genetic markers yg C sh bz wx of the short arm of chromosome 9. Appropriate screening of the ensuing seedlings, together with the results of appropriate testcrossing and scoring of the pollen, made it possible to isolate hypoploid plants in which the chromosome 9 carrying the dominant markers had lost at least the Wx gene is known to affect the chemical nature of the starch, both in endosperm and in pollen grains. The pollen produced by such plants was only influenced by the wx gene of the undamaged chromosome. The reversion rate of this gene—on the basis of the appearance of pollen grains giving the Wx chemical reaction (with iodine solution)— was measured and compared with the reversion rate of normally diploid wx wx plants. On the basis of results with yeast, the expectation was that the normal chromosome condition should have produced a higher reversion rate than the genotype in which the hypoploid condition associated with the deficiency of the genetically marked region provided no opportunity for pairing and crossing-over, which was supposed to be responsible for that high reversion rate. In maize the experimental data failed to confirm a similar behaviour. This finding suggests that the wx alleles investigated are likely to be due to base substitution. In such a case unequal crossing-over, causing bases losses and insertions in the DNA and, therefore, restoration of a sequence compatible with normal functioning, appears unlikely. Mutagenesis in these cases appears to rely only on mutational events sensu stricto and associated with a suppressor system. In maize such a requirement seems to be satisfactorily met with the controlling elements.