The procedure used to select point mutations in inbred lines involves the treatment of plants before meiosis with a mutagen and using the pollen as a screen to screen out undesirable mutations. Most of the mutations due to chromosome breakage events will be screened out due to pollen abortion. A few very small deficiencies may pass through the pollen, however many of these will produce smaller size pollen. Cytoplasmic mutations will be screened out since little or no cytoplasm passes through the pollen. This procedure permits the use of high dosages of the mutagen. Point mutations should occur more frequently at high dosages (however, unless the undesirable mutations are screened out the load of chromosome breakage events may be so high that it may be impossible to select the point mutations, in which case lower dosages of the mutagen should be used.) The procedure used most frequently involves treating the seed of an inbred line with a dosage of a mutagen which results in about 10 to 20 percent of the plants producing a tassel. Pollen is collected from these plants and crossed to the normal inbred line plants. Plant progeny from such crosses appear similar to the normal inbred line plants except for an occasional plant. Usually, we grow 50 plants from an individual cross involving a mutagen and self 10 to 15 plants. Inbred line Oh43 is being used in our studies. One type of mutation being sought are those which increase the vigor of the plant. One plant considerably larger than the others was selfed and found to be heterozygous for a recessive allele (a deficient type endosperm in this case). Plants homozygous for the mutant allele appeared to be the same size as the normal inbred line plants; the heterozygotes are larger. Several other plants (not all) classified as slightly taller were also found to be heterozygous for various visible recessive alleles. A series of various visible recessive alleles have been produced and a study is in progress to determine the vigor characteristics associated with these alleles in the relatively homozygous background of the inbred line. The data are not complete nor an analysis made at this time, however observation suggests that the most frequent characteristic associated with heterozygotes involving visible recessive alleles is early silking and early pollen shed.
E. J. Dollinger
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