| Neatby's virescent E. R. SEARS U. S. Department of Agriculture, Columbia, Missouri, U.S.A. The virescent gene v discovered by Neatby has been found to lie on chromosome III and when present in two doses to cause a retardation in the development of chlorophyll. In one dose, whether heterozygous or hemizygous, it has no obvious effect if the light intensity is normal; but when the light is poor, Vv and -v show a reduced amount of chlorophyll. In three doses the gene caused albinism. As Neatby noted, vv plants often have sectors of green or white tissue, and vv also gives rise to green or white offspring. Obviously these mutations might be due to a simple change in dosage of v to one or three as a result of non-disjunction of chromosome III. If, as seems likely, non-disjunction proves to be the sole or even major cause of mutation in vv, this gene could be useful as an indicator of the mitotic and meiotic stability of various varieties. Special interest attaches to v because of the fact that, unlike defective-chlorophyll genes in diploid plants, it is not simply a deficiency. It is a gene which actively interferes with chlorophyll production. Its normal allele V acts like a deficiency in dosage series, no effect being discernible of increasing its dosage from 0 to 4; yet V cannot be a deficiency, because it gave rise to v. It is possible that V is a gene concerned with chlorophyll production but non-essential because it is duplicated at other loci. Thus it would be free to mutate in ways that would not be possible in a diploid, where it would be lethal unless it continued to perform its chlorophyll-making function. In order to determine whether V may be concerned with chlorophyll production, an attempt is being made to eliminate both it and one of its duplicate genes, which presumably lie on the homoeologous chromosomes XII and XVI. Deficiencies for v (which are dominant in vv material) have been obtained by X-ray treatment, and will be combined with nulli-XII and nulli-XVI. If these two chromosomes, and only these, carry duplicates of V, the dosage of V will be reduced from 6 to 2, and chlorophyll production may be affected adversely. |