Phenotypic Stability in Rate of Maturation of Heterozygotes for Induced Chlorophyll Mutations in Tomato.

ONSIDERABLE attention has been directed in recent years to the study of C single-gene heterosis. Several workers have presented evidence to indicate that, occasionally, there is a cumulative action of divergent alleles such that the heterozygote is superior to either homozygote. In such cases, then, it appears that heterozygosity per se contributes to heterosis. GARBER and ROWLEY (1927) found that maize plants heterozygous for a recessive, defective endosperm mutation yielded significantly less than did their homozygous normal siblings. WENTZ and STEWART (1927) reported that soybeans heterozygous for a mutation which was semilethal in the homozygous condition, were not significantly different from their homozygous normal siblings with respect to yield. MANGLESDORF (1928) found that a defective endosperm mutation had a slight, but consistently deleterious effect in the heterozygous condition. The traits affected were earliness of blooming, length and weight of ears, and plant height. KARPER (1930) found that sorghum plants heterozygous for an albino seedling mutation were slightly, but not significantly taller than their homozygous normal sibs. The heterozygotes were also heavier than the homozygotes, but not significantly so. The two genotypes were equal with respect to earliness. ROBERTSON and AUSTIN (1935) spdied three recessive chlorophyll mutations in barley. Single gene heterozygotes were in no way inferior to their respective homozygous normal plants. Two of the genes were linked, and it was found that the double heterozygotes for these loci were significantly superior to the homozygous normal plants with respect to three different measurements of yield. JONES (1945) investigated five defective mutations in maize. The various heterozygotes were found to yield from 3 to 104 percent better than their respective homozygous siblings and were from 0 to 9 percent taller than homozygotes, depending on the mutation involved. Of special interest are the investigations of GUSTAFSSON and his co-workers on barley (1946, 1947, 1950a, 1950b, 1951). GUSTAFSSON (1951), working with the induced mutations, viridis and erectoides-29, demonstrated that, although viridis is lethal when homozygous, in the heterozygous condition in the dominant erectoides-