Single Gene Heterosis Associated with a Second Chromosome Recessive Lethal in Drosophila Melanogaster.

OR the reasons that heterosis has immense value in practical utilization in F agriculture and the evolutionary significance of heterosis in natural populations is important, a number of investigators have worked on this problem and proposed numerous hypotheses for the explanation of this phenomenon. The hypotheses which have been proposed can be classified into two categories: (a) dominance theory, in which the phenomenon of heterosis is said to be attributable to the separate action of dominant genes from both parents and (b) overdominance theory, in which heterozygosity, per se, is thought to be superior to both types of the homozygosity. However, RENDEL (1953), HAGBERG (1953) and others have pointed out that the above two theories need not be mutually exclusive (HEXTER 1955). In recent years, considerable attention has been paid to single gene heterosis which gives a direct proof of the overdominance theory. It is evident that the plan of breeding in plants and animals completely changes according to whether we accept the dominance or the overdominance theory. On the assumption that there is only one wild type allele for the mutant gene in question, the proof of single gene heterosis must rest on the elimination of the contribution of (a) the genetic background and (b) specific interaction between the genetic background and the gene in question. Up to now, two ways have been used in testing for single gene heterosis. In one way, an inbred line and its mutant derivative have been used because there is little difference in their genetic background. In this way, only the first condition above can be satisfied. The other way is only used in testing for heterosis in viability of animals. The dynamic observation of frequency of heterozygotes in random mating populations is made generation after generation to equalize the genetic background for each genotype. Again, this method does not satisfy the above second condition. In order to do this, the degree of heterosis should be tested in different genetic backgrounds. Recent findings in studies of heterosis in plants (SCHULER 1954; JONES 1957) have cast suspicion on the first method. Their results showed that the genetic