Selection in Experimental Populations of Drosophila Melanogaster with Different Genetic Backgrounds.

HE heterozygous carriers of some genetic variants are superior in fitness to Tthe corresponding homozygotes. The causation of this hybrid vigor or heterosis (euheterosis according to DOBZHANSKY 1952) constitutes an important but as yet little understood problem. One possibility is that some gene alleles are “overdominant” with respect to fitness, and thus give heterosis in heterozygous conditions with certain other alleles at the same locus. On the other hand heterosis may be caused by interaction of nonallelic, but usually more or less closely linked gene complexes sometimes referred to as “supergenes” ( DARLINGTON and MATHER 1949). Experimental discrimination between overdominance at a single locus and supergene interactions is a difEicult matter. TEISSIER ( 1943), BUZZATITRAVERSO (1952), GUSTAFSSON and NYBOM (1950), and MUKAI and BURDICK (1959,1960,1961) described instances of heterotic effects of spontaneously arisen or induced mutants, which seem to represent clear cases of overdominance. JINKS (1955), using the method of diallel crosses, was, however, able to detect interaction of epistatic nature in some previously reported instances of overdominance. ROBINSON and COMSTOCK (1955) attempted to evaluate the relative importance of dominant and overdominant genes in the production of heterosis in corn; the available data, however, do not permit a definite answer to be given. The experiments described in the present article were designed to study the possible occurrence of heterosis following hybridization of mutant and wild-type strains of Drosophila melanogaster. The results of these experiments were reported at the 1 l th International Congress of Genetics (POLIVANOV 1963).