Heterosis and Genetic Variance in Hybrids between Inbred Lines of Drosophila Melanogaster in Relation to the Level of Homozygosity.

LTHOUGH exploitation of heterosis in hybrid maize is so far the most successful A single achievement of applied genetics, understanding of the causes responsible for this phenomenon remains still unsatisfactory. Much of the work on heterosis has been purely empirical and descriptive. Analytical studies have been conspicuously rare, and this in spite of the fact that heterosis is a term which, as most geneticists agree, is being applied to a group of “scarcely related phenomenal’ (DOBZHANSKY 1950). Only in recent years has interest in the investigation of the genetic bases of heterosis asserted itself. A symposium on heterosis was held in 1950 (edited by GOWEN 1952). CROW (1952), DOBZHANSKY (1952), HAGBERG (1953), MATHER (1955), RENDEL (1953) and others have discussed the different genetic mechanisms which may bring heterosis about. Drosophila is the most important animal which has been used for experimental investigations of heterosis. Two main methods have been utilized. First, observations and experiments on the behavior of inverted sections of chromosomes in natural and laboratory populations have been described by DOBZHANSKY and his colleagues. Secondly, lines inbred by different methods are crossed, and different morphological and physiological characteristics of the hybrids and the inbreds measured and compared (ROBERTSON and REEVE 1952, 1954, 1955a, b; RASMUSON 1952; CLARKE and MAYNARD SMITH 1955; TANTAWY 1955 and others). The results obtained are variable for different traits and different situations. In order to understand these variations, careful analysis of the genetic systems by which the inbreeding has been brought about is indispensable. The experiments reported in the present communication have been designed to study the relationships between the rates of inbreeding and the observed inbreeding degeneration and restoration of heterosis by crossing. The relation between the genetic variances observed in the inbred lines and in the hybrids between them has also been studied.