The effects of some physical and biotic environments on heterosis of direct and associate genotypes in Drosophila melanogaster.

number of studies have shown that the manifestation of heterosis may be A dependent on physical and biotic environments. DOBZHANSKY et al. (1955) found that Drosophila pseudoobscura heterozygous for genes on the second chromosome was, in general, superior to homozygotes in viability. The relative viabilities of homozygotes and heterozygotes changed, however, with varying temperatures and with different kinds of yeasts used in the media. In Drosophila melanogaster, the superior fitness of ebony heterozygotes ( e /+ ) over the homozygotes has been well established. The literature has been reviewed by MOREE and KING (1961) who also reported that the superiority in larval viability of e/+ hybrids over e/e parents increased with increasing larval density. DAWOOD and STRICKBERGER (1964) found that the larval viability of the +/e hybrids was superior to that cf the wild-type flies when the two genotypes were competing in crowded cultures. More recently, DAWOOD and STRICKBERGER (1969) reported that the viability of e/+ larvae relative to the parental genotypes was affected by the age structure of the competing larvae, by larval density, and by the interactions between these two factors. Evidence of dependence of heterosis on environments in plants is also available. For example, temperature-dependent heterosis in plant weight of Arabidopsis thaliana has been reported by GRIFFING and LANGRIDGE (1963). More recently GRIFFING and ZSIROS (1971) reported that the heterotic response in plant weight of Arabidopsis thaliana was affected by nutrition, temperature, and plant associations. DOBZHANSKY (1 952) distinguished two kinds of heterosis: euheterosis, which is related to the relative fitness of the genotypes; and luxuriance, which describes the greater size and growth rate of the hybrids. Thus the experimental results in Drosophila reported earlier may be said to represent euheterosis while the evidence in Arabidopsis may be luxuriance. Both types of heterosis are important in their own right. However, there is little experimental work relating the two kinds of heterosis over a range of environmental conditions. The present communication reports a study of hybrid vigor in Drosophila nielanogaster over a range of physical and biotic environ-