Reversal effect of the genetic background on some small-eye mutants,with special reference to gene action of morphogenetic mutants inDrosophila

The facet number of heterozygous females of theBar-eye mutant ofDrosophila melanogaster varies from ±154 to ±665 according to the full eye strains combined with the mutant. TheBar character is commonly recognized as a semidominant, but its manifestation varies from nearly complete dominance to recessiveness according to the mode of the heterozygous combination. Virtually the same is true of the mutantLobe ² which is usually acknowledged as a complete dominant, but may be manifested as an incompletely dominant gene. Thew ^e andb gp strains decrease the eye size of the heterozygotes withBar, but increase the eye size of the heterozygotes withLobe ².Co-isogenic strains ofBar, bar-3 andeyeless ² mutants with Oregonisogenic increase the facet number to degrees 1.4–1.8 times of the original strains. However, the co-isogenicLobe ² strain with the same Oregonisogenic strain reduces the facets completely, and makes virtually all individuals eye-less. These observations indicate that the same genetic background may give an opposite effect to different small-eye mutants. Analysis was carried out for modifying genes ofbar-3. Modifiers were detected on all large chromosomes, besides a recessive major gene located on the 2nd chromosome of Oregon-isogenic strain.A part of this work was performed in the Division of Biology, California Institute of Technology, Pasadena.