STUDIES ON THE NATURE AND FUNCTION OF POLYGENIC LOCI IN DROSOPHILA. III. VEINLET MODIFIERS HAVING REGION-SPECIFIC EFFECTS UPON THE VEIN PATTERN

olygenic modifiers affecting the expression of the mutant veinlet were studied to determine whether each acts specifically upon one vein or wing region or whether they affect the venation pattern in some general way. Selection experiments showed that the L4 vein can be modified independently of the L2 and L3 veins. Similarly, the L2 vein can be shortened while the L3 is selected to be longer, although there is some interdependence between the L2 and L3 veins. Assays of heterozygous whole chromosome effects show that different chromosomes are involved in the responses of separate veins, and one polygenic locus causing a decrease in L4 vein length has been isolated. Substitutions of whole chromosomes from selection lines into unselected backgrounds of non-homologous mutants demonstrate that the selected modifiers have the same qualitative effects upon other mutants having similar phenotypes. These results support the hypothesis that the polygenic modifiers affecting veinlet expression function independently of the veinlet locus, presumably by influencing common steps in the developmental processes leading to the formation of individual veins.