Genetic Modification of a Temperature Sensitive Lethal in Drosophila Willistoni.

HE SURVIVAL value of a genetic factor is conditioned by its phenotypic Tmanifestation in all environments encountered by the species, as well as by its performance in combination with the rest of the genotype of the species. The hereditary norm of reaction can be analyzed by observing a given factor under various environmental conditions and in different genotypes. DOBZHANSKY and SPASSKY (1944) studied the expression of characters associated with the second and fourth chromosomes of Drosophila pseudoobscura by changing the environment as well as the residual genotype. They found one chromosomal type which had an almost normal viability a t 16x°C, was semilethal at 21°C and lethal a t 2555'C. Various degrees of viability of this chromosome were obtained at 21" by altering the genetic background. Another factor on the fourth chromosome exhibited temperature-conditioned visible characters which could be paralleled by changes in genetic residues. Viability influences of five systems of geographic modifiers were tested for a number of homo-zygotes for second and third chromosomes derived from natural populations. Some modi$ers caused larger viability changes than others, but the influence of a particular geographic mouifying system was not beneficial to all homozygotes nor deleterious to all. A temperature-sensitive lethal factor (ZWB), the viability effect of which can also be modified by alterations in the genotype, has been discovered in Drosophila willi-sloni. The recessive factor ZWB on the second chromosome was extracted from the Goyaz (Brazil) population of D. willistoni. At 25°C this factor is a lethal. The ZWB homozygotes are viable below 25' C and can be distinguished by their wing and eye abnormalities. The wings of the newly emerged flies are blistered and vesiculated, but in older flies they appear shrunken and slightly darker in color than normal wings. The wings are held in the unfolded position (divergent). The description of balloon wings in D. melanogaster fits this lethal more closely than any other described mutant (BRIDGES and BREHME 1944). Irregular curvature of the eyes also occurs in the ZWB homozygotes. A pure culture of the abnormal flies has been maintained a t 18OC. The balanced lethal stock of ZWB was outcrossed to a wild strain, "Standard"; the ZWB/+ flies obtained in the FI were mated and their offspring raised a t 25°C. Among the progeny of the FS there were flies with shrunken wings which had survived the lethal temperature. Repetition of this experiment revealed discrepancies in the …