Studies on the Bar Series of Drosophila. I. The Effect of the Gene Vestigial on Facet Number in Bar

Abstract

A general theory specifying the action of the genes in development was first proposed by GOLDSCHMIDT (1920), as the result of a series of studies on intersexuality in Lymantria dispar. The theory, elaborated more fully in 1927, considers the genes as affecting the rates of developmental processes. Since an effect on the rates of some processes limits the durations of others, the varying grades of expression of any character of the organism may be interpreted as the result of some alteration in either the rate or duration of one or more processes in the developmental chain leading from the genes to the character studied. As a formal explanation of the facts, there can be little doubt that the theory is tenable. It lacks, however, any extensive quantitative experimental verification.Any direct attack upon this problem necessarily involves the possibility of studying directly one or more of the processes in the chain leading from the genes to the adult characters. The difficulties attendant upon such an attempt are apparent. In a form like Drosophila, a direct attack appears impracticable at the present time. We may, however, approach the problem from the standpoint of interaction of genetic factors with each other, and with environmental agents known to affect the expression of various characters of the organism. Since the characters of an organism are the resultants of an interplay of its genes with environmental agents acting upon it throughout the course of development, we may, within certain limits, alter the course of the developmental processes by varying these two kinds of factors either independently or jointly. In working with different genotypes, preferably with biotypes differing only in respect to a single gene, quantitative results relating the character studied to the environmental variables may be obtained. Knowing the functional relations between character and environmental agent for a variety of genotypes, we then have data which may be used for the beginning of an analysis of the action of the gene.Numerous investigations of this type have been carried out on Drosophila melanogaster. A variety of characters which lend themselves to quantitative estimation, for example, facet number, wing area, bristle number, and others, have been studied in different mutant stocks. For the most part, temperature was the environmental agent used in the experiments. In an investigation on bristle number in Dichaete flies, PLUNK-GENETICS 20: 156 Mr 1935 157 * Males and females were not recorded separately.