410 Action of the Genetic Material 



The best information is derived from the so-called homoeotic mutants 

 in Drosophila, in which one type of segmental organ is changed into 

 another: an antenna into a tarsus or a whole leg, a wing into a leglike 

 structure or a haltere-like organ, a haltere into a wing, and so on. 

 Such mutants have been studied extensively in regard to genetics and 

 physiological genetics. A large number of experiments have been 

 performed with aristopedia, a mutant (ss") which transforms the 

 arista of the antenna into a part of a leg, a tarsus. In some alleles this 

 transformation is always complete; in others the penetrance is in- 

 complete and the expressivity highly variable from normal through 

 many transitions to aristopedia. The earlier experiments, based upon 

 a simple interpretation which I had proposed, consisting of combining 

 ss^ with other mutants affecting the legs or with mutants changing 

 developmental time, showed that timing of developmental features 

 played a role (Braun, 1940, 1942; Villee, 1943, 1944, 1945, 1946) but 

 that not all facts could be explained simply (Waddington, 1940a). 

 M. Vogt (1946, 1947) then showed that a much more compHcated 

 system was responsible for the results obtained by combining trans- 

 plantation techniques with genetical and environmental actions. In 

 development the first visible differentiation (segmentation) of the 

 antennal disc appears before the corresponding process in the legs, 

 and this both in + and ss*^ flies. The penetrance and expressivity of 

 the tarsus character of the antenna are increased with lower temper- 

 ature, and the transforming effect upon the arista, in different degrees 

 of expression, increases from the proximal to the distal. The tempera- 

 ture-effective period lasts from early development into the pupal 

 period, though the quantity of effect decreases during this period of 

 labile determination. Though the tarsal segments of both legs and 

 antenna differentiate late in aristopedia, there is one early difference: 

 in the antenna which transforms into a tarsus, the tip segment of the 

 anlage is larger from the beginning. 



It was then found that the transformation of the arista segment 

 into tarsus is linked with an intensive growth of this segment. A proof 

 for the causal meaning of this was found when an explanted disc of 

 ss'' was treated with colchicine to stop growth and was reimplanted; 

 now it formed an arista, no tarsus. The fact that a completely un- 

 specific interference with growth produces the specific effect suggests 

 that the action of the mutant ss"* is also an unspecific effect upon 

 growth of the arista segment. Here it should be mentioned also that 

 the aristopedia phenotype has been produced as a phenocopy by 

 different means (Bodenstein and Abdel-Malek, 1949; Rapoport, 1947; 



