Gene Action 



161 



the remainder. Either constitution typically 

 determines male differentiation. In accord- 

 ance with expectation the mosaic individuals 

 were males bvit frequently showed some 

 feminized genital structures (Fig. 42). Ap- 

 parently, the two alternative kinds of sex 

 factors in the nuclei of the mosaics control 

 the production of two kinds of diffusible 

 male determining substances which are com- 

 plementary in such a fashion as to cause, 

 jointly, female differentiation. 



Restricted diffusion of genetically con- 

 trolled substances is also involved in the 

 specific shape of the adult testes of different 

 species of Drosophila (Stern, '41). In some 

 species these organs are ellipsoidal, in others 

 spiral or helical. Normally, the uncoiled 

 larval testes attain the specific adult shape af- 

 ter having become attached to the vasa defer- 

 entia which originate independently. An in- 

 ductor-reactor relation in regard to spiraliza- 

 tion exists between vas and testis in "spiral" 

 species with transplanted ellipsoidal testes. 

 Interspecific transplantations of larval testes 

 show that the typical different adult testes 

 shapes are not autonomous responses to the 

 stimulus emanating from the vasa but that 

 the vasa of the different species cause dif- 

 ferential growth of testes. The vas of a "non- 

 spiral" species causes limited, more or less 

 equally distributed, growth of an attached 

 testis of even a spiral species, and the vas of 

 a spiral species causes unequally distributed 

 growth of an attached testis of even a non- 

 spiral species. These growth differentials are, 

 presumably, due to diffusion into the react- 

 ing testicular tissue of specifically distributed 

 growth promoting substances in the vas. An 

 analysis in terms of individual genes of the 

 differences between the different noninter- 

 breeding species is not possible. Therefore, in 

 the present state of our knowledge one may 

 designate the substances involved as "genom 

 dependent" rather than as gene dependent 

 (Hammerling, '46). 



'10). Examples of homeotic mutant genes 

 are aristapedia, which changes the arista of 

 the antenna to a leg, proboscipedia, which 

 changes mouth parts into leglike organs or 

 aristae, and bithorax, which changes halteres 

 into winglike appendages (Fig. 4f3A-D); 

 these and other homeotic mutants are listed 

 by Herskowitz ('49). It is not known how 

 these simply and typically inherited genes 

 produce striking morphogenetic effects. The 

 appendages of adult insects develop from 

 separate embryonic imaginal discs, and the 

 nature of the stimuli which normally cause 

 the differentiation of an antenna from a head 

 disc and of a leg from a thoracic disc has not 

 been established. It could be that the differ- 

 ent regions of the embryo or larva determine 

 the specific differentiation of originally toti- 

 potent discs by means of specific evocators. 

 It could also be true that more general prop- 

 erties imposed on the discs in the different 

 regions, for instance, different rates of growth 

 or of metabolism, lead to autonomous chan- 

 nelling of development within the disc into 

 one or another of the alternative courses. 

 Similarly, in a homeotic mutant, it is con- 

 ceivable that the unusual development of a 

 disc is determined by an evocator which is 

 not typical for the region of the specific disc, 

 or that a change in intrinsic general proper- 

 ties of the disc results in the homeotic dif- 

 ferentiation. A third scheme, proposed by 

 Goldschmidt ('38), assumes that at different 

 developmental periods different kinds of evo- 

 cators are present in the embryo or larva 

 and that only discs in specific stages of de- 

 velopment are able to react to these evoca- 

 tors. A mutant which shifts the rate of de- 

 velopment of a still totipotent disc so that it 

 becomes reactive at an earlier or later time 

 than normal will expose it to the evocator of 

 that period and thus cause the production of 

 a homeotic organ. For the mutant arista- 

 pedia, studies by Vogt ('46) have not sup- 

 ported this interpretation. 



HOMEOTIC MUTANTS 



GENETIC ASYMMETRIES 



In D. melanogaster, a number of mutant 

 genes have been found which direct the de- 

 velopment of embryonic anlagen of a body 

 segment into new channels so that they dif- 

 ferentiate into organs normally characteristic 

 of other segments (see Villee, '42). Such 

 mutant genes thus produce the kind of 

 changes which fall under the term homeosis 

 (Bateson, '94) and which have been dealt 

 with extensively by earlier students of mor- 

 phogenesis (Herbst, 1896-1901; Przibram, 



A special group of gene-controlled pheno- 

 types is that of hereditary asymmetries. Many 

 animals typically show specific asymmetries, 

 for instance, of the viscera in vertebrates 

 (Ludwig, '32). The geneticist cannot con- 

 tribute directly to the understanding of these 

 basic asymmetries. However, specific genes 

 are known which change the type of asym- 

 metry normally present or produce asym- 

 metries in structures which usually have a 

 symmetrical arrangement. 



