Gene Action 



159 



Drosophila it is not easy to discern, at pres- 

 ent, a primary site from which effects radi- 

 ate out both to the formation of pigment 

 in the eye and to specific shape of the sperm- 

 athecae. While such a developmental site 

 may still be fovmd, it is perhaps easier to 

 conceive of genie action occurring independ- 

 ently at two different sites, the eye anlagen 

 and the spermathecae. A similar interpreta- 

 tion has been considered in respect to the 

 gene locus W in mice, which controls both 

 pigmentation and hematopoiesis (E. S. Rus- 

 sell, '49). More than one site may also be 

 involved in the "myelencephalic blebs" mice, 

 since the effect of the mb gene on hair 

 growth has not been related to the appear- 

 ance of blebs but seems to be separate in 

 origin. 



The action of the same gene at different 

 sites ("repetitive genie action") (Stern and 

 Schaeffer, '43) raises the problem of multi- 

 ple or unitary primary action in a new form. 

 Assuming a single activity at each site, it 

 would be possible for this activity to be 

 identical in the several sites or to be differ- 

 ent from site to site. Some further comments 

 on these problems will be made when gen- 

 eral aspects of genie mechanisms of differ- 

 entiation are considered (see pp. 163-166). 



DEPENDENT AND AUTONOMOUS 

 DEVELOPMENTAL PROCESSES 



The site of genie action may or may 

 not be the site where its final developmental 

 effect becomes apparent. The example of 

 myelencephalic blebs in mice has shown that 

 the earliest known developmental site is in 

 the region of the brain, whereas the later 

 phenotypic effects are on eyes and feet. 

 The embryological analysis of the sequence 

 of events suggests strongly that the appear- 

 ance of blebs is the cause of the defects in 

 the distant eyes and feet. It would be pos- 

 sible, however, to argue that the gene mb 

 which leads to bleb formation is also an 

 agent in the anlagen of eyes and limbs which 

 makes them react in their abnormal way to 

 the migrating blebs. Such an argument, 

 while unlikely, can best be tested by experi- 

 ment. A classic experiment of this type re- 

 lates to a genetically dwarf strain of mice 

 (Smith and MacDowell, '30). In this strain, 

 a histological study of the pituitary gland 

 showed deficiencies in the anterior lobe. It 

 was obvious to suggest that the growth re- 

 tardation of the animals was caused by 

 means of a deficiency in the growth hor- 

 mones normally produced in the pituitary. 



It could be shown that hormonal deficiency 

 indeed is the essential factor and that 

 the slowly growing tissues which were of the 

 same abnormal genetic constitution as the 

 pituitary nevertheless had normal growth 

 potencies: artificial supply of growth hor- 

 mones by implantation of normal pituitaries 

 leads to successful growth. 



Such dependent, gene-controlled, differen- 

 tial development is undoubtedly frequent. 

 Particularly striking examples have been 

 provided by some lethal genotypes. Cultures 

 of some tissues from lethal brachyuric mouse 

 embryos (Ephrussi, '35), and from the homo- 

 zygous lethal Creeper fowl (David, '36), 

 have been successfully grown beyond the 

 normal life span of the doomed donor. Vari- 

 ous tissues of lethal Drosophila larvae have 

 survived and undergone development if trans- 

 planted to normal hosts (Hadorn, '48), or 

 present in small mosaic patches on nonlethal 

 individuals (Demerec, '34, '36). The death 

 of the whole, in these lethals, must be due 

 to lethal action in some particular region 

 or regions, an action which unavoidably 

 leads to death of the potentially viable tis- 

 sues. 



On the other hand, the existence of "cell- 

 lethals" discussed earlier (p. 152) shows that 

 the specific differentiation of cells often de- 

 pends on the action of their own genes. 

 This is obviously true even in dependent 

 differentiation, where a primary site of gene 

 action, as perhaps within the pituitary cells 

 of dwarf mice, must form the starting point 

 for secondary dependent events. In many 

 cases the final phenotypic effects are the re- 

 sults exclusively of genie action within the 

 cells concerned in the phenotype. Studies of 

 genetic mosaics (see p. 157) as well as of 

 transplants have demonstrated autonomous 

 differentiation in coat colors of mammals 

 (Bhat, '49), in eye and body pigmentation, 

 bristle shape, wing type, and many other 

 traits of insects. Particularly the gynanders 

 of Drosophila, and of many other insects, 

 are evidence of a cellular autonomy for they 

 show sharp phenotypic lines of demarcation 

 corresponding to the genetically diverse tis- 

 sue areas (Sturtevant, '32). 



There is no general rule as to when de- 

 pendent and when autonomous differentia- 

 tion may be expected. While most eye color 

 mutants in Drosophila behave autonomously, 

 the difference between vermilion and nor- 

 mal red eyes, or between cinnabar and nor- 

 mal red eyes, is due to the presence or 

 absence of circulating or diffusing substances, 

 kynurenin and hydroxykynurenin, respec- 



