Gene Action 



155 



verified over a wide range of phenotypes in 

 various organisms, e.g., Lepidoptera (Kiihn 

 and Henke, '29-'36), Drosophila (Gold- 

 schmidt, '35a,b, '37; Henke, v. Fink, and Ma, 

 '41), chickens (review by Landauer, '48) 

 and mice (Russell and Russell, '48). In 

 Drosophila, for instance, heavy shock of short 

 duration with high temperatures, or less 

 striking temperature increases for longer 

 periods, produce modifications, in genetically 

 normal individuals, of wings and halteres, 

 eyes and aristae, legs and body wall, and 

 bristles. The modifications are of numerous 



that the interference with development by 

 an external agent may well occur at the 

 same time, and perhaps in the same manner, 

 as the developmental shift under the in- 

 fluence of the mutant which is phenocopied. 

 In other instances, it has been demonstrated 

 that the sensitive period during which a 

 phenocopy may be induced is later than the 

 phase at which the mutant first shows its 

 developmental effect. Obviously in these 

 latter cases the phenocopy does not actually 

 imitate the course of events initiated by the 

 mutant allele whose effects it simulates. 



Hours: 2 

 Stage: 



Egg Blastoderm 



Segmentation 3rd Instar 



I I 



Prepupa Pupo 



Fig. 39. Sensitive periods of production of developmental anomalies in Drosophila melanogaster: tetraptera- 

 like wing modification of the halteres; abnormal abdomen; abnormal bristle growth on halteres (after Henke 

 and Maas, '46). 



types, as in the case of wings, where they 

 affect shape and size, venation and distri- 

 bution of bristles. 



Specific relations have been demonstrated 

 between the stage in development of the egg, 

 larva, or pupa, and the production of the 

 various modifications (see Fig. 39). Thus, a 

 period sensitive to moderate temperature ef- 

 fects occurs between two and four hours 

 after fertilization — before completion of the 

 blastoderm — resulting in the later morpho- 

 genesis of the metathoracic segment ap- 

 proaching that typical for the mesothorax. 

 Shortly later an embryonic sensitive period 

 may be used to initiate processes leading to 

 abnormal abdominal segmentation. Modifi- 

 cations of imaginal bristles may be induced 

 in newly hatched larvae, of wing shape in 

 larvae of the second instar, and of the eye 

 surface in those of the third instar. Likewise, 

 all these may be induced by treatment of 

 prepupae and pupae (Henke and Maas, '46). 



Goldschmidt introduced the name pheno- 

 copy for these modifications, since they seem 

 to imitate in genetically normal strains the 

 phenotypes known to be caused by mutant 

 allelic substitutions. Comparisons between 

 the development of mutant phenotypes and 

 of phenocopies have shown in some cases 



There is, however, an arbitrariness in choos- 

 ing a single specific mutant and comparing 

 it with a given phenotypic modification. It 

 is known that many different mutant loci 

 may cause the development of similar or 

 identical final phenotypes and it seems pos- 

 sible that every environmentally induced 

 developmental modification may have its 

 true genetic counterpart in one of the various 

 known, or not yet discovered, mutants. 



While it is necessary to be aware of the 

 usually somewhat arbitrary nature of desig- 

 nating a modification as the phenocopy of a 

 given mutant, it seems likely that a pheno- 

 copy will be found for each mutant, and a 

 mutant for each phenotypic modification. 

 Therefore, the terms "true" and "false" 

 phenocopies, which suggest that an environ- 

 mentally induced phenotype either corre- 

 sponds in its developmental origin to a mu- 

 tant phenotype ("true"), or only simulates 

 it ("false"), have only limited validity. 



TIME OF GENIC ACTION 



Gene-controlled events are known to occur 

 at various stages of development. One of the 

 earliest evidences for genie action are "size 

 genes" brought into the egg by the sperm in 



