Gene Action 



157 



In other examples of maternal (prefertil- 

 ization) effects the path from genie action 

 to external expression is much longer. Thus, 

 the sexual differentiation of the genital sys- 

 tem in the adult moth Lymantria dispar de- 

 pends on the genotype of the female parent 

 (Goldschmidt, '34), and so does adult testis 

 size in hybrids between Drosophila pseudo- 

 obscura and D. persimilis (Dobzhansky, '35). 

 Another rather well understood situation con- 

 cerns pigmentation of the larval eyes of the 

 moth Ephestia kuhniella (Caspari, '33; 

 Kiihn, Caspari, and Plagge, '35). Here, the 

 gene A is concerned with the elaboration of 

 kynurenin, a colorless precursor of certain 

 pigments. Usually caterpillars of the geno- 

 type aa do not form pigment unless the 

 mother was Aa. Furthermore, implantation 

 of AA tissues into aa females results in a 

 situation such that the newly hatched cater- 

 pillars from a mating of these females to aa 

 males have pigmented eyes and skin. Gradu- 

 ally, however, the pigment disappears again 

 during growth of the caterpillars. Appar- 

 ently, the effect of the gene A in the mother, 

 or the implantation of AA organs, leads to 

 storage of kynurenin or of a derived pig- 

 ment precursor in the egg, its presence be- 

 coming evident only when ocelli and skin 

 have been differentiated and its later disap- 

 pearance being due to depletion of the stored 

 precursor. 



In contrast to these examples in which 

 genie action precedes the developmental ef- 

 fect by a strikingly long period are others 

 in which genie action can be shown to occur 

 late in development, relatively shortly prior 

 to the appearance of a specific trait. A means 

 for studying such late gene activity is avail- 

 able in certain genetic mosaics (Sturtevant, 

 '32). If, in the course of development, somatic 

 mutation or chromosomal changes occur in 

 a cell and if, as a consequence, a new pheno- 

 type arises, then the conclusion is justified 

 that the developmental processes in this cell 

 and its progeny tissue were not independent 

 of, but susceptible to, control by the changed 

 genotype, although the processes themselves 

 may have previously been initiated geneti- 

 cally. Most cases of mosaics in Drosophila 

 and other insects, as well as some rare 

 somatic mosaics in birds and mammals, be- 

 long to this category. These phenotypic mo- 

 saics indicate either that the genie action sets 

 in normally after the time of the "muta- 

 tional" accident, or that even a shortened 

 active period is sufficient to produce an ef- 

 fect. Often this effect is as strong as if it had 

 been present from the beginning of develop- 



ment. Sometimes a weakened or otherwise 

 modified phenotype results. Here we may 

 witness the outcome of abbreviated gene ac- 

 tivity, or we raay wonder whether prior 

 action of the original genotype has left a 

 shortage of some specific substrate on which 

 the new genotype has to act. Still another 

 cause for a modified phenotype may lie in 

 some interaction between the genetically dif- 

 ferent tissues of such a mosaic organism 

 (see pp. 159-160). 



TYPE OF GENIC ACTION 



Most genes produce multiple effects in 

 development ("pleiotropy"). For instance, 

 in the so-called "Frizzle" fowl, animals ho- 

 mozygous for the Frizzle allele show not only 

 the abnormal feather curliness which has 

 given them their name but also the follow- 

 ing traits: "The heart rate is accelerated and 

 both ventricles of the heart are hypertro- 

 phied. The blood volvime is greater than 

 normal, and the spleen is enlarged. Weight 

 (relative to the body) of crop, gizzard, pan- 

 creas and kidneys, length of the coeca, ca- 

 pacity of the intestinal tract (from duo- 

 denum to large intestine) are in excess of 

 what is normal. The adrenals are hypertro- 

 phied. The white cells of the blood may show 

 a relative agranulocytosis and relative gran- 

 ulopenia. The metabolic rate is above nor- 

 mal, food consumption increased, water va- 

 porization reduced. The thyroids may be re- 

 duced in size or much enlarged. . . . The 

 ovaries mature late and may remain atrophic; 

 the testes frequently show interstitial edema 

 and tubular damage. . . . General body 

 growth falls below normal. Viability is re- 

 duced." (Landauer, '46.) 



Another example of multiple genie effect, 

 in a mammal, is furnished by the gene for 

 myelencephalic blebs in mice. Homozygous 

 mb individuals freqviently have (1) defec- 

 tive eyes, (2) polydactylous or syndactylous 

 feet, and (3) abnormal hair growth (Grvine- 

 berg, '43). 



In Drosophila, the most striking examples 

 for multiple effects of genes come from 

 studies specifically designed to investigate 

 the question of the freqviency of such mul- 

 tiple effects (Dobzhansky, '27; Dobzhansky 

 and Holz, '43; Schwab, '40). Mutant strains 

 which had been characterized by means of 

 observed effects on such external traits as 

 eye pigment or body coloration were ana- 

 lyzed for the type of growth of an internal 

 organ, the female spermatheea. In several 

 instances it was discovered that the external 



