:;ss 



CHAITIR 30 



one changed paternal X.) Among the SD- 

 containing sons the daughters produce, the 

 half receiving the unchanged maternal X can 

 distort, whereas those receiving the changed 



paternal X cannot. When either of these 

 kinds oi males are outcrossed to SP S!) 

 females, all SD-containing sons receive an 

 unchanged maternal X and. therefore, can 

 distort. Females producing .VD-carrying 

 sons of which only half distort, arc said to 

 he conditioned and to show conditional dis- 

 tortion: the mechanism of conditional distor- 

 tion is unknown. 



The case of SD shows some similarity to 

 the Ac-Ds example discussed. 4 SD causes 

 some kind of genetic change regulated by 

 Ac(SD). The activity of SD is modified by 

 St(SD) and is also conditioned by the X 

 chromosome. SD provides an excellent 

 example of the genetic control of muta- 

 bility. 



SD was initially obtained from a natural 

 population that showed no distortion because 

 SD's detrimental effect on the transmission 

 of its homolog was suppresed by a combina- 

 tion of factors. One was X-chromosome 

 conditioning fostered by inbreeding. In this 

 population selection apparently favored the 

 retention of SD + alleles resistant to distor- 

 tion as well as inversions or other structural 

 changes involving IIR which, in heterozy- 

 gous condition, prevented pairing and, hence, 

 distortion. SD is an example of meiotic 

 drive, a force capable of altering gene fre- 

 quencies in natural populations by the pro- 

 duction of functional gametes which do not 

 carry segregants in a one-to-one ratio. 



Episomes and Viruses as Mutagens 



Suppressed or variegated phenotypic effects 

 are known which are due to the placement 

 of heterochromatin near euchromatin. In 

 Drosophila. such position effects are frequent 

 after structural changes in chromosomes. 



4 See L. Sandler and Y. Hiraizumi (1961). 



Some cases of phenotypic suppression in- 

 volve special genetic elements — for example, 

 Segregation Distortion in Drosophila and 

 Dissociation in corn — associated with hetero- 

 chromatin. Some of these genetic factors are 

 capable of causing breakage and of changing 

 their location in the genome. Since such 

 factors and episomes have certain character- 

 istics in common, both should be studied and 

 compared with regard to phenotypic sup- 

 pression, organelle movement, and chromo- 

 somal breakage. 



The spontaneous mutation frequency from 

 auxotrophy to prototrophy is known for a 

 large number of alleles for various markers 

 in Salmonella. When auxotrophic bacteria 

 are infected with transducing phage grown 

 on the same genetic strain or on a bacterial 

 strain carrying a deletion (deficiency) for 

 the gene under test, the frequency of proto- 

 trophs is significantly increased/' Genes in- 

 duced to revert to prototrophy in this way 

 are called seljers. Although the mechanism 

 of reversion is not fully understood, the 

 presence of a transducing fragment which 

 synapses in a region near a selfer gene some- 

 how stimulates the mutability of the selfer. 

 Consequently, phage enhances the mutabil- 

 ity of bacterial genes. We have already seen 

 (p. 373) that the mutability of an extra- 

 nuclear gene is under control of nuclear 

 genes. 



In this connection we should also note 

 the following results involving the higher ani- 

 mals, including man: 



1 . The addition of Rous sarcoma virus to 

 normal rat cells in tissue culture produces an 

 increased incidence of chromosome breakage 

 over the control level. 



2. After herpes simplex virus is innocu- 

 lated into established human tissue-culture 

 lines, the incidence of chromosomal break- 

 age increases. 



"•See M. Demerec (1963), and A. L. Taylor 

 (1963). 



