Chapter 30 



THE GENETIC CONTROL 

 OF MUTATION 



C 



hapters 11, 12, and 14 dealt 

 with different units of muta- 

 tion, ranging from the larg- 

 est — genomic — changes to the smallest — 

 gene — changes. Although various external- 

 ly-applied environmental agents can produce 

 mutations of all kinds (Chapters 13 and 16), 

 we would like to know to what extent the 

 genotype regulates its own mutability. 



Mitosis is so precise that ordinarily the 

 genotype prevents the occurrence of genomic 

 and single, whole-chromosome changes in 

 successive generations of cells. Mitotic rate 

 and spindle orientation are controlled by 

 genes, just as are other aspects of mitosis. 

 In Ascaris, the genotype seems to prevent 

 mutation in a polycentric chromosome by 

 suppressing the action of all but one centro- 

 mere. 



In meiosis, crossing over occurs at points 

 that correspond precisely in two nonsister 

 strands, so that crossovers containing de- 

 ficient or duplicated segments are avoided. 1 

 In this way intrachromosomal euploidy is 

 maintained even though recombination be- 

 tween homologs is permitted. Synapsis and 

 chiasma formation in meiosis help distribute 

 the homologs in a way that prevents the gain 

 or loss of whole chromosomes, that is, aneu- 

 somy. Evidence for the genetic control of 

 synapsis is provided by collochores in Dro- 

 sophila (p. 185) and by the genes for asyn- 

 apsis found in maize (p. 190), many other 



1 See, however, G. E. Magni (1963). 

 383 



plant species, and Drosophila. Genes pro- 

 ducing spindles that diverge at the poles 

 during meiosis are known in both Drosophila 

 and maize. In general, the synthesis of new 

 genes is usually regulated to prevent sub- 

 stitution of improper genetic raw materials 

 for the proper ones, assuming both types are 

 present in the cell at the same time. 



It might be argued that the examples given 

 demonstrate only that reduced mutability 

 is an inevitable consequence of normal cell 

 operation. Although present genotypes may 

 appear to play a passive role, mitosis and 

 meiosis are not intrinsic properties of genes 

 or cells, and, therefore, during the course 

 of evolution the selection of genes for carry- 

 ing out these activities must have been an 

 active process aimed at reducing mutability; 

 that is, genes that could maintain genetic- 

 stability and permit replication and genetic 

 recombination via sexuality must have been 

 favored. 



Though the genetic controls so far men- 

 tioned lead to a reduction in mutability, it 

 should be realized that the genotype also per- 

 mits genetic changes to occur in the follow- 

 ing controlled or regulated ways: 



1 . The ploidy changes in a sexual cycle 

 — from diploid to haploid and back again — 

 are under genetic control. 



2. Mutational changes increase with mito- 

 tic activity (p. 193 ) ; since the rate of mitosis 

 is under genetic control (many cancer cells 

 are mutants whose mitotic rate has in- 

 creased), the genotype controls mutability 

 in this way. 



3. We have already mentioned (Chapter 

 11) certain modifications of meiosis — un- 

 doubtedly under genetic control also — lead- 

 ing to ploidy changes in the next generation. 



4. Even within the somatic tissues of a 

 multicellular organism, controlled genetic 

 change is permitted in cells whose chromo- 

 somes become polyploid (as in human liver), 

 or highly polynemic (as in the Dipteran 

 larval salivary gland). 



