17-6 THE YEAST CELL 



salivary X-chromosome of Drosophila copied from Bridges' ori- 

 ginal article. The extraordinarily high frequency of the double 

 bands ("doublets") is most striking; each of these presumably ori- 

 ginated by unequal crossing over. In addition, relatively large re- 

 gions are repeated. In division 3, subdivision C, bands 1, 2, and 

 3 are obviously duplications of bands 5, 6, and 7. In subdivision D, 

 bands 1 and 2 are duplicates of bands 3 and 4. In the enlarged re- 

 gion in division 2, subdivision B, two pairs of conspicuous ''repeats" 

 can be observed. Since Drosophila have undergone relatively few 

 changes over geological periods as evidenced by the discovery of 

 typical forms in amber, it is apparent that this replication of chro- 

 mosomes was well advanced hundreds of thousands of years ago. 

 The frequently occurring recessive mutations characteristic of 

 wild type Drosophila are practically all losses rather than pro- 

 gressive improvements. The losses occasionally have an advan- 

 tage in fitting the organism into an ecological niche and thus have 



H 



Fig. 17-3 A Diagram Showing how Unequal Crossing— Over 

 Could Produce a Duplication of one Locus and a Slightly Longer 



Ch 



romosome . 



a selective advantage. Dobzahnsky (1946) has produced evidence 

 supporting this view by showing that seasonal fluctuations occur 

 in Drosophila genotypes and that populational shifts occur with a 

 predominance of one genotype at one season and the alternative 

 type at another season. Loss mutations offer a certain advantage 

 to the lactobacilli which are unable to synthesize the vitamins and 

 amino acids found in milk; this may reduce the synthesizing "load* 

 on metabolism of the organism (Lwoff, 1943). Such deficiencies 

 also serve to "lock" the organism into its particular niche, making 

 it completely dependent on milk and thus forcing it exclusively to 

 developing mutant forms increasingly better adapted to this sub- 

 strate (if the teleology may be excused). 



