GENES, MOLECULES AND PROCESSES 



arc the red and blue pigments, and anthoxanthins, which are the 

 ivory and yellow flavoncs and flavonols. These two classes have the 

 same basic structure and both are assumed on chemical grounds to 

 be built up by parallel processes from sugars. What does genetics 

 tell us of these processes ? 



In Pliarhitis nil, two gene mutants, c and ca, interfere with 

 production of both anthocyanin and anthoxanthin. They must, 

 therefore, act before the synthetic chains branch. Other mutants, 

 a and r, have a direct effect only on the anthocyanins. They act after 

 the chains branch, as do most of the known pigment genes. Now if 

 a gene acting on one branch sequence is reduced in activity we must 

 suppose that an extra supply of materials will be available for the 

 other sequence. Blockage should lead to diversion. This possibility 

 has been tested in the autotetraploid Dalilia variabilis where every 

 gene can be tested in five doses [a^, Aa^, A^az, A^a, A^). The gene 

 B, according to Lawrence and Price, produces anthocyanin, I 

 produces the anthoxanthin apigenin. Bh^i^ has, of course, antho- 

 cyanin pigmentation as has Bh^Ii^. But, still keeping Bh^, with 

 I^i or I^ the anthocyanin is much reduced. Thus B and / are evidently 

 competing for a precursor. Further, I shows the effect, even more 

 strongly, with another anthocyanin gene A; Aa^I^ produces almost 

 no anthocyanin at all. There is also evidence that each of these 

 genes A, B, I and a fourth, Y, appears to contribute to a pool of 

 common precursor. In fact, of course, they must increase the supply 

 by increasing the demand: they act backwards, as well as forwards, 

 in development. 



When we say that the gene works backwards this is, of course, a 

 figure of speech. It means that what is happening in the cytoplasm 

 reacts on the nucleus. The utilization in the cytoplasm of materials 

 produced by genes in the nucleus stimulates the activity of these 

 genes. The reaction of nucleus and cytoplasm must be reciprocal, 

 so far as the feeding of the nucleus is concerned. There seems little 

 doubt that it must also be reciprocal so far as the feeding of the 

 cytoplasm is concerned. 



Yet another point is shown by the anthocyanin genes A and J5 in 

 Dahlia. In small doses they produce only cyanin, which is regarded as 

 the primitive or precursor type of anthocyanin since it is commonest 

 in the leaves, while its derivatives are commonest in the flowers, 



164 



