204 BIOCHEMICAL SYSTEMATICS 



oversimplifies the situation and conveys to the casual reader the idea 

 that the gene categories are possibly biochemically as well as pheno- 

 typically analogous when, in fact, it is demonstrable that in numerous 

 cases they are not. 



As a result of a series of biochemical genetic studies involving 

 numerous plants several types of biochemical differences attributable 

 to single gene differences have been reported. (Since many of these 

 have been confirmed several times by different workers, only cases of 

 some special interest will be identified by citation.) In a number of 

 instances genes are known to govern the substitution pattern of the 

 B ring, that is, the number of hydroxyl groups present. Sometimes, 

 for example in Streptocarpus (Lawrence et al., 1939), a dominant gene 

 governs the formation of malvidin instead of pelargonidin. In this 

 case, it is possible that the gene permits the addition of one or more 

 OH groups in the B ring (or a precursor thereof ) and thus provides a 

 site for methylation so that a single gene may appear to govern a 

 more complex biochemical process than is actually the case. A similar 

 situation probably occurs in Impatiens (Alston and Hagen, 1958). Of 

 course, it is possible that the gene governs methoxylation, but present 

 evidence does not permit a choice between these alternatives. It is in- 

 teresting that, to the writers' knowledge, there is no report in the 

 hterature of a gene which governs substitutions in the A ring other 

 than the glycosidic pattern. It is highly probable that such genes 

 exist, since hirsutidin (a 7-methoxy malvidin), gossypetin ( a flavonol 

 with an 8-hydroxy substitution) and other compounds with atypical 

 A ring substitution patterns exist. 



Numerous instances of the occurrence of single genes which 

 affect the glycosidic pattern are known, and as noted previously the 

 diglycoside is dominant to the monoglycoside. 



There are several instances known of single genes which 

 govern acylation. Abe and Gotoh (1956) reported a dominant gene 

 governing acylation with p-hydroxy cinnamic acid in the eggplant, 

 and Harborne (1956) reported an interesting situation in Solarium 

 in which a single gene appeared to govern three biochemical differ- 

 ences in the same anthocyanin: a change in substitution of the B ring, 

 a change in the glycosidic pattern, and acylation of the glycoside 

 with p-coumaric acid. 



There are numerous examples of genetic mechanisms which 

 involve interactions between anthocyanins and other classes of 

 flavonoids. In Dahlia, the classic example of such interaction 

 (Lawrence and Scott-Moncrieff, 1935), one factor, I, governs flavone 

 synthesis at (apparently) the expense of anthocyanin. The authors 

 concluded from these results that a precursor, hmited in amount, was 



