INTRODUCTION TO BIOCHEMICAL SYSTEMATICS 49 



general. Although these two taxonomic levels do not impose any 

 absolute restrictions upon the particular biochemical approach, and 

 there is no mutual exclusion, it is important to emphasize some funda- 

 mental differences. 



Biochemical systematics at the major taxonomic category 

 level involves the use of classical studies of such substances as alka- 

 loids, and so on. For example, certain plant families tend to produce 

 alkaloids, while others do not. Within those famihes which do, one is 

 likely to find a certain class of alkaloid, and related genera are apt to 

 form a particular example of this type of alkaloid. The basic rationale 

 is that of associating specific secondary products of restricted occur- 

 rence with specific groups of plants. Some groups of secondary prod- 

 ucts, such as anthocyanins, are rather too widespread to be of great 

 value although we shall find that, even here, the distribution of un- 

 usual types is meaningful in systematic terms. 



Biochemical systematics as applied to minor categories may 

 be approached in diverse ways. It is theoretically capable of the ut- 

 most refinement, as will be discussed later in this section. Experimen- 

 tal chemical systematics is most likely to make a contribution at this 

 level. One form is the work by Turner and Alston on Baptisia which 

 has been referred to earlier in this chapter. It may be assumed as a 

 valid generalization that emphasis is shifting from the major to in- 

 clude the minor category level. It has only been within the last few 

 years that certain ultimate goals have even been conceived. A few 

 examples here will serve to illustrate that definite progress is being 

 made in directions undreamed of ten years ago. 



From the area of serology, an exceedingly interesting situa- 

 tion has been reported by Suskind (1957). In Neurospora crassa 

 a number of tryptophan-deficient mutants (td series) have been studied. 

 Evidence from serological studies indicates that a protein closely related 

 to tryptophan synthetase (the functioning enzyme) is present in a tryp- 

 tophan-requiring mutant. In fact, several td mutants have been 

 studied serologically, and those which exhibit serological cross reactivity 

 are referred to as CRM (cross reaction mutant). Some mutants (for ex- 

 ample, td) show no serological difference from the wild type allele 

 while others, although exhibiting a cross reactivity, demonstrate a de- 

 gree of reactivity indicating a serological difference. It is particularly 

 interesting to note that most CRM mutants can be suppressed while 

 CRM-less mutants cannot be suppressed (Suskind, 1961). 



The basic method is to obtain rabbit antibody (using partially 

 purified preparations of tryptophan synthetase) which neutralizes 

 enzyme activity. Tests, using td mutants, were conducted to deter- 

 mine whether or not they could yield a substance capable of combin- 



