3 I (', VITAMIN RF.QUIR I. Ml NTS 



1. The required vitamin is not available in the environment. 



2. The derangement of metabolism associated with the deficiency is 

 so great that even with the missing factor available in the environment 

 the mutant is at a selective disadvantage and cannot propagate itself. 



The second of these hypotheses is the more attractive, inasmuch as 

 even the plant pathogens, living in close contact with higher plant 

 cells, do not seem to develop strains deficient for certain vitamins. 



18. VITAMINS, TAXONOMY, AND ECOLOGY 



Comparative physiology often affords clues to taxonomic and phylo- 

 genetic relationships. Vitamin deficiencies do not, so far as can be 

 determined, offer such clues. From the frequency of induced muta- 

 tions, one would expect that vitamin deficiencies occur at random 

 within species, genera, and higher categories; this appears to be true. 

 Marked differences between the vitamin demands of strains of a single 

 species are often encountered, e.g., Trichophyton spp. and other animal 

 pathogens (5, 80, 81, 152), Lenzites trabea (HO), Boletus granulatus 

 (169), and Glomerella cingulata (270). In culture, both Fusarium 

 avenaceum (214) and Ustilago major (15) give rise to clones which 

 differ in requirements from the parent strain. 



Within a genus, again, species differ widely in vitamin demands; 

 examples include the genus Ophiostoma and its asexual form Cera- 

 tostomella (66, 67, 215, 217, 224), Trichophyton (31, 80, 222, 223), 

 Ustilago (245), Candida (241), Boletus (169), Marasmius (145), and 

 Absidia (213). 



In the light of this intra- and interspecific variation, it is difficult to 

 imagine that vitamin needs per se can separate taxons or can yield 

 valid evidence of phylogeny. 



The theory of Lwoff (150) and Knight (122), that metabolic defi- 

 ciencies have arisen in the evolution of microorganisms by loss of 

 functions originally present, has been generally accepted. If we grant, 

 in view of the argument just made, that this loss of function has not 

 followed phylogenetic or taxonomic divisions, we may ask if there is 

 any relation of vitamin needs to ecology. 



First, it appears that some genera as isolated from nature rarely have 

 vitamin deficiencies. Only a tentative listing can be made at present, 

 in view of the methodological difficulty in establishing the absence of a 

 requirement: Aspergillus, Chaetomium, Fusarium, Mucor, Penicil- 

 lium, Rhizopus, Streptomyces, and ZygorhyncJius (213). Exceptions 

 occur in most of these genera, i.e., some isolates are deficient or par- 



