120 



Comparative Animal Physiology 



becomes a dietary requirement only when there is a deficiency of its precursor. 

 Several amino acids, such as citrulline and diiodotyrosine, are of limited oc- 

 currence and are not building units of protein; hence they differ from the basic 

 nineteen amino acids named above. 



The ciliate Tetrahymena geleii strain W has been cultured in mixtures of 

 amino acids and no persistent growth occurs if any of the following is omitted: 

 isoleucine, leucine, lysine, methionine, phenylalanine, tryptophane, valine, 

 histidine, threonine, and arginine (Table 22). Methionine can be replaced 

 by homocystine as a sulfur-containing amino acid. '^^ For most effective 

 culture either serine or glycine is also necessary: these probably are not re- 

 quired for synthesis of the organism's proteins, but rather release the growth 

 inhibitions imposed by some of the other amino acids. The interactions among 

 various amino acids are complex; cystine, glycine, and serine stimulate growth, 

 whereas alanine, aspartic acid, glutamic acid, hydroxyproline, proHne, and 

 tyrosine in excess inhibit growth, but the stimulators can release the inhibition. 



TABLE 22. 

 TETRAHYMENA GELEll-MEDlUM USED BY KIDDER AND DEWEY'" 



Also there are strain differences in growth on mixtures of the essential amino 

 acids and strain E appears to require serine. -^^ Thus, it appears that, al- 

 though some flagellates can live well on single amino acids, some ciliates 

 require mixtures comparable to those required to support mammals. 



Several insects have been grown on mixtures of amino acids as the nitrogen 

 source. Larvae of the mosquito Aedes aegypti have been maintained with fair 

 but not normal growth in a mixture of eleven amino acids (Table 21); phenyl- 

 alanine and tyrosine are interchangeable, but if both are omitted no growth 

 occurs; also citrulline can be substituted for arginine.^" Growth of l)wsophila 

 larvae on an amino acid mixture has been reported. ^-^ Carpet beetles have 



