GENETIC CONTROL 7 



and which allow them to make the normal pigment. Implantation of cinna- 

 bar into vermilion and vice versa showed that the transformations are due 

 to two substances : v+ which causes the change from vermilion to cinnabar, 

 and cn+ from cinnabar to wild. 



A series of remarkable studies by the groups of Ephrussi, Tatum and 

 Butenandt led to the discovery that v+ is kynurenine, a product of the 

 oxidation of tryptophan ; cn+ was later identified as hydroxykynurenine, a 

 further step in tryptophan oxidation. Both substances are used up during 

 the transformation of the eye disks, they are intermediates in the bio- 

 synthesis of normal eye pigments (Kikkawa, 1941). It was clear that 

 mutation of the two genes each blocks one specified step in the oxidation 

 of tryptophan. Ephrussi suggested (1942) among other possibilities that the 

 genes involved control the formation of specific oxidation enzymes. 



Further study of eye pigment synthesis from the enzymological point of 

 view looked very difficult, but it was clear that the type of approach was very 

 promising and that great developments both in genetics and in biochemistry 

 would follow once a material more amenable to enzymological studies 

 is found. 



rf^ 



CO — CHg CH COOH /'%^/-'^^ ^^2 CH COOH 



OH 



Kynurenine Hydroxykynurenine 



Fig. 7. 



Neurospora crassa was such a material. The vegetative form of this 

 mould, the mycelium, which can be grown easily in fairly large batches, 

 produces asexual spores, the conidia. These make it possible to keep col- 

 lections of various strains and to grow unlimited amounts of a given 

 mycelium, the enzymes of which will easily be studied by classical bio- 

 chemical methods. On the other hand, Neurospora offers many features 

 which are of great interest for genetical studies. The mycelium is haploid ; 

 therefore, problems of dominance in the expression of the genetic char- 

 acters are avoided. Sexual reproduction is easily obtained by fusion of 

 mycelia of opposed mating types ; the zygote develops into an ascus con- 

 taining eight haploid spores from which the mycelium is obtained (Beadle, 

 1947). 



Auxotroph mutants, that is mutants which require for growth a sub- 

 stance that the wild strain can dispense with, proved especially suitable for 



B 



