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STERLING EMERSON 



leaving the requirement for lysine. These heterocaryons were more vigorous 

 than the original lysineless strain, but no more vigorous than the pure double 

 mutant strains extracted from the heterocaryons. 



In studies on reverse mutation in a leucineless strain (33757), Ryan and 

 Lederberg (1946) found that heterocaryons, whose nuclei differed only in 



HOURS 



Fig. 12.8 — Growth curves of heterocaryons between lysineless (/y, +) and lysineless, 

 arginineless (/y, arg) strains of Neurospora at 35° on minimal medium. Curve 1 : heterocar- 

 yon in which both nuclear types were of mating type A; curves 2 to 5: heterocaryons made 

 up of nuclei of different mating types (/v, +, A and /v, arg, a) — cf. Beadle and Coonradt 



' (1944). 



that some carried the wild type allele and some the mutant allele of the 

 leucineless gene, almost invariably reverted to the homocaryotic condition. 

 By the time growth had reached the end of a tube containing minimal me- 

 dium, nothing but wild type nuclei remained. In tubes containing limiting 

 concentrations of leucine, nothing but leucineless nuclei were present after a 

 short period of growth. This was under conditions where the growth rate of 

 the leucineless strain is considerably less than that of wild type. Under both 

 of these conditions, the heterocaryon is at a strong disadvantage compared 

 to its components. It is not known whether or not there is a particular con- 

 centration of leucine which would favor the heterocaryon. 



