12-17 THE YEAST CELL 



asci containing 4 light and 4 dark progeny, or 8 dark progeny. All 

 the asci in a given perithecium contain either four light or four 

 dark or 8 dark cultures. Lindegren has shown that in Neurospora 

 all the asci in a single perithecium (when matings are made as they 

 are in the Glome rella work) arise from a single pair of nuclei as- 

 sociated at the initiation of the perithecium. On this fact, the work 

 in Glomerella can be interpreted in terms of mutation from homo- 

 thallic light to dark, and the association either or two mutant nuclei 

 or one light and one dark nucleus at the initiation of the perithecium. 

 The homothallic light mutates to dark and two kinds of perithecia 

 are produced, the light/dark, or dark/ dark. Mutation occurs so 

 frequently that the light/light combination is only rarely found. 



PENICILLIUM AND ASPERGILLUS 



The discovery of penicillin has resulted in considerable interest 

 in variations among species of Penicillium in penicillin production 

 Derx proved by mating single ascospore cultures isolated from 

 Penicillium luteum that this species is heterothallic, but it is also 

 homothallic. Penicillium notatum, from, which penicillin is cur- 

 rently produced, is a form species rather than a true species. It 

 probable originated from Penicillium ascospores and is perpetu- 

 ated in nature asexually. The existence of heterothallic species 

 in Penicillium suggest that the sexual mechanism may be exploited 

 to obtain penicillin -producing strains. 



Pontecorvo has devised a technique for the analysis of homo- 

 thallic fungi which was applied to Aspergillus nidulans but 

 which could easily be applied to Penicillium luteum. 



"The technique as developed for A. nidulans is as follows: 



1) A ''balanced heterokaryon" is formed between two strains dif- 

 fering by two pairs of "biochemical" alleles and one or more pairs 

 of other allelesCbiochemical" or "morphological")- Indicating 

 with L/1 and h'/V the former, and A/a and B/b the latter, a bal- 

 anced heterokaryon will carry two kinds of nuclei (I) and (n), for 

 instance (L,l', A, B) and (1,L', a, b). The balanced heterokaryon 

 will be able to grow on a medium which does not supply either 

 nutrilite required by genotypes 1 and 1'. 



2) The heterokaryon forms zygotes by fusion of any two nuclei. 

 Selfed zygotes (1,1) and (n, U\ and crossed zygotes (I, n) will be 

 produced with frequencies (p+q)^ (where p is the proportion of nu- 

 clei (I) and q of nuclei (n) in the heterokaryon) provided fusion of 

 nuclei is at random, p and q can be determined independently; 

 e.g. in A. nidulans by sampling the uninucleate conidia. 



3) The zygotes produce haploid ascospores following meiosis. As 

 regards the "biochemical" alleles these spores will be of four kinds: 

 L, L'; L, 1'; 1, L'; and 1, 1'. On a medium supplying neither of the 

 nutrilites required by the 1 or 1' genotypes, only L,L' recombinants 



