NONCHROMOSOMAL GENES 



or a Q or a 



261 



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S cS 



FIGURE 9.9. Origin and inheritance of "barrage" in Podospora. Mycelia of Podo- 

 spora produce male and female structures as in the related fungus Neurospora, and 

 consequently reciprocal crosses may be made between two strains of opposite mating 

 type. In crosses of S X s, the progeny segregate 2S : 2s^, with the total loss of the 

 s phenotype, and appearance in its place of the new property s^. In reciprocal crosses of 

 s' X S, the progeny segregate 2:2 normally. However, in crosses of s^ X s, the Fj 

 progeny all resemble the female parent, thereby showing nonchromosomal inheritance of 

 the difference between s and s^ Chromosomal genes segregate normally in these 

 crosses (after Rizet, Marcou, and Schecroun, 1958, Bull. Soc. Fran^aise Physiol. 

 Vegefale, 4:136). 



tion at meiosis, a highly unusual occurrence, resembles the paramutation 

 phenomenon recently discovered in maize. No cytoplasmic component 

 has been identified in paramutation, but one may surely wonder, none- 

 theless, whether the system has an explanation like that of barrage. 



An interesting aspect of the Podospora work is the evident infectivity 

 of the nonchromosomal determinants. The relationship between in- 

 fectivity and heredity is receiving widespread attention at present, and 

 has fundamental implications both in heredity and in disease. Let us 

 turn then to a consideration of other infectious hereditary agents which 

 are also extrachromosomal. 



