260 CELL HEREDITY 



The close phenotypic interaction of chromosomal and nonchromosomal 

 genes is illustrated in this system by a chromosomal gene, A, which 

 increases the streptomycin-resistance level of sr-500 strains and of 

 chromosomal sr-100 strains as well. 



Other examples of nonchromosomal genes have been investigated in 

 the molds, for example, genes which determine senescence and barrage in 

 Podospora. Senescence, the death of aging mycelia, is a common occur- 

 rence in many molds. In Podospora, it is possible to make reciprocal 

 crosses between the fruiting bodies of aging and of young mycelia, and 

 thereby to study the inheritance of senescence. It is found to be mater- 

 nal. Further evidence of its nonchromosomal nature comes from my- 

 celial fusions between young and senescent hyphae. In the absence of 

 nuclear migration, there is rapid infection of the senescent mycelium 

 by hereditary determinants which convert it back to the youthful state, 

 as shown by subsequent crosses. Thus, in these strains, senescence is the 

 result of the loss or alteration of a nonchromosomal hereditary determi- 

 nant, capable of propagation by infection through a mycelial fusion. 



In the barrage phenomenon (Figure 9.9), strains of Podospora having 

 mycelia which form a line at the point of contact are found to differ by a 

 single gene pair, S/s. However, in crosses of strain S x s, the s phenotype 

 is lost and in its place a new class of progeny appear in which the my- 

 celium is compatible both with S and with s. This new phenotype, 

 called s\ must be attributed to a genetic alteration since it is stable in 

 vegetative growth and in crosses. However, it shows strictly maternal 

 inheritance. 



Although the / property is stable, occasional mutations occur, always 

 back to s. Most interesting, the / property can be lost by infection. 

 When mycelial grafts are made between / and s strains, the s property 

 rapidly spreads through the / mycelium, in the absence of nuclear 

 migration, converting the / strain completely back to s. It is inferred 

 that the / property results from the loss of a cytoplasmic determinant 

 from the s strain, owing to interactions with S cytoplasm at meiosis and, 

 as a result, the s mycelium becomes compatible with S. Evidently, this 

 determinant can rapidly re-establish itself in an / mycelium after re- 

 introduction by fusion. 



Barrage is of particular interest because it combines the features of an 

 apparent gene mutation occurring at meiosis with maternal inheritance 

 of the mutation, and infectivity of a cytoplasmic factor. The apparent 

 gene mutation turns out upon analysis not to be chromosomal at all, but 

 rather it is a cytoplasmic change resulting from gene interaction in the 

 ascus at the time of meiosis. Formally, the apparent 100 per cent muta- 



