NONCHROMOSOMAL GENES 257 



terminant in the segregational petite strains. In order to obtain a normal 

 diploid from the fusion of a segregational with a vegetative petite, the 

 segregational petite must contribute the nonchromosomal factor which 

 the vegetative petite lacks. Thus, in this system, it is shown that non- 

 chromosomal genes are maintained and replicated despite their inactive 

 status, just as are the chromosomal genes. 



In the mutants of Neurospora with altered cytochrome activities, the 

 morphological picture is quite different. Here it seems likely that some 

 structural alteration in mitochondrial organization is involved in the 

 phenotype. No mitochondria comparable to those of wild-type strains 

 are found in the poky mycelia, although cytochrome activity is associated 

 with sedimentable membranes which are present throughout the cyto- 

 plasm. Thus, the altered cytochrome activity may be a secondary con- 

 sequence of some more basic defect. 



A related finding of interest comes from the mycelial fusion of wild- 

 type strains with poky, forming what have been called heterocyto- 

 somes because they contain two kinds of cytoplasm. Initially these 

 fused mycelia grow at the wild-type rate, but gradually they slow down, 

 and eventually become typically poky mycelia. In subsequent crosses, 

 only the poky factor is transmitted. Thus, the mutant elements replace 

 the wild type, and the nuclear composition has no effect upon the out- 

 come. This result is surprising since one would expect to find selection 

 operating against the mutant. In heterocytosomes composed of two 

 mutant strains, poky and mi-S (a different cytochrome-deficient non- 

 chromosomal mutant), the growth rate is slow, indicating that although 

 these mutations differ in some way, as shown by their cytochrome 

 activity patterns, they are not complementary. Another pair of non- 

 chromosomal mutants, mi-S and mi-4, do complement in the heterocytosome. 



Taken together, these studies indicate that mitochondrial heredity can 

 be influenced by mutation of either chromosomal or nonchromosomal 

 genes. However, as with chloroplasts, there is as yet no definitive in- 

 formation on the location or identity of the responsible nonchromosomal 

 determinants. 



NONCHROMOSOMAL HEREDITY INVOLVING NO KNOWN STRUCTURES 



In the early investigations of nonchromosomal heredity in higher 

 plants, a large number of morphological phenotypes were involved, es- 

 pecially in interspecies crosses which provided a wide diversity of traits 



