200 



STERLING EMERSON 



to be. The walls between them have perforations which permit both cyto- 

 plasm and nuclei to move from cell to cell. If all nuclei are identical, their 

 movement and distribution is probably of minor importance, but if they are 

 not identical there may be effects of considerable consequence arising from 

 irregularities in nuclear distribution. 



There are two ways in which a mixture of different kinds of nuclei within 

 a single cell may come about. In the growth resulting from a sexually pro- 

 duced ascospore, or from a uninucleate asexual microconidium, all nuclei 

 are directly descended from a single haploid nucleus. Barring mutation, they 

 should all have the same genetic constitution. After the growth has become 



Strain X 



Fig. 12.1 — Heterocaryon formation resulting from hyphal fusion (a diagram). 



multinucleate, if a mutation should occur in one nucleus, the descendants of 

 that nucleus would then have a different genetic constitution from the re- 

 maining nuclei in the common cytoplasm, and a condition of heterocaryosis 

 would exist. The second way in which heterocaryons arise is from the direct 

 fusion of branches or hyphae of different strains, with the subsequent in- 

 termingling of their nuclei. By the latter method, heterocaryons of pre- 

 determined genetic constitution can be made at will. 



The controlled production of heterocaryons is shown diagrammatically in 

 Figure 12.1. Strain X is represented as having black nuclei to distinguish 

 them from the nuclei of strain Y, which are pictured as being white. After 

 fusion between hyphae, nuclei of strain Y may migrate into cells of strain X, 

 and those of X into Y. It is possible that different hyphal tips, growing from 

 this common mass of cells, will have different relative numbers of the two 

 sorts of nuclei, as illustrated by the ratios 1:7, 1:1, and 7:1 in three of 



