THE FORMATION OF HYPHAL FUSIONS 73 



Hansen and Smith x mixed two strains of Botrytis cinerea (a 

 fungus in which hyphal anastomoses between various strains are 

 readily formed), analysed their progeny and, as a result of their 

 work, suggested that " by the mechanism of anastomosis nuclei of 

 one strain may migrate into the cells of other strains and thus give 

 rise to cells and spores containing two or more kinds of genetically 

 different nuclei " ; and these authors also concluded that " variable 

 forms of Fungi Imperfect i may owe their instability, not to mutation, 

 but to nuclear heterogeneity (heterocaryosis), and that this condition 

 " can be brought about both in vivo and in vitro by nuclei of one strain 

 entering the cells of another strain through anastomoses, and that 

 the re-assortment of these diverse nuclei is accomplished by the 

 mechanism of anastomosis and unequal cell divisions." 



Whether or not the explanation given by Hansen and Smith for 

 the results of their observations is the correct one remains to be 

 determined by further work. 



In Ustilago zeae, although each yeast cell contains a single 

 nucleus, discontinuous variations (saltations, mutations), as the 

 work of Stakman 2 and others has shown, occur ver}^ frequently. 

 In this fungus at least, heterocaryosis of the kind postulated by 

 Hansen and Smith cannot possibly be the cause of the large amount 

 of variation. 



If, in Botrytis cinerea, one nucleus passed via an anastomosis into 

 a cell of another strain containing say five nuclei, so that the cell 

 became heterocaryotic, and from this cell a mycelium originated 

 which produced numerous spores having six nuclei like those of our 

 original cell, then conceivably we might obtain a new strain in which 

 all the spores would be alike ; but such an even and uniform 

 distribution of nuclei in the spores could scarcely be brought about 

 without the mechanism of conjugate nuclear division. So far as 

 I know no such conjugate nuclear division has been observed in 

 Botrytis or in any of its relatives. 



The fact that there are a great many strains in Botrytis cinerea 



1 H. N. Hansen and R. E. Smith, " The Mechanism of Variation in Imperfect 

 Fungi : Botrytis ciyierea," Phytopathology, Vol. XXII, 1932, pp. 953-964. 



2 E. C. Stakman, J. J. Christensen, C. J. Eide, and B. Peturson, " Mutation and 

 Hybridization in Ustilago zeae," Minn. Agr. Exp. Sta. Tech. Bull., No. 65, 1929. 



