42 VARIATIONS IN THE LOWER FUNGI 



with two haploid nuclei in each cell. Such a dikaryophyte presents 

 special problems in genetics peculiar to the fungi. Eventually in 

 some of these binucleate cells the nuclei fuse, and the resulting diploid 

 nucleus divides to give rise to the haploid basidiospores or sporidia. 

 Hyphal fusions are not, however, confined to the Basidiomycetes. 

 They have been observed to occur in a variety of fungi. Vegetative 

 anastomoses occur between neighboring filaments of a single plant, 

 and between neighboring colonies of imperfect fungi. Such fusions 

 do not result in sexual spores. Laibach =^° and Kohler ^^ have called 

 attention to the morphologic and physiologic similarity between these 

 vegetative anastomoses and hyphal fusions in the smuts. 



The genetic significance of such hyphal fusions is not yet clear. 

 Hansen and Smith ^^ noted the occurrence of hyphal fusions between 

 different single-spore races of Botrytis cinerea, and believed that they 

 resulted in mycelia containing nuclei from both of the parent strains. 

 In this species, as in numerous other imperfect fungi, the conidia as 

 well as the hyphal cells are multinucleate and these authors state 

 that "a multinuclear spore is, therefore, not an individual but, in 

 reality, a colony, and it can, therefore, not give rise to a genetically 

 pure culture unless all of its nuclei are genetically identical." They 

 observed variants in the offspring of cultures derived from hyphal 

 fusions occurring between two different races, and suggested that 

 such strains owe their instability not to mutation but to nuclear 

 heterogeneity. In later work they "ctossed" two species, B. Allii 

 and B. Ricini, and obtained three types which they considered to be 

 new varieties or new species. Davidson, Dowding, and Buller ^ ob- 

 served the occurrence of hyphal fusions in several species of dermato- 

 phytes {Microsporum Audouini, M. Canis, Trichophyton mentagro- 

 phytes). They found that fusions occurred readily between differ- 

 ent strains of the same species, but not between different species, and 

 suggested this as a criterion for species identification. They did not 

 observe variants, and Emmons also failed to observe any evidence 

 of hybridization when different variants of M. gypseum were seeded 

 together. It should be noted here that Spring-^ failed to find any 

 . evidence of heterothallism in dermatophytes. 



Practical Considerations. What does all this signify to the prac- 

 ticing bacteriologist? It need not disturb his routine work very 

 much. It is not intended to give the impression that all fungi will 

 transform into new types on continued cultivation. This is a tend- 

 ency which has been observed in 'many species of the lower fungi 

 studied by medical and industrial mycologists. But, as with the 



