120 



WILLIAM J. ROBBINS 



Vigor in Heterocaryons 



Observations of Dodge (1942) on heterocaryosis in Neurospora are of 

 interest to the general problem of heterosis. Dodge inoculated three petri 

 dishes, one with his Dwarf 16 strain of Xeurospora telrasperma, one with race 

 C-8, and the third with mixed mycelium or conidia of both the dwarf and the 

 C-8 races. He observed that the mycelium of the mixed culture grew much 

 more rapidly and produced more abundant conidia than the mycelium of 

 either the dwarf or the C-8 races (Fig. 7.3). 



Fig. 7.3 — Heterocaryotic vigor in Neurospora telrasperma. Growth in 34 hours at room 



temperature in petri dishes. The myceUum of the two heterocaryotic races {16 -\- C 4 and 



16 + C8) has nearly covered the medium in the dishes; C4 and C8 have not grown halfway 



across the medium and Dwarf 16 has made no visible growth. 



When two races of Xeurospora telrasperma are grown together, there is a 

 migration of nuclei through the openings at the points of hyphal anas- 

 tomoses. The races need not be of opposite sex. After nuclear migration, the 

 cells of the resulting mycelium are heterocaryotic. They contain two kinds of 

 haploid nuclei. The greater vigor of the mixed culture referred to above ap- 

 pears to be the result of the presence in a common cytoplasm of two kinds of 

 nuclei. 



Heterocaryotic vigor does not always accompany heterocaryosis. Dodge 

 (1942) observed heterocaryotic vigor when the two races, Dwarf 16 and C-4, 

 were grown together. But heterocaryosis for races C-4 and C-8 did not result 

 in increased vigor in the mixed culture. Not all dwarf races act as race 16 

 does. Some of them evidence heterocaryotic vigor with both C-4 and C-8, 



