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by leaf hopper transmission) may be interpreted in terms of interfer- 

 ence between a virulent and an avirulent strain if we postulate, not 

 that the virus is a "mixture" of two different, independent strains, but 

 that there are two mutant forms of virus in equilibrium in recovered 

 plants, because of mutations in both directions, of selection pressures, 

 or both. Recovery would correspond to the condition in which the mild 

 virus predominates, although both viruses are present. Graft trans- 

 mission from a recovered plant would give a mild disease because of 

 massive transmission of a virus population already in equilibrium, 

 with an opportunity for the mild mutant to prevent a quick spread of 

 the virulent one. We should suppose that the insect transfers the severe 

 disease, not necessarily because it selects the virulent virus, but be- 

 cause it transmits it in such a way that its spread is not checked by 

 concurrently inoculated mild virus. This in turn could be due to se- 

 lective growth of the virulent virus in the insect, or to transmission of 

 small virus amounts with chances for one virus type only to be in- 

 troduced in each cell or group of cells, or to lower ability of the mild 

 virus to spread if inoculated in small amounts. In tomatoes, which 

 recover less frequently, the severe virus may be less easily overcome 

 by the mild mutant because of any one of a number of conceivable 

 possibihties. 



This suggestion is not intended to "explain" the results, but only 

 to point out how genetic considerations may be useful in suggesting 

 alternative interpretations of complex cases in virus-host relationship. 



