180 L. M. BLACK 



developmental stages in the vector. He believed that because of the sensi- 

 tivity of the method the eclipse period appeared to be real and that there was 

 evidence for the existence of two developmental stages, infective (mature) 

 and noninfective (vegetative) virus. The only one of the propagative viruses 

 for which it is possible, at present, to estimate the sensitivity of the insect 

 injection method is the wound tumor virus. It has already been estimated 

 that the lowest concentration of particles of this virus that will react serolo- 

 gically is about 10^ per milliliter and that this represents a virus preparation 

 from insects diluted to 10~^*^, when the concentration in insects themselves is 

 taken as unity. The end point for infectivity in insect injection experiments is 

 greater. One infection has been obtained at a dilution of 10~^ and 8 at 10~^. 

 Since the volume of solution injected into insects averaged about 0.1 fi\. 

 (Black and Brakke, 1952), this would indicate that infection was obtained, in 

 the one case, with about 10-*^ virus particles and, m the other cases, with 

 about 10^-^. The former figure is an estimate of the minimum number of 

 wound tumor virus particles necessary for successful inoculation of the vector 

 by injection. The data indicate, however, that successful inoculation with 

 10^*^ particles is only attained if many trials are made; it would be safer to 

 consider 10^*^ particles as a practical minimum number with present tech- 

 niques. These minimum numbers are much lower than those for any other 

 infectivity assay of virus that infects plants. Nevertheless, the calculation 

 indicates that the insect injection technique is not sensitive enough to 

 ascertain whether or not an eclipse period really exists — a possibHty which 

 was considered in Maramorosch's (1953a,b) discussions. It indicates, instead, 

 that a considerable increase in fully formed infective virus occurs during that 

 part of the incubation period that has been suggested as the eclipse period, 

 before the virus concentration reaches a level great enough to be detected. 

 This does not mean there is not an eclipse period during the cycle of propaga- 

 tive viruses in insects; it does indicate that, at present, we have no direct 

 evidence for such an eclipse period. What evidence there is on the reproduction 

 of other viruses would suggest that an ecUpse period is a part of the virus 

 cycle of reproduction in the plant and insect, before and after the end of the 

 incubation period. The presence of soluble viral antigen of wound tumor in 

 both insect and plant supports this probability. 



In speculating upon the possible reasons for the high mortahty in the 

 second passage of virus in series through the vector, Maramorosch (1953a,b) 

 suggested that the aster yellows virus, in its behaviour toward the vector, 

 might be comparable to a lysogenic phage in its relationship to the bacterial 

 host. However, except for a few days at the beginning of the incubation 

 period, the insect is the best source of infective virus of which we know. This 

 is the opposite of the lysogenic relationship. The recent finding of a cytopatho- 

 genic effect in the vector itself is also contrary to the lysogenic relationship. 



