CYCLES OF PLAKT VIRUSES IN INSECT VECTORS 173 



Maramorosch (1958b) has demonstrated that if the Rio Grande strain of 

 corn stunt virus is first acquired by the vector Dalbulus maidis, it cannot 

 subsequently transmit the Mesa Central strain. However, in the leafhopper, 

 protection in the reverse direction is incomplete and, in plants simultaneously 

 infected with both strains, symptoms of the Mesa Central strain may be 

 overcome and suppressed by those of the Rio Grande strain. 



Recently, Littau and Maramorosch (1956, 1958) have reported the first 

 discovery of a cytopathogenic effect of a virus in its leafhopper vector. Such 

 an effect was searched for previously but unsuccessfully by Kunkel (1926), 

 Dobroscky (1931a,b), and Fukushi (1934). Littau and Maramorosch reported 

 that the aster yellows virus caused changes, which are more readily observed 

 in the male than in the female, in the fat body cells of Macrosteles fascifrons. 

 In these ceUs, the cytoplasm becomes sparse, the nuclei become stellate, and 

 the cell membranes become indistinct. This provides evidence against the 

 possible multiplication of aster yellows virus in the presumed symbionts 

 (Black, 1953b, p. 411) which occur in another tissue, the mycetome. Finally, 

 Jensen (1958) demonstrated that the average life of leaf hoppers oiColladonus 

 montanus that acquired and transmitted peach yeUow leaf roll virus was 

 shorter (22 days) following acquisition than that (55 days) of insects that 

 failed to transmit. This finding indicates that this virus multipKes in and is 

 pathogenic to its leafhopper vector. 



The cumulative evidence from these many studies now clearly indicates 

 that there are many leafhopper-borne viruses which infect their leafhopper 

 carriers as surely as they infect their plant hosts although in most cases the 

 infection of the leafhopper is inapparent. 



IV, Evaluation of Kinds of Evidence for Multiplication 



The above history of the problem of multiplication of certain viruses in 

 both plants and insects reveals a considerable variety of approaches. It 

 would seem worthwhile at this stage briefly to recapitulate and reconsider 

 these techniques, keeping in mind their possible application to other viruses. 

 Some of the techniques have been enormously expensive in both time and 

 effort; it would be an advantage to use the less costly approaches if they are 

 adequate. It should be remembered that all techniques must ensure that any 

 increase in virus that is demonstrated cannot be due to fresh acquisition of 

 virus from plants. 



There are two techniques which have been used, which admit of no other 

 interpretation than multiphcation in the vector. They are: 



(1) Serial passage of virus from insect to insect by injection technique 

 until the dilution attained exceeds with certainty the maximum dilution of 

 the starting material that can be successfully inoculated. This technique 



