CYCLES OF PLANT VIRUSES IN INSECT VECTORS 181 



Kunkel (1938, p. 22) considered multiplication as the basis for the specific 

 relationship that exists between the aster leafhopper and the aster yellows 

 virus. The specificities in the relationships of leafhoppers and their viruses 

 have been discussed earlier (Black, 1953c). Since then an even more specific 

 relationship has been described by Maramorosch (1958a), who found that 

 Macrosteles laevis, the vector of European aster yellows virus, would not 

 transmit strains of aster yellows virus of either the eastern United States or 

 California, although these were transmitted by M. fascifrons. Work by 

 Maramorosch (1952b) has a bearing on the question of multiplication being 

 the basis for specificity. Preliminary evidence was obtained suggesting that 

 corn stunt virus could be transferred serially from vector to vector. 

 Maramorosch also obtained evidence that this virus and aster yellows virus 

 were able to maintain themselves, for at least 19 days, in certain nonvector 

 species of leafhoppers that had acquired them by feeding on diseased plants. 

 The fact that he could not detect virus in the nonvectors 2 days after acquisi- 

 tion, but could detect it 19 days after acquisition, may mean that some 

 multiplication of virus occurred in the nonvector species. However, attempts 

 to recover the two viruses after three blind serial passages in nonvector 

 leafhoppers failed, a result which indicated no multiphcation or limited 

 multiplication in the nonvectors. If multiplication does occur in nonvectors, 

 then the concept that multiplication determines specificity of transmission in 

 these viruses must undergo some modification. Also, curly top virus, for 

 which the weight of evidence appears to be against multiplication, is one of 

 the most specific in its vector relationships. 



Black (1941, p. 126) discussed the implications deriving from any demon- 

 stration that a plant virus could multiply in its insect vector. He considered 

 that it would provide strong evidence for the relationship of plant and 

 animal viruses and would serve as a connecting hnk between the two groups. 

 In fact, he considered that a virus multiplying in both insects and plants was 

 reaUy both a plant and animal virus. He also reasoned that it did not support 

 any precursor hypothesis of virus multiplication which required, in any 

 medium supporting virus replication, the existence of an element of approxi- 

 mately viral size, which, on undergoing a change involving a comparatively 

 small fraction of its total structure, became the active virus. He thought that 

 virus growth might be akin to the assimilation of simple compounds by 

 organisms. 



Kunkel (1911, p. 769) considered that aster yellows virus must be classified 

 as an obhgate heteroecious parasite, like the plant rusts, in that it must 

 alternate between widely different hosts. In the case of this virus the obliga- 

 tory alternation in nature is between an insect and a plant species. 



Black (1953b) emphasized the bearing of these viruses upon virus classifica- 

 tion, expressing the opinion that the current primary divisions of bacterial, 



