CYCLES OF PLANT VIRUSES IN INSECT VECTORS 179 



Young and Frazier, 1954). C. tenellus from North Africa were shown to inter- 

 breed with C. tenellus from the United States and to transmit the North 

 American virus (Freitag et al., 1955); finally, the curly top virus was foimd in 

 Turkey and shoMTi to be transmissible by C. tenellus of the United States 

 (Bennett and Tanrisever, 1957). These findings support the hypothesis that 

 both the curly top virus and the vector were introduced into North America 

 from the Mediterranean region. 



The specific relations of this circulative virus are emphasized by the 

 demonstrated vector specificities of varieties of the virus. The variety distans 

 of Argentina is not transmitted by C. tenellus but by Agalliana ensigera 

 (Bennett et at., 1946); the variety hrasiliensis from Brazil is not transmitted 

 by Agalliana ensigera but by Agallia alhidula (Bennett and Costa, 1949); and 

 another Brazilian variety, solanacearum, is not transmitted by Agallia 

 alhidula but by Agalliana ensigera and Agallia sticticollis (Costa, 1952). All 

 these fuidings indicate that a high degree of specificity may be found in 

 circulative viruses. 



The curly top virus studies also revealed genetic differences in transmissive 

 ability of individual insects in C. tenellus (Bennett and Wallace, 1938). In 

 other words, even if the virus does not multiply in the vector, the frequency 

 with which individual infective insects transmit the virus and the proportion 

 of insects that become infective under given conditions is genetically deter- 

 mined. 



VI. Significance of Biological Cycles 



Kunkel (1926) considered that the incubation period of the propagative 

 aster yeUows virus in the vector is a period of multiphcation. Titration of the 

 virus content during the incubation period provides direct evidence that this 

 is so (Black, 1941; Maramorosch, 1953a). In two cases, Maramorosch (1953b), 

 was able to get transmission of aster yeUows virus on the day the vectors 

 were injected, but not on subsequent days. The fact that this occurred only 

 when the maximum possible dose of virus was injected supports the idea, 

 advanced by Kunkel (1937) in his interpretation of his heat-induced mcuba- 

 tion periods, that the virus must reach a certain concentration before 

 transmission can occur. Multiplication during the incubation period is sup- 

 ported by the evidence that the aster yeUows virus could be detected at low 

 concentration at the fourth day of an 18-day incubation period but increased 

 at least 100-fold between the beginning and end of the incubation period 

 (Black, 1941). Maramorosch's results (1953a, 1956) support these findings. 



Maramorosch (1953a,b) considered that the failure to detect virus by the 

 insect injection technique during the first few days of the incubation period 

 might indicate the existence of an eclipse period similar to that occurring 

 during phage reproduction. This would imply that the virus exists in different 



