CYCLES OF PLANT VIRUSES IN INSECT VECTORS 167 



these experiments. In one case he obtained transovarial passage of virus to 

 the sixth generation of progeny. The virus was retained for at least 374 days 

 and infections were obtained in about 1200 rice plants by 82 infective leaf- 

 hoppers derived from a single viruliferous female. One egg from this female 

 gave rise in subsequent generations to leafhoppers that infected more than 

 1000 plants. No consistent or progressive reduction in the infective ability 

 was detectable throughout the generations, and Fukushi concluded that, 

 since the original amount of virus in a leaf hopper, and especially in an egg, 

 must have been extremely small, it seemed necessary to assume the multi- 

 plication of the virus in the leafhopper to account for the results. Usually, 

 there was an incubation period of 7 to 38 days before newly hatched nymphs 

 began to infect plants, and, after the insects became infective, they remained 

 so for as long as 88 days. Fukushi considered it likely that the incubation 

 period after hatching represented the time required for the virus to multiply 

 in the insect. The frequency of transmission varied from that in a few 

 viruliferous females, which infected no plants during their entire life but 

 produced infective progeny, to that in others, which infected plants consis- 

 tently on consecutive days. The infective capacity of some leafhoppers was 

 reduced or lost, especially in old age. 



In leafhoppers which did not transmit during their life, but which produced 

 infective progeny, Fukushi suggested that the virus might have been inhibited 

 in its multiplication and localized by chance m the ovarian tubules. He 

 noticed that when nonviruliferous females were mated with viruliferous males, 

 the virus-free progeny had a greater abiUty to acquire and transmit than 

 did those derived from nonviridiferous parents. He also noted that an average 

 of 85 % of the offspring of infective parents proved to be infective while, on 

 the average, only 60 % of those derived from infective females and nonviruli- 

 ferous males became infective. He postulated a dominant genetic factor for 

 susceptibility in the leafhopper to explain these differences. Such a genetic 

 factor was also suggested to explain the striking difference between the 

 percentage (12 %) of infective progeny from nonviruliferous parents, that 

 (68 %) from crosses between infective females and nonviruliferous males, and 

 that (92 %) from infective males and infective females when such progeny 

 were reared on dwarf diseased rice plants. Fukushi considered that the long 

 incubation period in the leafhoj^per, varying from 10 to 73 days, was too long 

 to represent the time necessary for virus merely to migrate through body 

 fluids and tissues. 



In 1941, Kunkel reported in detail his experiments on the cure of Vinca 

 rosea and Nicotiana rustica plants infected with aster yellows. The treatments 

 were carried out at higher temperatures, usually 38 to 42°C., and for longer 

 periods of time, usually 2 or 3 weeks, than were necessary for the inactivation 

 of the virus in leafhoppers. These experiments demonstrated that the virus 



