CYCLES OF PLANT VIRUSES IN INSECT VECTORS 163 



these conclusions. According to Fukushi, the entomologists at that time were 

 unaware of the virus nature of the disease and considered that the ability to 

 produce dwarf disease in rice was a character specific to the leaf hoppers native 

 to Shiga prefecture. Fukushi, on the other hand, convincmgly demonstrated 

 that the disease-producing agent was transmitted to the progeny when the 

 female was viruliferous and the male nonviruliferous, but not when the 

 female was nonviruliferous and the male viruliferous — in other words, that 

 the virus passed through the egg but not through the sperm. He also deter- 

 mined that nymphs hatching from eggs laid by noninfective females in 

 diseased plants were nonviruhferous. From these experiments, Fukushi 

 concluded that the virus entered the eggs in the ovary. 



Later, Fukushi (1934) reported that, in spite of the fact that this virus was 

 regularly transmitted through the egg and that inoculation feedings as short 

 as 5 minutes were sometimes successful, extensive tests indicated that 

 the virus was, sometimes at least, difficult for the vector to acquire. For 

 example, in one set of acquisition feeding periods varying from 3 to 50 days, 

 only 25 of 1300 leaf hoppers were demonstrated to have acquired the virus. 

 In such insects the incubation period varied from 10 days to 2 months. 

 Nymphs which acquired the virus transovariaUy sometimes transmitted the 

 virus immediately after emergence, but usually an incubation period of 1 to 

 14 days elapsed before newborn nymphs became infective. 



In the same paper, Fukushi reported that virus inclusion bodies occurred 

 in the chlorotic tissue of a number of grass species affected by rice stunt. 

 However, although he examined thousands of sections, he was unable to find 

 any such bodies or any other change of etiological significance in the sahvary 

 glands, alimentary canal, egg follicles, ovarian tubules, mycetome, or other 

 organs of viruhferous vectors. If either Kunkel in 1926 or Fukushi in 1934 had 

 been able to detect any deleterious effects of the virus upon the vector, it 

 seems likely that others would have been much more ready to accept the 

 idea of plant virus multiplication in the msect vector. 



In 1935, Fukushi reported his first experiment on the passage of rice stunt 

 virus through the egg of its leaf hopper vector to several succeeding generations 

 without access to virus in diseased plants. A single viruliferous female was 

 mated to a nonviruliferous male and allowed to oviposit eggs in rice plants. 

 At the moment of hatchuig, the young nymphs, which averaged 0.06 mg. in 

 weight, were removed from the plant in which the egg had been inserted and 

 transferred to a healthy young rice seedling. This process required patient 

 observation but ensured that the yomig insect had not sucked up virus from 

 the plant on which its viruliferous mother had fed and oviposited. Sub- 

 sequently, each nymph was transferred daily to a healthy young rice seedling 

 throughout its life of 2 months or so, a procedure which ensured that the 

 nymph could not acquire fresh virus from plants. Fukushi had obtained 



