Abstract
A variant of σ virus defective for maturation functions was studied. Stabilized flies for this virus were called ultra-ρ flies. They were not CO2-sensitive and extracts were not infectious. The presence of the virus was detected because it conferred to imagos a characteristic immunity against a superinfecting σ virus: these flies were not immunized against a superinfecting virus of the same group, such as vesicular stomatitis virus.
Wing discs were taken from larvae of two ultra-ρ strains (U-ρ46 and U-ρ751) and were exposed to superinfection by implantation into hosts infected with a non-defective σ virus. The blastemas were then implanted into a detector host able to support virus multiplication until the symptom appeared showing superinfection. Control experiments were made with originally virus-free discs. We have thus shown that the characteristic ultra-ρ immunity is present in the imaginal wing disc of ultra-ρ larvae. It is concluded that embryonic blastema cells contain ultra-ρ virus genomes.
In tests on the persistence of immunity through successive transfer generations the results differed with the ultra-ρ strain used. The detector hosts implanted with U-ρ46 blastemas were classified as early CO2-sensitive and CO2-resistant; the number of CO2-resistant hosts did not decrease and the U-ρ46 immunity was therefore stable. On the other hand, the detector hosts implanted with U-ρ751 blastemas were of three classes: early CO2-resistant and late sensitive hosts decreased as a function of time indicating that U-ρ751 immunity was unstable. This instability suggests that blastemas giving late sensitivity are mosaics of ultra-ρ cells and virus-free cells.