The late functions of Drosophila sigma virus.

Several criteria can be used to characterize the temperature-sensitive mutants of sigma virus: the lability of infectious centers initiated by the efficient viral particles following the inoculation; the localization of the temperature-sensitive period; the hereditary transmission of infection in a maternal Drosophila line at high temperature; the evolution of the CO2 sensitivity symptom and of the viral titer per fly for hereditarily infected flies kept at a permissive temperature and shifted to a restrictive one. Experimental data concerning the last two criteria are shown and discussed here. From all the criteria, three classes of ts mutants can be defined: the first class (prototype ts4) are the mutants blocked in hereditary transmission at high temperature. These mutants are affected during the whole viral cycle but the viral maturation is not directly affected. The second class (ts9, haP27) are the mutants directly affected in viral maturation. For these latter, the hereditary transmission in a maternal line is efficient at high temperature, and only a late temperature sensitive period is observed. The third class (prototype haP7) are the mutants for which only an early temperature-sensitive period is observed. In fact, for these mutants, the viral information is strongly heat-sensitive at the beginning of viral cycle. This defect is rapidly corrected, and the hereditary transmission, late viral functions and viral maturation are as efficient as those of wild type virus, at high temperature. Using temperature-sensitive mutants similar to the wild type virus at permissive temperature, a formal description of the viral cycle at 20 degrees C in the standard flies can be obtained. The long period necessary to perform an entire viral cycle (about 60 hours) is a result of the restrictive action on late viral functions at 20 degrees C of the Oe allele of the ref(3)O host gene present in the standard flies.