[Adaptation of a Sindbis virus population to "Drosophila melanogaster" (author's transl)].

The evolution of a Sindbis virus population during serial passages in drosophila has been studied. The adaptation of Sindbis virus to this unusual host was achieved within about twenty passages. Of course, the viral population went on evolving: a thermosensitive mutant appeared which excluded the original virus; later, a new type, different in plaques morphology, was observed: it did not exclude the former type but remained balanced with it. The adaptation of Sindbis virus to drosophila was revealed in two ways: first, in an improvement of the efficiency of infection in flies without any alteration of the plating efficiency on vertebrate cells; second, in an earlier and earlier invasion of flies which resulted from an accelerated rate of early virus production by the cells. A relationship between those two modifications is suggested. The adaptation of Sindbis virus to drosophila was probably the result of a rather high number of mutationnal steps which did not involve any concomitant change in the thermosensitivity of the viral population. An interpretation is presented.