Abstract
1. The NSR-spirochetes injected to Drosophila simulans killed sons in the embryonic stage and the SR condition was established in this species by transmission of the spirochetes from mother to daughter through generations.
2. Inter-species hybrid males between D. melanogaster and D. simulans were killed by the NSR agent mostly at the stages of larvae and pupae, while the hybrid females were not affected.
3. Daughterless gene (da) killed the hybrid females in the embryonic stage.
