CYTOGENETICAL LOCALIZATION OF ZYGOTIC-HYBRID-RESCUE (ZHR), A DROSOPHILA-MELANOGASTER GENE THAT RESCUES INTERSPECIFIC HYBRIDS FROM EMBRYONICLETHALITY

Hybrid females from crosses between Drsophila melanogaster males and f emales of its sibling species, D. simulans, D. mauritiana, or D. seche llia die as embryos. This lethality is believed to be caused by incomp atibility between the X chromosome of D. melanogaster and the maternal cytoplasm. Zygotic hybrid rescue (Zhr) prevents this embryonic lethal ity and has been cytogenetically mapped to a proximal region of the X chromosome of D. melanogaster, probably in the centromeric heterochrom atin. We have carried out high resolution cytological mapping of Zhr u sing deficiencies and duplications of the X heterochromatin. Deletions of the Zhr+ gene from the hybrid genome exhibit the Zhr phenotype. On the contrary, addition of the wild-type gene to the hybrid genome cau ses embryonic lethality, regardless of sex. The Zhr locus has been nar rowed down to the region covered by Dp(1;f) 1162 but not covered Dp(l; f ) 1205, a chromosome carrying a duplication of heterochromatin locat ed slightly distal to the In(l)sc8 heterochromatic breakpoint.