Characterization of a reproductive isolation gene, zygotic hybrid rescue, of Drosophila melanogaster by using minichromosomes

Abstract

Hybrids carrying the wild type allele of zygotic hybrid rescue (zhr) of Drosophila melanogaster and the maternal cytoplasm of its sibling species (D. simulans, D. mauritiana or D. sechellia) are embryonic lethal, irrespective of their sex. The zhr gene has been localized in the X heterochromatin, slightly distal to the In(1)sc8 breakpoint, a location where highly repetitive satellite DNA exists. A free duplication minichromosome, Dp(1;f)1162, which is derived from the In(1)sc8 chromosome by a massive interstitial deletion has a significant, but not complete, effect on the hybrid lethality. In order to examine the organization of zhr cytogenetically, we synthesized four deletions from Dp(1;f)1162 by γ-ray irradiation. Three of them, Dp(1;f)1162S2, Dp(1;f)1162S3, and Dp(l;f)1162S5, showed a hybrid lethal effect not significantly different from Dp(1;f)1162, but one, Dp(1;f)1162S4, showed a significantly weaker hybrid lethal effect. Interestingly, hybrids were completely lethal when two copies of Dp(1;f)1162S4 were present. That the lethal effect is proportional to the number of the minichromosomes supports the model that a class of repeated sequences around the Dp(1;f)1162S4 breakpoint may be involved in the zhr gene activity. In the future, the mini-chromosome could be useful in the cloning and characterization of this reproductive isolation gene.