CHARACTERIZATION OF A REPRODUCTIVE ISOLATION GENE, ZYGOTIC HYBRID RESCUE, OF DROSOPHILA-MELANOGASTER BY USING MINICHROMOSOMES

Hybrids carrying the wild type allele of zygotic hybrid rescue (zhr) o f Drosophila melanogaster and the maternal cytoplasm of its sibling sp ecies (D. simulans, D. mauritiana or D. sechellia) are embryonic letha l, irrespective of their sex. The zhr gene has been localized in the X heterochromatin, slightly distal to the In (1)sc(8) breakpoint, a loc ation where highly repetitive satellite DNA exists. A free duplication minichromosome, Dp (1;f)1162, which is derived from the In (1)sc(8) c hromosome by a massive interstitial deletion has a significant, but no t complete, effect on the hybrid lethality. In order to examine the or ganization of zhr cytogenetically, we synthesized four deletions from Dp(1;f)1162 by gamma-ray irradiation. Three of them, Dp (1;f)1162S2, D p (1;f)1162S3, and Dp (1;f)1162S5, showed a hybrid lethal effect not s ignificantly different from Dp(1;f)1162, but one, Dp(1;f)1162S4, showe d 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 minichrom osomes supports the model that a class of repeated sequences around th e Dp (1;f)1162S4 breakpoint may be involved in the zhr gene activity. In the future, the minichromosome could be useful in the cloning and c haracterization of this reproductive isolation gene.