Rescue from the abnormal oocyte maternal-effect lethality by ABO heterochromatin in Drosophila melanogaster.

The euchromatic maternal-effect mutation abnormal oocyte (abo), of Drosophila melanogaster interacts with regions of heterochromatin known as ABO, which reside on the X, Y and second chromosomes. Here, we show that survival of progeny from abo females depends in part upon the maternal dosage of ABO heterochromatin. A comparison was made of the recovery of genotypically identical progeny from abo mothers bearing sex chromosomes of various ABO contents. The results show that the recovery of daughters was decreased if mothers were ABO-/ABO-. However, no decrease was observed if mothers were ABO+/ABO-. In addition, the survival of daughters was greater when they received an ABO-X chromosome from an ABO-/ABO+ mother rather than the father. We suggest that these results reflect a complementation or interaction between the ABO-deficient X and the ABO heterochromatin in the maternal genome. This proposed interaction could occur early in oogenesis in the mother or prior to completion of meiosis I in the fertilized egg. To determine if zygotic dosage of ABO heterochromatin might also be important at very early stages of embryogenesis, we examined the timing of zygotic rescue by paternally donated ABO heterochromatin using a second mutation, paternal loss (pal). Homozygous pal males produce progeny which lose paternally derived chromosomes during the early zygotic divisions. Zygotes that have lost a paternal sex chromosome in a fraction of their nuclei will be mosaic for the amount of ABO heterochromatin. By monitoring the recovery of pal-induced mosaics from abo and abo+ females, we could determine the temporal and spatial requirements for ABO function. Results show that the survival of progeny from the abo maternal-effect lethality was increased if ABO heterochromatin was present prior to the pal-induced loss event. Analysis of mosaic patterns did not reveal a specific lethal focus. We conclude from these results that ABO heterochromatin serves its vital function prior to completion of the early cleavage divisions in progeny of abo mothers.