WOLBACHIA-INDUCED DELAY OF PATERNAL CHROMATIN CONDENSATION DOES NOT PREVENT MATERNAL CHROMOSOMES FROM ENTERING ANAPHASE IN INCOMPATIBLE CROSSES OF DROSOPHILA-SIMULANS

The behavior of parental chromosomes during the first mitosis of Droso phila simulans zygotes obtained from unidirectional incompatible cross es is described and it is demonstrated that the condensation of parent al chromatin complements was asynchronous. The timing of paternal chro matin condensation appeared to be delayed in these embryos, so that co ndensed maternal chromosomes and entangled prophase-like paternal fibe rs congressed in the equatorial plane of the first metaphase spindle. At anaphase the maternal chromosomes migrated to opposite poles of the spindle, whereas the paternal chromatin lagged in the midzone of the spindle. This resulted in dramatic errors in paternal chromatin inheri tance leading to the formation of embryos with aneuploid or haploid nu clei. These observations suggest that the anaphase onset of maternal c hromosomes is unaffected by the improper alignment of the paternal com plement. Since the first metaphase spindle of the Drosophila zygote co nsists of twin bundles of microtubules each holding one parental compl ement, we suspect that each half spindle regulates the timing of anaph ase onset of its own chromosome set. In normal developing embryos, the fidelity of chromosome transmission is presumably ensured by the rela tive timing required to prepare parental complements for the orderly s egregation that occurs during the metaphase-anaphase transition.