Reciprocal Inheritance of Centrosomes in the Parthenogenetic Hymenopteran Nasonia vitripennis . U.K. Tram , W. Sullivan. Dept Biol, Univ California, Santa Cruz, Santa Cruz, CA.

   Of the mysteries surrounding virgin birth, the origin of the centrosome is perhaps the most puzzling. The centrosome is the microtubule organizing center of the cell and is usually assembled from the sperm basal body and maternally supplied products. In the insect order Hymenoptera, normal development can occur in the absence of fertilization. Unfertilized eggs develop as males while fertilized eggs develop as females. Little is known concerning the origin and inheritance pattern of centrosomes in Hymenoptera. Here we address this issue using real-time analysis of live embryos to follow centrosome, spindle, and nuclear behavior during initial development in Nasonia vitripennis. We show that centrosomes are assembled before the first mitotic division but that they are inherited differently in unfertilized and fertilized eggs. In both, large numbers of asters appear at the cortex of the egg after the completion of meiosis. In unfertilized eggs, these asters migrate inward and two become stably associated with the female pronucleus. Once this occurs no other asters associate with the female pronucleus and the remaining cytoplasmic asters rapidly disappear. In the fertilized egg, the sperm brings in paternally derived centrosomes that remain tightly associated with the male pronucleus. When pronuclear fusion occurs, the diploid zygotic nucleus is associated only with paternally derived centrosomes. None of the cytoplasmic asters associate with the zygotic nucleus and, as in unfertilized eggs, they rapidly degenerate. Unfertilized male eggs inherit maternal centrosomes while fertilized female eggs inherit paternal centrosomes. This is the first system described in which centrosomes are reciprocally inherited. Of particular interest, analysis of this parthenogenetic system demonstrates that selection and migration of the female pronucleus is independent of the sperm and its aster.