Drosophila parthenogenesis: a model for de novo centrosome assembly

The Drosophila egg contains all the components required to properly execute the early mitotic divisions but is unable to assemble a functional centrosome without a sperm-provided basal body. We show that 65% of unfertilized eggs obtained from a laboratory strain of Drosophila mercatorum can spontaneously assemble a number of cytoplasmic asters after activation, most of them duplicating in a cell cycle-dependent manner. Such asters are formed by a polarized array of microtubules that have their Asp-associated minus-ends converging at a main focus, where centrioles and typical centrosomal antigens are found. Aster assembly is spatially restricted to the anterior region of the oocyte. When fertilized, the parthenogenetic egg forms the poles of the gonomeric spindle by using the sperm-provided basal body, despite the presence within the same cytoplasm of maternal centrosomes. Thirty-five percent of parthenogenetic eggs and all unfertilized and fertilized eggs from the sibling bisexually reproducing D. mercatorum strain do not contain cytoplasmic asters. Thus, the Drosophila eggs have the potential for de novo formation of functional centrosomes independent of preexisting centrioles, but some control mechanisms preventing their spontaneous assembly must exist. We speculate that the release of the block preventing centrosome self-assembly could be a landmark for ensuring parthenogenetic reproduction.