MICROTUBULE ORGANIZATION DURING THE EARLY DEVELOPMENT OF THE PARTHENOGENETIC EGG OF THE HYMENOPTERAN MUSCIDIFURAX UNIRAPTOR

The origin of the zygotic centrosome is an important step in developme ntal biology. It is generally thought that sperm at fertilization play s a central role in forming the functional centrosome which subsequent ly organizes the first mitotic spindle. However, this view is not appl icable in the case of parthenogenetic eggs which develop without the s perm contribution. To clarify the problem of the origin of the zygotic centrosome during parthenogenetic development, we studied a hymenopte ran, Muscidifurax uniraptor. Antitubulin antibody revealed that after activation several asters assembled in the egg cytoplasm. The number o f asters varied in relation to the cell cycle. They became visible fro m anaphase of the first meiotic division and increased in number as me iosis progressed, reaching a maximum at the first mitosis. From anapha se-telophase of the first mitosis they decreased in number and were no longer found during the third mitotic division. To elucidate the natu re of these asters we performed an ultrastructural study with transmis sion electron microscopy and immunofluorescence with antibodies agains t anti-gamma-tubulin and CP190. In this way we showed the presence in these asters of centrosomal components and centrioles. Our observation s suggest that the cytoplasm of Muscidifurax eggs contains a pool of i nactive centrosomal precursor proteins becoming able to nucleate micro tubules into well-defined asters containing centrioles after activatio n. (C) 1998 Academic Press.