Oogenesis in turbellarians of the genus Geocentrophora studied by light and electron microscopy. V. Oocyte nuclear structures containing factors of pre-mRNA splicing and pre-rRNA processing

An immunoelectron study of nuclear distribution of pre-mRNA splicing and pre-rRNA processing factors was carried out for oocytes of two turbellarian species: the Baikal endemic Geocentrophora wagini and a cosmopolitan G. baltica. Using monoclonal antibodies against Sm-epitope of small nuclear RNPs (snRNPs) and SR-protein SC35, it has been shown that on different stages of oocyte growth splicing factors (snRNPs and SC35) are distributed within the whole nucleus. A fibrogranular material located near heterochromatin clumps is labeled with these antibodies. A fibrillar part of this material seems to represent perichromatin fibrils. The features of intranuclear distribution of splicing factors in Geocentrophora oocyte nuclei and their ultrastructural features suggest that pre-mRNA synthesis and splicing may occur up to the end of diplotene. In Geocentrophora oocyte nuclei a few nuclear bodies (NBs) were found. Splicing factors (snRNPs and SC35) and fibrillarin were revealed in these NBs. Homology of Geocentrophora oocyte NBs to coiled bodies of oocyte and somatic cell nuclei of other animals is discussed. During diplotene, Geocentrophora oocyte nucleoli were found to lose their granular component and to change to large fibrillar structures named "postnucleoli". The postnucleoli contain both fibrillarin and non-nucleolar spliceosomal components (snRNPs and SC35). Geocentrophora oocyte postnucleoi are compared with similar structures of mammalian oocyte nuclei, taken as an example of morphological convergence of nuclear structure organization in phylogenetically distant animal species.