On some structural and evolutionary aspects of rDNA amplification in oogenesis of Trachemys scripta turtles
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On some structural and evolutionary aspects of rDNA amplification in oogenesis of Trachemys scripta turtles Asya Davidian 1 & Elena Koshel 2 & Alexander Dyomin 1,3 & Svetlana Galkina 1 & Alsu Saifitdinova 4 & Elena Gaginskaya 1 Received: 21 March 2020 / Accepted: 12 August 2020 # Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract The features of rDNA amplification have been studied in oocytes of the red-eared slider Trachemys scripta using a number of specific histochemical and cytomolecular methods. A single nucleolus in early diplotene oocytes is associated with the nucleolus organizer region (NOR). With oocyte growth, the number of nucleoli increases dramatically and reaches hundreds by the lampbrush chromosome stage (pre-vitellogenesis). RNA-polymerase I, fibrillarin, and PCNA immunodetection in the amplified nucleoli and FISH of the 5’ETS probe to the oocyte nuclear content suggest pre-rRNA and rDNA synthesis in the nucleoli at all stages studied. This implies a continuous reproduction of the nucleoli during oocyte development from early diplotene up to vitellogenesis. The data obtained offer a different way for rDNA amplification and formation of extrachromosomal nucleoli in turtle oocytes compared with the amplified nucleoli formation in amphibian and fish oocytes. In the Sauropsida clade of Archelosauria, which includes turtles, crocodiles, and birds, rDNA function is known to be suppressed in avian oogenesis during the lampbrush stage (Gaginskaya et al. in Cytogenet Genome Res 124:251–267, 2009). Keywords Reptilia . Red-eared slider . Oocytes . Extrachromosomal nucleoli . rDNA amplification
Introduction Oogenesis is a particular type of cell differentiation responsible for the formation of a mature egg containing a maternal stock of macromolecules to ensure initial embryo development. Among others, a lot of ribosomal RNA (rRNA) are required to create an apparatus for massive protein synthesis in the embryo cells, which rapidly increase in number during cleavage and blastula formation, as was studied in detail in amphibian embryogenesis (in particular Davidson 1986). During the early development of Xenopus laevis, the nucleolus organization is not associated with the transcription process per se but rather with the presence of maternal unprocessed rRNAs (Verheggen et al. 1998, 2000). The source of
* Elena Gaginskaya [email protected] 1
St Petersburg University, St Petersburg 199034, Russia
2
ITMO University, St Petersburg 197101, Russia
3
Saratov State Medical University, Saratov 410000, Russia
4
Herzen State Pedagogical University of Russia, St Petersburg 191186, Russia
the maternal rRNA can be endogenous (a function of the oocyte inherent genome) or exogenous (a function of the genomes inherent to associated cells). Respectively, the peculiarity of a nucleolus organizer (NOR) functionality in the oocyte genome and the origin of the maternal rRNA stockpile in the oocyte define some important features related to the oogenesis type. Hypertranscriptional oog
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