Relationships between the structural and functional organization of the turtle cell nucleolus

Area of Science:

• Cell Biology
• Evolutionary Biology
• Molecular Biology

Background:

• The nucleolus is crucial for ribosome formation in eukaryotic cells.
• Eukaryotes typically exhibit either tricompartmentalized or bicompartmentalized nucleoli.
• Turtles, an amniote group, display a unique bipartite nucleolar organization.

Purpose of the Study:

• To investigate the functional organization of the turtle nucleolus in detail.
• To understand the evolutionary implications of the turtle nucleolus's bipartite structure.
• To elucidate the dynamics of rRNA processing within the turtle nucleolus.

Main Methods:

• In vitro cell culture of turtle liver and spleen cells.
• Electron microscopy and silver-stained Nucleolar Organizer Region (Ag-NOR) staining.
• DNA labeling, actinomycin D and 5,6-dichlorobenzimidazole riboside treatments, BrUTP pulse-chase experiments, and 3D confocal microscopy.

Main Results:

• Turtle nucleoli are primarily composed of a fibrillar zone and a granular zone.
• The fibrillar zone contains DNA and condenses upon treatment with specific inhibitors.
• rRNA dynamics extend from UBF-positive sites in the fibrillar zone to the granular zone without UBF release.

Conclusions:

• The turtle nucleolus's bipartite organization reflects a less ordered ribosome biogenesis pathway.
• This structure may represent an evolutionary reversion within amniotes.
• Further research is needed to fully understand the implications of this unique nucleolar organization.