The nucleolus: when 2 became 3.

Though the nucleolus is considered today as a multifunctional domain, its primary function is ribosome biogenesis. We have shown at the ultrastructural level that there are primarily two types of nucleolar organization: nucleoli containing three components in amniotes and two components in all other eukaryotes. In a recent report we made the additional, and surprising, finding that both types of nucleolar arrangement are found among living reptiles, viz. a bicompartmentalized nucleolus in turtles and a tricompartmentalized nucleolus in lizards, crocodiles and snakes. This latter organization occurs regardless of the species, the tissue or the developmental stages analyzed. These results are compatible with the view that the transition between bipartite and tripartite nucleoli coincided with the emergence of the amniotes within the Reptilia. They also support the previous hypothesis that turtles are primitive reptiles. The emergence in amniote vertebrates of a third nucleolar compartment might have imparted novel regulatory functions to the nucleolus, as well as perhaps, expanding the adaptability of ribosome synthesis to an ever changing environment, thus, enhancing the overall fitness of amniotic vertebrates.