Nucleolar structure across evolution: The transition between bi- and tricompartmentalized nucleoli lies within the class Reptilia

Abstract

Two types of nucleolus can be distinguished among eukaryotic cells: a tri-compartmentalized nucleolus in amniotes and a bi-compartmentalized nucleolus in all the others. However, though the nucleolus' ultrastructure is well characterized in mammals and birds, it has been so far much less studied in reptiles. In this work, we examined the ultrastructural organization of the nucleolus in various tissues from different reptilian species (three turtles, three lizards, two crocodiles, and three snakes). Using cytochemical and immunocytological methods, we showed that in reptiles both types of nucleolus are present: a bi-compartmentalized nucleolus in turtles and a tri-compartmentalized nucleolus in the other species examined in this study. Furthermore, in a given species, the same type of nucleolus is present in all the tissues, however, the importance and the repartition of those nucleolar components could vary from one tissue to another. We also reveal that, contrary to the mammalian nucleolus, the reptilian fibrillar centers contain small clumps of condensed chromatin and that their surrounding dense fibrillar component is thicker. Finally, we also report that Cajal bodies are detected in reptiles. Altogether, we believe that these results have profound evolutionarily implications since they indicate that the point of transition between bipartite and tripartite nucleoli lies at the emergence of the amniotes within the class Reptilia.

Alu element‐containing RNAs maintain nucleolar structure and function

“…The nucleolus is a nuclear subcompartment that is found in almost all eukaryotic cells (Lamaye et al , ), while Alu elements are primate specific (Liu et al , ). This raises the question whether the mechanism revealed here in studying human cells can also operate in other eukaryotic organisms.…”

Reading the Evolution of Compartmentalization in the Ribosome Assembly Toolbox: The YRG Protein Family

“…The nucleolus has kept evolving along the evolution of chordates. For example, in Amniota (reptiles and mammals) the nucleolus has three subcompartments, instead of the two present in the rest of the eukaryotes [37,38]. The emergence of Gnl3 might have facilitated this evolution (Fig 2), complementing the function of Gnl2 and Gnl3l in the nuclear/nucleolar ribosomal biogenesis and maintenance.…”

Functional ultrastructure of the plant nucleolus

“…There is also a controversy concerning the presence of argyrophilic proteins. These proteins, including nucleolin, were identified in FCs of some animals (Ploton et al 1987 ; Hozák et al 1992 ), however, in other animals, nucleolin was not observed although silver-stained proteins were localized in FCs (Lamaye et al 2011 ). With respect to plants, although there are prevailing contentions that AgNOR proteins are absent from these nucleolar regions (Motte et al 1988b ; Moreno et al 1989a , 1989b , 1990 ; Wei et al 2003 ), some studies revealed silver grains in FCs of plants at the ultrastructural level after silver impregnation (Medina et al 1983b , 1986 ).…”