Nucleolar organizer regions, their function and polyploidy in fishes

Birstein VJ. Vasilev VP.

The problems of nucleolar organiser regions (NORs) and nucleoli numbers, and rRNA content in fishes have been discussed. The caryotypes ot four Actpenserids (Acipenser guldenstaedti, A. ruthenus, A. stellatus and Huso huso) have been studied. Diploid species two active NORs per karyotype and two nucleoli per nucleus; tetraploids, which were intensively diploidized, have two (mean number) NORs and nucleoli, and other tetraploids have 4-10 NORs. Octoploids always have more than two NORs and nucleoli. Although in tetraploid fishes the rRNA gene numbers is approximately two to three times that in diploids. rRNA content per cell is almost constant in all fishes, and there is a correlation between the number of active NORs or RNA genes and the nuclear volume. A similar situation was found in di- and tetraploid anurans, although it is evident that anurans have other mechanism of NORs and rRNA level regulation. Among the other vertebrates a high number of NORs per caryotype and nucleoli per nuclei is characteristic only for mammals. It has been proposed that after appearance of warm-blooded animals in the course of vertebrate evolution the mechanisms of control of rRNA transcription and processing changes. In fishes the processes of tetraploidization and subsequent diploidization are a way of increasing of polymorphism of protein and rRNA genes structure.