Comparative nucleolar ultrastructure analysis and life-history traits in three zooplankton species

The nucleolus is the most conspicuous ribonucleoprotein membrane-less nuclear subdomain where ribosome production occurs, and its ultrastructural morphology and organization reflect the metabolic activity of the cell. At this level, three regions are observed that correspond with the ribosome biogenesis process: the fibrillar centers (FC), the dense fibrillar region (DFC), and the granular region (GC). Nucleoli that show the three regions are denominated tripartite nucleoli and bipartite nucleoli when the FC are not evident. In this study, we examined nucleolar morphology at the ultrastructural level in three zooplankton species representing different evolutionary lineages and reproductive strategies: the rotifers Mniobia sp. and Euchlanis dilatata , and the cladoceran Moina macrocopa . Conventional Transmission Electron Microscopy (TEM) techniques with general contrast (Ur-Pb) and EDTA regressive staining, preferential for ribonucleoproteins, were used to analyze the nucleolar material. The results showed differences in nucleolar architecture among the obligately asexual rotifer Mniobia sp., which exhibited a bipartite nucleolar structure, in contrast to E. dilatata and M. macrocopa , both of which are cyclic parthenogens that show a tripartite nucleolar organization. These findings suggest that variations in nucleolar architecture could be linked to reproductive strategies and life-history traits in zooplankton species.