It is also remarkable that the tripartite nucleolar organization is not general since the nucleoli of Drosophila and insects lack FCs (Knibiehler et al. 1982; Knibiehler et al. 1984) . It has been proposed that this diVerence in organization could be linked to the evolution of the rDNAs, in particular to the size of the intergenic sequences (Thiry and Lafontaine 2005) .