Study of nucleolar organizer region in Palinurus elephas (Crustacea, Decapoda)

Among Palinuridae, Palinurus elephas (Fabricius, 1787) is the most karyologically studied species, but the knowledge on this family and on Decapoda in general is very scanty. The P. elephas chromosome complement is made up of a high and variable number of chromosomes (138-150) and the occurrence of supernumerary chromosomes has been pointed out. In this work we have studied the nucleolar organizer region (NOR); the number and localization of which are important sources of information for cytogenetic analysis, especially in Decapoda in which technical constraints in obtaining good chromosomal preparations make chromosome banding difficult to perform. Chomosomal preparations of P. elephas specimens have been obtained from testicular tissue by a modified air-drying method. Ribosomal genes have been localized by 45S rDNA FISH on medium sized chromosomes and on some small chromosomes, entirely labeled. The staining with chromomycin A3, fluorochrome which binds preferentially to GC-rich DNA, has pointed out bright fluorescent signals corresponding to the FISH signals, showing a GC-richness of this region. Moreover the treatment with silver staining, that stains trascriptionally active NORs, has shown a larger number of Ag-positive regions than FISH signals. This could be due to the presence of silver stainable centromeric proteins not involved in NOR