Physical mapping of the 5S rRNA genes in the common sea urchin, <i>Paracentrotus lividus</i> (Echinodermata: Echinoidea), by in situ hybridization

Locations of the 5S rRNA genes (rDNA) are usually considered to be helpful, as generally invariable chromosome markers, especially if chromosome banding is not possible. The genes have been defined in about 90 invertebrate species, and were found linked to other multigene families in genomes of some nematodes and arthropods (Drouin and Moniz de Sa, 1995; Barzotti et al., 2000). They were also found co-localized by FISH with major rDNA clusters (NORs) and telomeric repeats in some annelids and molluscs (Vitturi et al., 2002, 2004). Here we report for the first time the chromosomal localization of 5S rDNA in a species of echinoderms, Paracentrotus lividus, which, compared to other echinoderms, has an exceptionally low chromosome number, 2n = 36 (Boveri, 1902). Its karyotype is composed of one pair of large submetacentric chromosomes, a pair of sex-specific heteromorphic chromosomes, and 16 pairs of subtelocentric chromosomes (Lipani et al., 1996). Materials and methods