Cytogenetic characterization of Brachidontes pharaonis ( Fischer P., 1870): karyotype, banding and fluorescent in situ hybridization (Fish) (Mollusca: Bivalvia : Mytilidae)

The mussel Brachidontes pharaonis (Fisher P., 1870) (Bivalvia: Mytilidae) h as a diploid chromosomal set of 28 made up of 14 pairs of which eight are m ono-armed (ST) and six hi-armed (M+SM). Fourteen bivalents occur in spermat ocytes both at pachytene and metaphase-I. The use of combined silver and CM A(3) staining reveals that nucleolus organizer regions (NORs) are located t erminally on the long arm of a small subtelocentric chromosome pair (pair 1 4) and are compartmentalized in GC bast pairs. A Paracentrotus lividus (Ech inodermata) 4.3 kilobase (kb) rDNA probe (prR14) consisting of sequences fr om the 3' end of 18S rDNA to the 3' end of 26S rDNA was used to map the rDN A loci of B. pharaonis by fluorescence in situ hybridization; The results o btained with this technique confirm that NORs are located terminally on two small subtelocentric chromosomes (pair 14) and establish that the differen ce in dimension of homologous NORs is due to difference in the number of rD NA copies.