Cytogenetics in the sacoglossan Oxynoe olivacea (Mollusca : Opisthobranchia): karyotype, chromosome banding and fluorescent in situ hybridization

Developing embryos and sexually mature follicles of the male portion of ovo testis proved to be a suitable material as a source of cleaving cells for a dvanced cytological investigations on the sacoglossan species Oxynoe olivac ea Rafinesque, 1819 (Mollusca: Opisthobranchia). O, olivacea has a diploid chromosomal number of 30 made up of 15 pairs of which six are metacentric/s ubmetacentric (M/SM), four subtelocentric (ST) and five on the borderline b etween SM and ST. Correspondingly, 15 bivalents occur in spermatocytes at M etaphase I. Constitutive heterochromatin is scarce and restricted to small C-bands seen in five pachytene bivalents. The use of combined silver staini ng and fluorescent in situ hybridization (FISH) with a Paracentrotus lividu s (Echinodermata) 4.3 kilobase (kb) rDNA probe (prR14) consisting of sequen ces from the 3' end of 18S rDNA to the 3' end of 26S rDNA, revealed that nu cleolus organizer regions (NORs) are situated terminally on one arm of a sm all metacentric pair. The telomeric (TTAGGG)(n) sequence did not hybridize with termini of O. olivncea chromosomes.