KARYOTYPE, NUCLEOLUS ORGANIZER REGIONS AND CONSTITUTIVE HETEROCHROMATIN IN OSTREA-ANGASI (MOLLUSCAE, BIVALVIA) - EVIDENCE OF TAXONOMIC RELATIONSHIPS WITHIN THE OSTREIDAE

Chromosome preparations from gill tissue of the Australian flat oyster Ostrea angasi Sowerby were studied with conventional Giemsa staining, C-banding and silver staining techniques. A diploid complement of 2n = 20 was observed, consisting of five metacentric, three submetacentri c and two subtelocentric pairs. Constitutive heterochromatin was distr ibuted as large centromeric blocks in Chromosome Pairs 3, 6, 8, 9 and 10. The nucleolus organizer regions (NORs) were located terminally on long arms of Chromosome Pairs 9 and 10. This allowed the identificatio n of homologous chromosomes in submetacentric and subtelocentric pairs . Intraspecific variability in NOR pattern as revealed by differences in the number of silver-stained NORs (Ag-NORs) per cell was found to b e very common. Comparison of the patterns of karyotype, C-band and Ag- NORs between species of the larviparous oysters for which data have be en published demonstrate that the chromosomal structure of the endemic Australian and New Zealand species O. angasi shows little similarity to the Southern Hemisphere oysters O. (Eostrea) puelchana Orbigny and Tiostrea chilensis (Philippi) and the Indo-West Pacific oyster O. dens elamellosa, but very high resemblance to the European species O. eduli s.