Chromosomal location of the major ribosomal RNA genes in Crassostrea virginica and Crassostrea gigas

The chromosomal location of the major ribosomal RNA genes (rDNA) was studied in the eastern oyster Crassostrea virginica (Gmelin, 1791), and the Pacific oyster C. gigas (Thunberg, 1793) by fluorescence in situ hybridization (FISH). Probes were made by PCR amplification of the intergenic transcribed spacers between the 18S and 5.8S (ITS1), and between 5.8S and 28S RNA genes (ITS2), and labeled by PCR incorporation of DIG-11-dUTP. FISH with either probe on interphase nuclei showed two strong signals in both species. In C. virginica, FISH signals were located on the short arms of Chromosome 2, the second longest, with a centromere index of 0.39. In C. gigas, the rDNA probe hybridized to the long arms of Chromosome 10, the shortest chromosome. In both species, the FISH signals were confined to the telomere region of the chromosomes. All Crassostrea species have a haploid number of 10, and C. virginica and C. gigas share an almost indistinguishable karyotype. The size of the rDNA-bearing chromosome is the first major difference reported between the two karyotypes. This result suggests that considerable chromosomal evolution is possible in oysters despite the highly conserved haploid number.