CHROMOSOMAL ASSIGNMENT OF rDNA IN THE AMERICAN AND PACIFIC OYSTER BY FLUORESCENCE IN SITU HYBRIDIZATION

¹ Haskin Shellfish Research Laboratory, Institute of Marine and Coastal Sciences, Rutgers University, 6959 Miller Avenue, Port Norris, NJ 08349 USA

² Department of Biological Sciences, University of Science in Philadelphia, 600 South 43rd Street, Philadelphia, PA 19104 USA

The chromosomal location of rDNA was studied in the American oyster (Crassostrea virginica) and Pacific oyster (Crassostrea gigas) by fluorescence in situ hybridization (FISH). A rDNA probe (about 500bp) was made by PCR amplification of the transcribed spacer between the 18S and 5S genes, and labeled by PCR incorporation of DIG-11-dUTP. FISH on interphase nuclei showed two strong signals in both species. In the American oyster, the hybridization signals were located on the short arms of Chromosome 2, the second longest with a centromere index of 0.39. In the Pacific oyster, however, the same sequence was located on the long arms of Chromosome 10, the shortest chromosome. In both species, the signals were at the telomere region of the chromosome arms. All oyster species have a haploid number of 10, and the American and Pacific oysters share an almost indistinguishable karyotype. The rDNA location is the first major difference in organization between the two oyster genomes. This result suggests that major genomic reorganization can occur under seemingly conserved karyotypes.