Chromosome preparation from gill tissue of the tropical oyster (Crassostera belcheri Sowerby, 1871) collected from an oyster farm in Kantang District, Trang Province, southern Thailand was studied by conventional staining and Ag-NOR baining techniques. The result revealed that the diploid chromosome number is 2n=20 and the fundamental number (NF) is 40. The karyotype has revealed 10 large metacentric and 10 medium metacentric chromosomes. One pair of nucleolar organizer regions (NORs) was apparent on the short arm telomeric region of chromosome pair 10. Basic information on the cytogenetics of the C. belcheri may be useful for future studies on interspecific hybridization and polyploidy. The karyotype formula of C. belcheri is as follows: m m 10 10 2 (diploid) 20=L +M n

A discovery of nucleolar organizer regions (NORs) polymorphism and karyological analysis in the crystal eye catfish, Hemibagrus wyckii (Bleeker, 1858) from Nong Khai and Sing Buri Provinces, Thailand, were investigated. The mitotic chromosome preparation was prepared by directly from kidney cells of five male and five female specimens. Conventional and Ag-NOR staining techniques were applied to stain the chromosomes. The results shown that the diploid chromosome number of H. wyckii was 2n=62 and the fundamental numbers (NF) of both sexes were 110. The karyotype consists of 14 large metacentric, 14 large submetacentric, 8 large acrocentric, 8 medium metacentric, 4 medium submetacentric, 6 medium telocentric, and 8 small telocentric chromosomes. No strange size chromosomes related to sex were observed. In addition, the interstitial nucleolar organizer regions (NORs) were clearly observed at the long arm of the chromosome pair No. 12. This is the first report on polymorphism of NORs in H. wyckii. The result revealed that a heteromorphic of one male and one female had a single NOR-bearing chromosome of chromosome pair No. 12 (12a12b), while four males and four females had two NOR-bearing chromosomes of the chromosome pair No. 12 with a homomorphic (12a12a). The karyotype formula for H. wyckii is as follows: m sm a m sm t t 14 14 8 8 4 6 8

A first report on nucleolar organizer regions (NORs) and karyological analyses in the Chevey s sheetfish (Micronema cheveyi) from the Chao Phraya Basin, Thailand, was performed. The mitotic chromosome preparation was directly prepared from kidney cells of ten male and eight female specimens. Conventional staining and Ag-NOR banding techniques were applied to stain the chromosomes. The results shown that the diploid chromosome number of M. cheveyi was 2n=78 and the fundamental numbers (NF) were 96 in both sexes. The karyotype composed of 4 large metacentric, 6 large submetacentric, 10 large acrocentric, 32 medium telocentric, and 26 small telocentric chromosomes. No heteromorphic sex chromosomes were revealed. In addition, the telomeric NORs were clearly observed at the short arm adjacent to the telomere of the third chromosome pair (submetacentric chromosomes). The karyotype formula for M. cheveyi is as follows: m sm a t t 4 6 10 32 26

Conventional karyotyping and detection of nucleolar organizer region (NOR) were performed on a tropical oyster, Crassostrea iredalei from Nakhon Si Thammarat province, Thailand. Chromosome preparations were made from gill tissues. Conventional Giemsa s staining and Ag-NOR banding were performed. The results revealed that the chromosome number was 2n= 20 and fundamental number (NF) was 40, the karyotype consisting of 12 large metacentric and eight medium metacentric chromosomes. The NORs were located terminally on the long arms of the chromosome pair 10.