Interpopulational Variation of NOR Positions and Karyotypic Analysis of Siamese Catfish (Pseudomystus siamensis) in Thailand

Abstract

Interpopulational variation of nucleolar organizer region (NOR) position and karyotypic analysis of Siamese catfish (Pseudomystus siamensis) in Thailand were studied. The samples were taken from three locations (8 populations); the mitotic chromosome preparations were conducted directly from kidney cells. Conventional and Ag-NOR staining techniques were applied to stain the chromosomes. The results showed that the diploid chromosomes number was 2n=50 and the fundamental number (NF) was 96 in both sexes. The karyotype of P. siamensis showed two new cytotypes for this species. The chromosome of cytotype A and B showed the presence of metacentric, submetacentric, acrocentric and telocentric autosomes as 22–18–6–4 and 20–20–6–4, respectively, without any apparent sex chromosomal heteromorphism. Data obtained from chromosomal banding technique further suggests an interpopulation NOR polymorphism. Such variation is possibly caused by paracentric inversion. In addition, a pair of the long arms of chromosome pair 2′ in cytotype A and chromosome pair 12′′ in cytotype B showed clearly observable NORs. The karyotype formulas of cytotype A and B for P. siamensis could be deduced as:
Cytotype A: 2n (diploid) 50=L₆^m+L₆^sm+L₂^a+M^m₁₆+M₁₂^sm+M^a₄+M^t₂+S^t₂
Cytotype B: 2n (diploid) 50=L₂^m+L₆^sm+M^m₁₄+M₁₂^sm+M^a₆+M^t₂+S₄^m+S₂^sm+S^t₂.