Karyotyping and chromosomal Ag-NOR banding of Siamese fighting fish (Betta splendens)

Siamese fighting fish (Betta splendens، Regan، 1909) is a freshwater fish native to southeast Asia that has attracted considerable attention in Iran as an ornamental fish. The purpose of this research was karyotyping of this fish by in vivo method as well as its Ag-NOR chromosomal banding. Chromosomal spreads were obtained from hematopoietic (head kidney and spleen)، gill and testicular tissues by splash and squash (stamping) methods on cold slides، which were then stained by 25% Giemsa. In addition، sequential staining nucleolus organizer regions (NORs) were performed by Ag-NO3 staining. Chromosome number in diploid and haploid cells in this species were counted 2n=42 and n=21، respectively. Fundamental number was NF=68. 4 pairs of NORs which were found in methaphase plates. Chromosomal formula consisted of 1 pair of metacentric، 3 pairs of submetacentric، 9 pairs of subtelocentric (acrocentric) and 8 pairs of telocentric chromosomes. The chromosomal formula was similar in both sexes، however، comparing male and female karyotypes، the chromosome pair number 17 was heteromorphic. In this study، the number of chromosomes (2n) was similar but chromosomal formula and arm number (FN) were different from those in the previous studies. Metacentric chromosomes (pair 1) and presence of a pair of heteromorphic chromosomes in the two sexes (pair 17) are reported in this species for the first time. In the case that chromosomes pair 17 are sex chromosomes، a WZ sex determination system can be suggested for this species where females are heterogametic (WZ) and males are homogametic (ZZ) sexes.