The first identification of constitutive heterochromatin and NOR-bearing chromosomes in the prehistoric bony fish whiting, Merlangius merlangus, using the latest short-term method

This study is the first to characterize the chromosomes and cytogenetic features of Merlangius merlangus (Linnaeus, 1758), a typical commercial marine species in the Black Sea and the sole member of the order Gadiformes, which first appeared around 16 million years ago. Samples were collected from professional fishermen working in the coastal waters of Fatsa in autumn 2023. Postmortem fish samples were stored in PB-MAX culture medium for 2.5 hours, after which slides were prepared by conventional air-drying. These slides were stained with Giemsa, C-, GTG-, and AgNO3, respectively. The metaphases were examined and analyzed with AKAS image analysis software. Results showed that Merlangius merlangus has a diploid number (2n) of 20 chromosome pairs, with a karyotype comprising 18 pairs of bi-armed chromosomes (including five pairs of metacentric and four pairs of submetacentric) and 22 uni-armed chromosomes (with eight pairs of subtelocentric and three pairs of acrocentric). One metacentric chromosome with C+ regions, two different metacentric chromosomes with G+ regions, and one acrocentric chromosome with NOR regions were identified. The study aimed to determine the karyology and cytogenetic features of Merlangius merlangus, a species not previously studied cytogenetically, using sequential staining and banding techniques. As a migratory bottom-dwelling fish like other Gadidae, it is characterized by a high number of double-armed chromosomes and is consistently found in the Black Sea. This research will help identify 45S rDNA gene regions across populations from various geographic locations.