Karyotype of the Mormyrid Baby Whale Fish (Osteoglossiformes: Mormyridae): Brienomyrus Brachyistius in Oluwa River, Nigeria

Abstract

The mormyrid fishes of the order Osteoglossiformes are abundant in the freshwaters of tropical Africa; the enormous species diversity of the group requires many systematics tools to comprehend this diversity better. Cytogenetics data can complement other systematics procedures to accomplish the above-stated objective, but information on many mormyrids' karyotypes is unavailable. In this study, Brienomyrus brachyistius from the Oluwa River was studied to enhance the understanding of the karyotype organization in the Mormyridae. Metaphase chromosomes were processed from the fish kidney following a conventional cytogenetic procedure; the fish was subjected to an intra-peritoneal colchicine injection at the rate of 1ml per 100g of fish mass. Hypotonic treatment was in 0.56% KCl solution, and cells fixation and washes were in 3: 1 methanol: acetic acid. The slides were stained with Giemsa solution, while the constitutive heterochromatin was revealed from the previous Giemsa stained slide using the C-banding sequential method. Capturing of metaphase images was done with an Olympus BX50 microscope. The karyotype of 2n = 50 (50sta) obtained in the species was the first mormyrid karyotype composed of 2n = 50 acrocentrics, supporting the proposed ancestral karyotype for the Mormyridae.