First Report on the Karyotype of the Freshwater Weakly Electric Fish; Marcusenius brucii (Osteoglossiformes: Mormyridae) in Asejire Reservoir, Osun River Basin, Nigeria

Abstract

The weakly electric fishes of the family Mormyridae are one of the dominant fish groups in Africa; they are represented in all the major freshwater bodies in the tropical regions of the continent, except the Southernmost Cape Provinces, the Sahara and the Northernmost Maghreb. The Mormyridae boasts 225 species in 22 genera, making it the most speciose Osteoglossiformes family. Such great diversity requires many systematics approaches for good comprehension; to this end, cytogenetics provides valuable data for a better definition of this diversity. However, cytogenetics data on mormyrids are still scarce, thus limiting its usefulness in the biodiversity study of the group. In this study, the chromosome composition of a mormyrid species, Marcusenius brucii, was investigated to reveal its chromosome number and standard karyotype for the first time; to enhance the understanding of the karyotype organization of the group. The fish was sourced from a local fisherman in Asejire Reservoir, Osun River Basin, Nigeria. Metaphase chromosomes were prepared from the fish kidney after in vivo injection with 0.05% colchicine solution. Slide preparation and karyotyping followed conventional cytogenetics techniques. The study showed that M. brucii karyotype consists of a diploid number of 2n = 50 dominated by two-armed chromosomes. While the chromosome number is in harmony with the general chromosome number in the order Osteoglossiformes and the hypothetical diploid number for the family Mormyridae, it differed mainly in its possession of bi-armed elements. The phylogeny of Marcusenius is yet to be resolved; this, coupled with the limited karyotyped individuals in the genus, makes the karyotype evolution of the genus difficult to infer. This scenario will improve as the cytogenetics knowledge of the genus increases.

Keywords: Cytogenetics, chromosomes elephantfishes, karyotype, mormyrids, Osteoglossiformes

DOI: 10.7176/JNSR/13-18-03

Publication date:October 31^st 2022