Karyotype of the african weakly electric fish, <i>Gymnarchus niloticus</i> (Osteoglosiformes: Gymnarchidae) from Oluwa River, Nigeria

Abstract

Bonytongue fishes of the order Osteoglossiformes provide an interesting clade for evolutionary study considering their wide distribution and their basal position in the general Teleostei phylogeny. Lack of adequate information on the cytogenetics of this group of fishes has been a limiting factor to the understanding of karyotype evolution of the clade. In this study, karyotype of Gymnarchus niloticus, the only species in the family Gymnarchidae collected from Oluwa River was investigated in order to increase the knowledge of karyotype pattern in the family. Metaphase chromosomes were obtained from the anterior portion of the kidney after injecting the fish intraperitoneally with 0.05% colchicine. Slide preparation followed the conventional Giemsa staining technique while digital images of the chromosomes were processed using Mirosoft Excel and Photoshop. The result revealed a karyotype of 2n = 54 (26m+14sm+14sta) which was significantly different in chromosome number and in chromosome macrostructure from the karyotype of 2n = 34 (34m/sm) reported for G. niloticus from Lekki Lagoon. The wide difference in chromosome number and in chromosome structure observed in G. niloticus from these two locations is not a common occurrence among closely related fish species. These two karyotypes therefore raise a question of the possibility of another species in the family Gymnarchidae.

First cytogenetic information for five Nilotic elephantfishes and a problem of ancestral karyotype of the family Mormyridae (Osteoglossiformes)

“…Based on the simultaneous phylogenetic analysis of molecular data and chromosome number, Canitz et al (2016) recognized karyotype with n = 24 as the most parsimonius ancestral state for the order Osteoglossiformes, while the haploid chromosome number of n = 24-25 was inferred for the most recent common ancestor of the family Mormyridae. Their analysis, however, did not include the most recent cytogenetic data for several osteoglossomorph clades (Ráb et al 2016;Barby et al 2018;Jegede et al 2018;Cioffi et al 2019;de Oliveira et al 2019). Moreover, the recent genomic data evidence for the ancestral Euteleostomi karyotype of 50 chromosomes with domination by acrocentric elements (Nakatani et al 2007;Sacerdot et al 2018;de Oliveira et al 2019).…”

Derived karyotypes in two elephantfish genera (Hyperopisus and Pollimyrus): lowest chromosome number in the family Mormyridae (Osteoglossiformes)

“…Along with the family Mormyridae, a substantial reduction of chromosome numbers seems to occur in the related lineages of the cohort Osteoglossomorpha. Very interesting data on Gymnarchus niloticus Cuvier, 1829, the only representative of the family Gymnarchidae and a sister group of Mormyridae, reveal unexpectedly different karyotype structures -2n = 34 (26m + 8sm) and 2n = 54 (26m + 14sm + 14sta) -in the two Nigerian populations separated by a distance of less than 200 km (Hatanaka et al 2018;Jegede et al 2018). Notopteridae is a sister group of Mormyridae + Gymnarchidae (Lavoué, Sullivan 2004;Nelson et al 2016).…”

Preliminary cytogenetic study of <em style="mso-bidi-font-style: normal;">Ctenopoma kingsleyae</em> from Esa-Odo water reservoir, Osun State, Nigeria

“…The fish were too small to access the anterior portion of the kidney, thus gill filaments were removed and placed in a beaker containing 0.56% potassium chloride for a period of 40 minutes. A 50 mL syringe without needle was used in squashing the filament to obtain homogenous cell suspension (Jegede et al, 2018). The supernatant was carefully poured into centrifuge tubes and centrifuged at 1000 rpm for 10 minutes.…”