Karyotype of the West African Spotted Babywhale Mormyrid Fish, Pollimyrus adspersus in the Oluwa River, Ondo State, Nigeria

Pollimyrus is a non-monophyletic and paraphyletic species-rich Mormyridae genus that is still cytogenetically understudied. Here, the chromosome composition of Pollimyrus adspersus in the Oluwa River, Nigeria, was investigated for the first time to determine its karyotype and highlight the cytogenetic relationship between it and the two earlier studied species in the genus. Mitotic cells were arrested at the metaphase stage after a 0.05% peritoneal colchicine injection at 0.05ml per 10g of fish weight for 1 hour. Metaphase chromosomes were extracted from the fish kidney following the standard procedure. Slide staining was done using 6% Giemsa solution. Chromosome metaphases were snapped and processed in Photoshop. Chromosomes were characterised as metacentric, submetacentric, and acrocentric based on centromeric position. Chromosome homologs were paired and arranged in descending order of size. The chromosome number of 2n = 50 all acrocentric; 2n = 50 (50a), recovered from the fish, represents one of the least rearranged Mormyridae chromosome conditions, and was significantly different from its Ethiopian cogenerics; P. nigricans and P. isidori. The present study provides cytogenetic Evidence supporting an earlier morphologically based study that separated the studied species and its Ethiopian counterparts into different clades. Given previous cytogenetic and morphological data and the result presented herein, the appropriateness of retaining the species in the Pollimyrus genus should be reviewed. However, further studies incorporating molecular genetics, electrical organ discharge, and cytogenetics are essential for a more robust approach to the genus taxonomy. Keywords: Osteoglossiformes, Bony tongues, Non-monophyletic, Cytogenetics, Clade