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

Abstract

The African weakly electric elephantfish family Mormyridae comprises 22 genera and almost 230 species. Up-to-date cytogenetic information was available for 17 species representing 14 genera. Here we report chromosome number and morphology in Hyperopisus bebe (Lacepède, 1803) and Pollimyrus isidori (Valenciennes, 1847) collected from the White Nile system in southwestern Ethiopia. Both taxa displayed the diploid chromosome number 2n = 40, but they differed in fundamental numbers: FN = 66 in H. bebe and FN = 72 in P. isidori; previously the same diploid chromosome number 2n = 40 was reported in an undescribed species of Pollimyrus Taverne, 1971 (FN = 42) from the same region. Our results demonstrate that not only pericentric inversions, but fusions also played a substantial role in the evolution of the mormyrid karyotype structure. If the hypothesis that the karyotype structure with 2n = 50–52 and prevalence of the uni-armed chromosomes close to the ancestral condition for the family Mormyridae is correct, the most derived karyotype structures are found in the Mormyrus Linnaeus, 1758 species with 2n = 50 and the highest number of bi-armed elements in their compliments compared to all other mormyrids and in Pollimyrus isidori with the highest number of bi-armed elements among the mormyrids with 2n = 40.

Integration of Maps Enables a Cytogenomics Analysis of the Complete Karyotype in Solea senegalensis

“…The centric fusions that occurred in S. senegalensis account for three of the nine biarmed chromosomes (considering subtelocentrics as bi-armed), so pericentric inversions have played a central role in the evolution of the Senegalese sole karyotype. Inversions are important rearrangements that lead to polymorphism maintenance events in populations through balancing selection and are also important in the speciation process [49,50]. In addition, there is much evidence to support the view that inversions are involved in the environmental adaptation of species and that inversion polymorphisms within a species are related geographically [51,52].…”

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

“…Karyotyping was according to the criteria of Levan et al (1964); the chromosomes were arranged and paired in descending order of size. 2020) proposed 2n = 48-50 and 2n = 50 all acrocentrics as the ancestral karyotype for the Mormyridae; most mormyrids karyotyped so far displayed chromosome numbers within this range; but till now, none has exhibited a 2n = 50 (50a) (Simanovsky et al, 2021a). The diploid number of 2n = 50 composed of uni-armed elements; 2n = 50 (50sta), FN = 50, obtained in this study for Brienomyrus brachyistius in Oluwa River, Nigeria, makes it the first mormyrid to display the Simanovsky et al's., (2020) proposed Mormyridae ancestral karyotype.…”

“…However, the conventional cytogenetic approach is still valuable and can provide insight into the species diversity of some fish groups (Reviewed in Cioffi et al, 2019;Jegede et al, 2018). The conventional cytogenetic procedure has been the major approach to investigating the chromosome composition of the Mormyridae (Krysanov & Golubtsov, 2014;Simanovsky et al, 2020;Simanovsky et al, 2021a;Simanovsky et al, 2021b;Jegede, 2022). However, karyotype knowledge of the mormyrids is still sparse: till now, karyotype information is known for only 14 genera and 19 species of the 22 genera and over 255 mormyrids' species (Simanovsky et al, 2021a); the paucity of cytogenetic information on this fish group has been a limiting factor to its application to their evolution and phylogeny.…”