Research Article |
Corresponding author: Sergey Simanovsky ( sergey.a.simanovsky@gmail.com ) Academic editor: Alicja Boroń
© 2020 Sergey Simanovsky, Dmitry Medvedev, Fekadu Tefera, Alexander Golubtsov.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Simanovsky S, Medvedev D, Tefera F, Golubtsov A (2020) First cytogenetic information for five Nilotic elephantfishes and a problem of ancestral karyotype of the family Mormyridae (Osteoglossiformes). Comparative Cytogenetics 14(3): 387-397. https://doi.org/10.3897/CompCytogen.14i3.52727
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The elephantfish family Mormyridae is the most diverse lineage of the primitive teleostean clade Osteoglossomorpha distributed in inland waters of all continents except Antarctica and Europe. The family Mormyridae is endemic to Africa and includes 22 genera and almost 230 species. The evolutionary radiation of mormyrids most probably should be attributed to their capability of both generating and receiving weak electric signals. Up-to-date cytogenetic studies have revealed substantial karyotype differentiation among the nine investigated elephantfish species and genera (a single species studied per each genus). In the present study, karyotypes of five species representing five mormyrid genera (four unexplored ones) collected from the White Nile system in southwestern Ethiopia are described for the first time. The results show substantial variety of the diploid chromosome and fundamental numbers: 2n = 48 and FN = 54 in Brevimyrus niger (Günther, 1866), 2n = 50 and FN = 72 in Cyphomyrus petherici (Boulenger, 1898), 2n = 50 and FN = 78 in Hippopotamyrus pictus (Marcusen, 1864), 2n = 50 and FN = 76 in Marcusenius cyprinoides (Linnaeus, 1758), 2n = 52 and FN = 52 in Mormyrops anguilloides (Linnaeus, 1758). Karyotype structure in the latter species seems to be close to the ancestral condition for the family. This hypothesis is discussed in the light of available data on karyotype diversity and phylogeny of mormyrids.
Africa, chromosomes, karyotype evolution, Brevimyrus, Cyphomyrus, Hippopotamyrus, Marcusenius, Mormyrops
The elephantfish family Mormyridae belongs to one of the most primitive groups of teleostean fishes, the cohort Osteoglossomorpha (
First cytogenetic data on the osteoglossomorphs and particularly mormyrids were published by
There is a coherent hypothesis about phylogenetic position of the family Mormyridae among other Osteoglossomorpha (
Meanwhile, only a small fraction of the total mormyrid diversity (less than 5% of species) has been yet studied cytogenetically. New findings may correct the existing views on their karyotype evolution. In the present study, new data for five mormyrid species from northern East Africa are presented using cytogenetic analysis (chromosome number and morphology). Relevance of these data to undrstanding of karyotype evolution within the family Mormyridae is considered.
The fifteen individuals studied represent five species of different genera – Brevimyrus niger (Günther, 1866), Cyphomyrus petherici (Boulenger, 1898), Hippopotamyrus pictus (Marcusen, 1864), Marcusenius cyprinoides (Linnaeus, 1758) and Mormyrops anguilloides (Linnaeus, 1758) – of the elephantfish family Mormyridae (Table
Species, fish standard length (SL), numbers of individuals (N) and metaphases (Nmt) studied, and collection site.
Species | SL, mm | N | Nmt | Collection site |
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Brevimyrus niger | 81–87 | 3 (1♀, 2♂) | 32 | Tida River |
Cyphomyrus petherici | 69–153 | 5 (3♀, 2♂) | 54 | Alvero River |
Hippopotamyrus pictus | 197 | 1 (♂) | 11 | |
Marcusenius cyprinoides | 196–217 | 3 (2♀, 1♂) | 30 | |
Mormyrops anguilloides | 409–498 | 2 (1♀, 1♂) | 21 | |
413 | 1 (♀) | 17 | Baro River |
Before preparation fish were treated intraperitoneally with 0.1% colchicine for 3–4 hours. Then fish were euthanized with an overdose of tricaine methanesulfonate (MS-222), identified based on morphological key characters (
Chromosome preparations were obtained from anterior kidney according to
Chromosome spreads were analysed under “Axioplan 2 Imaging” microscope (Carl Zeiss, Germany) equipped with “CV-M4+CL” camera (JAI, Japan) and “Ikaros” software (MetaSystems, Germany). Karyotypes were established according to the centromere position following the nomenclature of
Brevimyrus niger has a karyotype with 2n = 48 (Fig.
No distinguishable sex chromosomes were observed in complements of the four species in which individuals of both sexes were studied (B. niger, C. petherici, M. cyprinoides, and M. anguilloides), while the only male of H. pictus was karyotyped (Table
Data for all mormyrid taxa studied cytogenetically in the present study and earlier are presented in Table
Cytogenetically studied elephantfishes of the family Mormyridae. Diploid chromosome number (2n), karyotypic formula, fundamental number (FN) and geographic origin.
Taxon | 2n | Karyotypic formula | FN | Origin | References |
Subfamily Petrocephalinae | |||||
Petrocephalus microphthalmus Pellegrin, 1909 | 50 | 2sm + 48a | 52 | Ogooué Basin, Gabon |
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Subfamily Mormyrinae | |||||
Brevimyrus niger (Günther, 1866) | 48 | 4m + 2sm + 42a | 54 | White Nile Basin, Ethiopia | This study |
Brienomyrus brachyistius (Gill, 1862) | 48 | 1m + 4sm + 2st + 41a | 53 | Unknown (fish store) |
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Campylomormyrus rhynchophorus (Boulenger, 1898) | 48 | 26m + 4sm + 18a | 78 | Unknown (laboratory stock) |
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Cyphomyrus petherici (Boulenger, 1898) | 50 | 18m + 4sm + 28a | 72 | White Nile Basin, Ethiopia | This study |
Gnathonemus petersii (Günther, 1862) | 48 | 10m + 6sm + 32a | 64 | Unknown (fish store) |
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48 | 18m + 2sm + 28a | 68 | Unknown (fish store) |
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Hippopotamyrus pictus (Marcusen, 1864) | 50 | 24m + 4sm + 22a | 78 | White Nile Basin, Ethiopia | This study |
Ivindomyrus opdenboschi Taverne et Géry, 1975 | 50 | 10m + 2sm + 38a | 62 | Ntem River, Gabon |
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Marcusenius cyprinoides (Linnaeus, 1758) | 50 | 22m + 4sm + 24a | 76 | White Nile Basin, Ethiopia | This study |
Marcusenius moorii (Günther, 1867) | 50 | 4sm + 46a | 54 | Ntem River, Gabon |
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Mormyrops anguilloides (Linnaeus, 1758) | 52 | 52a | 52 | White Nile Basin, Ethiopia | This study |
Paramormyrops sp.7 | 50 | 2m + 6sm + 42a | 58 | Ebeigne, Woleu River, Gabon |
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Pollimyrus prope nigricans (Boulenger, 1906) | 40 | 2m + 38a | 42 | White Nile and Omo-Turkana basins, Ethiopia |
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Stomatorhinus walkeri (Günther, 1867) | 50 | 2sm + 48a | 52 | Ogooué Basin, Gabon |
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Brevimyrus niger shares the karyotype with 2n = 48 with three other mormyrid taxa, but differs from two of them – Campylomormyrus rhynchophorus with FN = 78 and Gnathonemus petersii (Günther 1862) with FN = 64 or 68 – by a smaller number of biarmed elements (FN = 54). For third taxon, Brienomyrus brachyistius, the unbalanced karyotype with FN = 53 was described in a single specimen (
The karyotype with 2n = 50 was found to be dominating in both presently and previously studied mormyrids (three and five taxa, respectively). Cyphomyrus petherici (FN = 72), Hippopotamyrus pictus (FN = 78) and Marcusenius cyprinoides (FN = 76) have more biarmed elements in their compliment than any other mormyrid studied except Campylomormyrus rhynchophorus (FN = 78). Congeneric Marcusenius cyprinoides and M. moorii (Günther, 1867) sharing the same chromosome number differ substantially in their karyotype structure. Up to recently Cyphomyrus petherici was considered as belonging to the genus Pollimyrus (
Mormyrops anguilloides has a karyotype unique for the mormyrids studied and composed of 52 uni-armed chromosomes. There are two mormyrids – Petrocephalus microphthalmus Pellegrin, 1909 and Stomatorhinus walkeri (Günther, 1867) – with 2n = 50 and FN = 52. Karyotypes of these three taxa dominated by the uni-armed elements seem to be close to each other and to a hypothetical ancestral karyotype of the family Mormyridae. Mutial trasnformation of these karyotypes could occur in a few evolutionary steps (Fig.
Most parsimonious scenarios of the early karyotype evolution within the family Mormyridae including three variants (A–C) of karyotype structure in a hypothetic ancestor (HA) and three studied lineages (the genera Petrocephalus, Stomatorhinus and Mormyrops) with least advanced karyotype structure within the family. The alternative transformations of karyotype structure are joint with a dashed line. The solitary submetacentric pairs in Petrocephalus and Stomatorhinus are suggested to be not syntenic.
Based on the simultaneous phylogenetic analysis of molecular data and chromosome number,
Based on available data the most parsimonius scenarios of the early karyptype evolution in Mormyridae are presented in Figure
We believe that further cytogenetic studies of various mormyrid taxa may shape the existing views on the karyotype evolution within this diverse group of fish. Looking for the probable interspecific variation of the karyotype structure within the three phylogenetically basal groups (the genera Petrocephalus, Myomyrus, Mormyrops) is of special interest.
We gratefully acknowledge the JERBE coordinator Andrey A. Darkov (IEE RAS, Moscow) for logistic support, Sergey E. Cherenkov (IEE) for sharing field operations and assistance in collecting material, Eugeny Yu. Krysanov (IEE) for precious help at different stages of our work, Alicja Boroń (University of Warmia and Mazury in Olsztyn), Mikhail V. Mina (Koltsov Institute of Developmental Biology RAS, Moscow), Claudio Oliveira (São Paulo State University, São Paulo) for critical comments on the manuscript. This work is financially supported by the Russian Foundation for Basic Research Project no. 18-34-00638 for SS and benefits also (at the stage of manuscript preparation) from the Russian Science Foundation Project no. 19‐14‐00218 for AG.