Research Article |
Corresponding author: Sergey Simanovsky ( sergey.a.simanovsky@gmail.com ) Academic editor: Patrícia Parise-Maltempi
© 2021 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 (2021) Derived karyotypes in two elephantfish genera (Hyperopisus and Pollimyrus): lowest chromosome number in the family Mormyridae (Osteoglossiformes). Comparative Cytogenetics 15(4): 345-354. https://doi.org/10.3897/compcytogen.v15.i4.67681
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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.
Africa, chromosomes, karyotype evolution, chromosome fusions, Hyperopisus, Pollimyrus
The African weakly electric elephantfishes comprise the family Mormyridae including 22 genera and almost 230 species (
A problem of the ancestral karyotype for the family Mormyridae was discussed by
Indeed, such karyotype structure is found in the two genera (Petrocephalus Marcusen, 1854 and Mormyrops Müller, 1843) appearing among the basal groups in molecular phylogenies of the family Mormyridae (
The karyotype structure with chromosome number unusually low for mormyrids was reported by
In the present study, we address the uniqueness of the low chromosome numbers in mormyrids; H. bebe and the second species of the genus Pollimyrus were cytogenetically analyzed (for chromosome number and morphology). Based on the obtained and previous results, the two types of karyotype structure most derived from a hypothetical ancestral condition within the family Mormyridae were defined.
Fishes were collected in Ethiopia within the framework of the Joint Ethiopian-Russian Biological Expedition (JERBE) with permission from the National Fishery and Aquatic Life Research Center under the Ethiopian Institute of Agricultural Research and the Ethiopian Ministry of Innovation and Technology. The experiments were carried out in accordance with the rules of the Severtsov Institute of Ecology and Evolution, Russian Academy of Sciences.
Three individuals (two females and a male) of each of the two species – Hyperopisus bebe (Lacepède, 1803) (standard length, SL 131–356 mm) and Pollimyrus isidori (Valenciennes, 1847) (SL 54–60 mm) – were karyotyped; total numbers of complete metaphase plates studied for each species were 30 and 33, respectively. Fish were sampled in the Gambela Peoples’ Region, a regional state in western Ethiopia at two sites in November of 2017: P. isidori from the Baro River downstream of the City of Itang (8°10'47"N, 34°15'2"E) and H. bebe from the Alvero River downstream of the Abobo Dam (7°52'23"N, 34°29'48"E). Both rivers belong to the Sobat River drainage discharging into the White Nile in South Sudan. Fish were caught with gill (H. bebe) and cast (P. isidori) nets, delivered in 80-l plastic containers into the field laboratory, where they were kept in permamently aerated water for several hours before treatment.
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, measured to an accuracy of 1 mm, dissected for gonad examination and tissue sampling, and preserved in 10% formaldehyde. Vouchers are deposited at the Severtsov Institute of Ecology and Evolution (Moscow) under provisional labels of JERBE.
Chromosome preparations were obtained from anterior kidney according to
Hyperopisus bebe has karyotype with 2n = 40 (Fig.
Karyotypes of Hyperopisus bebe and Pollimyrus isidori after conventional Giemsa staining. Scale bars: 10 μm.
For comparative purposes, all the currently available data on the karyotype structure in mormyrids are given in Table
Cytogenetically studied elephantfishes of the family Mormyridae arranged in accordance with increasing (1) diploid chromosome number – 2n and (2) fundamental number – FN; karyotypic formulas most close to that in a hypothetic ancestor of the family are highlighted with bold.
Taxon | 2n | Karyotypic formula | FN | Origin | References |
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2n = 40 | |||||
Pollimyrus prope nigricans (Boulenger, 1906) | 40 | 2m + 38a | 42 | White Nile and Omo-Turkana Basins, Ethiopia |
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Hyperopisus bebe (Lacepède, 1803) | 40 | 24m + 2sm + 14a | 66 | White Nile Basin, Ethiopia | This study |
Pollimyrus isidori (Valenciennes, 1847) | 40 | 26m + 6sm + 8a | 74 | White Nile Basin, Ethiopia | This study |
2n = 48 | |||||
Brienomyrus brachyistius (Gill, 1862) | 48 | 1m + 4sm + 2st + 41a | 53 | Unknown (fish store) |
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Brevimyrus niger (Günther, 1866) | 48 | 4m + 2sm + 42a | 54 | White Nile Basin, Ethiopia |
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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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Campylomormyrus rhynchophorus (Boulenger, 1898) | 48 | 26m + 4sm + 18a | 78 | Unknown (laboratory stock) |
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2n = 50 | |||||
Petrocephalus microphthalmus Pellegrin, 1909 | 50 | 2sm + 48a | 52 | Ogooué Basin, Gabon |
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Stomatorhinus walkeri (Günther, 1867) | 50 | 2sm + 48a | 52 | Ogooué Basin, Gabon |
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Marcusenius moorii (Günther, 1867) | 50 | 4sm + 46a | 54 | Ntem River, Gabon |
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Paramormyrops sp.7 | 50 | 2m + 6sm + 42a | 58 | Woleu River, Gabon |
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Ivindomyrus opdenboschi Taverne et Géry, 1975 | 50 | 10m + 2sm + 38a | 62 | Ntem River, Gabon |
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Cyphomyrus petherici (Boulenger, 1898) | 50 | 18m + 4sm + 28a | 72 | White Nile Basin, Ethiopia |
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Marcusenius cyprinoides (Linnaeus, 1758) | 50 | 22m + 4sm + 24a | 76 | White Nile Basin, Ethiopia |
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Hippopotamyrus pictus (Marcusen, 1864) | 50 | 24m + 4sm + 22a | 78 | White Nile Basin, Ethiopia |
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Mormyrus caschive Linnaeus, 1758 | 50 | 20m + 14sm + 16a | 84 | White Nile Basin, Ethiopia |
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Mormyrus hasselquistii Valenciennes, 1847 | 50 | 20m + 14sm + 16a | 84 | White Nile Basin, Ethiopia |
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Mormyrus kannume Fabricius, 1775 | 50 | 20m + 14sm + 16a | 84 | Omo-Turkana Basin, Ethiopia |
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2n = 52 | |||||
Mormyrops anguilloides (Linnaeus, 1758) | 52 | 52a | 52 | White Nile Basin, Ethiopia |
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The chromosome set of the undescribed species reported by
Pollimyrus appears the third mormyrid genus for which the data on intrageneric variation of the karyotype structure are available (Table
Pericentric inversions are considered as the main type of chromosomal rearrangements in mormyrid karyotype evolution by
One may suggest that just tandem fusions played an important role in reduction of chromosome number to 2n = 40 at least in Pollimyrus prope nigricans with FN = 42 (Table
We gratefully acknowledge the JERBE coordinator Andrey Darkov (IEE RAS, Moscow) for logistic support, Sergey Cherenkov (IEE) for sharing field operations and assistance in collecting material, Eugeny Krysanov (IEE) for precious help at different stages of our work. 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.
Sergey Simanovsky https://orcid.org/0000-0002-0830-7977
Dmitry Medvedev https://orcid.org/0000-0001-8560-8186
Alexander Golubtsov https://orcid.org/0000-0002-8317-7527