Research Article |
Corresponding author: Sergey A. Simanovsky ( sergey.a.simanovsky@gmail.com ) Academic editor: Grazyna Furgala-Selezniow
© 2022 Sergey A. Simanovsky, Dmitry A. Medvedev, Fekadu Tefera, Alexander S. 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 SA, Medvedev DA, Tefera F, Golubtsov AS (2022) First cytogenetic data on Afrotropical lutefishes (Citharinidae) in the light of karyotype evolution in Characiformes. Comparative Cytogenetics 16(2): 143-150. https://doi.org/10.3897/compcytogen.v16.i2.79133
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The Afrotropical lutefish family Citharinidae (Citharinoidei, Characiformes) comprises three genera with eight species in total. Although Citharinidae have been studied in terms of taxonomy and systematics, no cytogenetic information was available for any representative of the family. Furthermore, only one species out of 116 in Citharinoidei (Distichodus affinis Günther, 1873) has been studied cytogenetically. Here, we report the karyotypes of Citharinus citharus (Geoffroy St. Hilaire, 1809) from West Africa and Citharinus latus Müller et Troschel, 1844 from Northeast Africa. The former has the diploid chromosome number 2n = 40 and the fundamental number FN = 80, while the latter has 2n = 44 and FN = 88. Hence, these karyotypes consist exclusively of bi-armed chromosomes. Such karyotypes were previously found in D. affinis and in many lineages of Neotropical species of another suborder of Characiformes, Characoidei. In contrast, the karyotypes dominated by uni-armed elements are typical for a number of phylogenetically basal lineages of Afrotropical and Neotropical Characoidei. We discuss the importance of our data on Citharinidae for the understanding of the karyotype evolution within the order Characiformes.
Africa, Characoidei, chromosomes, Citharinoidei, Citharinus, karyotype evolution
Characins, the order Characiformes, are classified into two suborders: Citharinoidei and Characoidei. The former includes two Afrotropical families: Citharinidae with eight species in three genera and Distichodontidae with 108 species in 16 genera, while the latter suborder (Characoidei) contains more than 2,000 species in two Afrotropical (Alestidae and Hepsetidae) and 20 Neotropical families (
There is no cytogenetic information about any citharinid species, whereas the karyotype of the only distichodontid species, Distichodus affinis Günther, 1873, was analyzed by
Here, we present the first data on the karyotypes of two species of the genus Citharinus Cuvier, 1816. We then cytogenetically compare these species with the nearest studied relative, D. affinis, and other characins. Finally, we discuss the importance of these data for the understanding of the karyotype evolution within the order Characiformes.
Seven individuals of an undetermined sex (UD) of Citharinus citharus (Geoffroy St. Hilaire, 1809), standard length (SL) of 61–91 mm, and three individuals (a female, a male and a UD individual) of C. latus Müller et Troschel, 1844, SL = 63–84 mm, were karyotyped. For each individual, at least 10 complete metaphases were analyzed to establish the diploid chromosome number and the karyotype structure. The total numbers of complete metaphase plates studied for each species were 101 and 42, respectively. Citharinus citharus were purchased from the Nigerian aquarium fish dealers through the mediation of the company Aqua Logo Engineering (https://www.aqualogo-engineering.ru) in October of 2021, while C. latus individuals were collected in southwestern Ethiopia by the Joint Ethiopian-Russian Biological Expedition (JERBE) from the Alvero River just downstream of the Abobo Dam (7°52'23"N, 34°29'48"E) in November of 2017. This river belongs to the Sobat River drainage discharging into the White Nile in South Sudan. Nigerian fish were kept in the Moscow laboratory in a 100-l aquarium with permamently aerated and filtered water for one to ten days before treatment. Ethiopian fish were caught with a cast net and 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 0.025% colchicine (0.01 ml / 1 g of their weight) for 1–2 hours (for C. citharus, in laboratory conditions) or 0.1% colchicine for 3–4 hours (for C. latus, in field conditions). Then, fish were euthanized with an overdose of tricaine methanesulfonate (MS-222), identified, measured with an accuracy of ±1 mm, dissected for gonad examination and tissue sampling, and preserved in 10% formaldehyde. Species identification was done based on the morphological characters (mostly, the number of scales in the lateral line for C. citharus and the relative size of adipose fin for C. latus, according to
Chromosome preparations were obtained from C. citharus following
The chromosome spreads were analysed using an Axioplan 2 Imaging microscope (Carl Zeiss, Germany) equipped with a CV-M4+CL camera (JAI, Japan) and the Ikaros software (MetaSystems, Germany). Final images were processed using the Photoshop software (Adobe, USA). Karyotypes were established according to the centromere position following the nomenclature by Levan et al. (1964). Chromosomes were classified as metacentric (m) or submetacentric (sm), grouped according to their morphology and ordered by the decrease of their size. To determine the fundamental number (FN), metacentrics and submetacentrics were considered bi-armed.
The karyotype of C. citharus has 2n = 40 and consists of 26 metacentrics (m) and 14 submetacentrics (sm), the fundamental number FN = 80 (Fig.
Metaphase chromosome plates (left) and karyotypes (right) of Citharinus citharus and C. latus after conventional Giemsa staining. Scale bar: 10 μm.
Citharinus citharus has nine chromosome pairs (nos. 1–3 and 14–19) noticeably larger than others, while C. latus has seven large chromosome pairs (nos. 1–4 and 16–18). This difference could be explained by two fusions of four pairs of smaller chromosomes (if the karyotype of C. latus is considered ancestral) or fissions of two pairs of larger chromosomes (if the karyotype of C. citharus is considered ancestral). However, another possible scenario would be an independent origin of karyotypes of the two Citharinus species. Namely, D. affinis exhibits 2n = 48 (
Of note, all the three Citharinoidei species with studied karyotypes – both Citharinus species presented here and D. affinis studied by
In comparison, the suborder Characoidei that is better studied cytogenetically demonstrates a wide variation in karyotype structures even in its basal groups. Specifically, karyotypes with exclusively bi-armed chromosomes are found in the family Crenuchidae (
Importantly, our new data on the two Citharinoidei karyotypes suggest a revision of the current hypothesis about the ancestral chromosome number of the order Characiformes. Namely, based almost exclusively on the cytogenetic data from the other suborder of Characiformes, Characoidei, the chromosome number 2n = 54 was suggested to be ancestral for the whole order (Oliveira et al. 1998,
On the other hand, some authors recently proposed a new hypothetical molecular phylogeny of the ray-finned fishes where the suborder Citharinoidei is separated into an order Cithariniformes and considered as a sister group to Characiformes + Siluriformes (
We gratefully acknowledge the JERBE coordinator Andrey A. Darkov, and also Konstantin S. Morshnev, both from the Severtsov Institute of Ecology and Evolution (IEE), for their support with logistics; Sergey E. Cherenkov (IEE) for helping with the field operations and for his assistance in collecting the C. latus individuals; Nikolay A. Veretennikov (IEE) for his help in keeping the C. citharus individuals in the Moscow laboratory; Sviatoslav Sidorov (The Francis Crick Institute) for his critical comments and his help with the editing and proofreading of the manuscript; and Eugeny Yu. Krysanov (IEE) for his precious help at different stages of our work. This study was supported by the Russian Foundation for Basic Research Project no. 18-34-00638 (S.A.S.) and at the stage of manuscript preparation also benefits from the Russian Science Foundation Project no. 19-14-00218 (A.S.G.).
Sergey A. Simanovsky https://orcid.org/0000-0002-0830-7977
Dmitry A. Medvedev https://orcid.org/0000-0001-8560-8186
Alexander S. Golubtsov https://orcid.org/0000-0002-8317-7527