First cytogenetic information for five Nilotic elephantfishes and a problem of ancestral karyotype of the family Mormyridae (Osteoglossiformes)

Abstract

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.

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

“…Pollimyrus appears the third mormyrid genus for which the data on intrageneric variation of the karyotype structure are available (Table 1). In this genus the pronounced divergence between species is similar to the situation in Marcusenius Gill, 1862, where two species studied have the same diploid chromosome number, but different karyotypic formula -M. moorii (Günther, 1867) has 4sm + 46a, M. cyprinoides (Linnaeus, 1758) has 22m + 4sm + 24a (2n = 50 for both) (Ozouf-Costaz et al 2015;Simanovsky et al 2020). On the contrary, among three species of the genus Mormyrus Linnaeus, 1758 no difference in their karyotype structure was found (Simanovsky et al 2021).…”

“…Usage of the name Pollimyrus prope nigricans (Boulenger, 1906) has been substantited by Krysanov and Golubtsov (2014). Division of the family Mormyridae into two subfamilies Petrocephalinae (including the single genus Petrocephalus) and Mormyrinae (including all other mormyrid genera), as well as usage of the names Brienomyrus brachyistius (Gill, 1862), Campylomormyrus rhynchophorus (Boulenger, 1898) and Paramormyrops sp.7, have been discussed by Simanovsky et al (2020). The karyotypes most similar to a hypothetical ancestral condition within the family based on arguments considered above are highlighted with bold in the Table 1.…”

“…The African weakly electric elephantfishes comprise the family Mormyridae including 22 genera and almost 230 species (Eschmeyer et al 2021;Froese and Pauly 2021). To date, the representatives of 14 mormyrid genera have been studied cytogenetically (Uyeno 1973;Krysanov and Golubtsov 2014;Ozouf-Costaz et al 2015;Canitz et al 2016;Simanovsky et al 2020Simanovsky et al , 2021. The diploid chromosome numbers in most elephantfishes vary between 48 and 52 with the mode 50 (Simanovsky et al 2020).…”

“…To date, the representatives of 14 mormyrid genera have been studied cytogenetically (Uyeno 1973;Krysanov and Golubtsov 2014;Ozouf-Costaz et al 2015;Canitz et al 2016;Simanovsky et al 2020Simanovsky et al , 2021. The diploid chromosome numbers in most elephantfishes vary between 48 and 52 with the mode 50 (Simanovsky et al 2020). While a single studied species of the genus Pollimyrus Taverne, 1971 exhibited 2n = 40 (Krysanov andGolubtsov 2014).…”

“…A problem of the ancestral karyotype for the family Mormyridae was discussed by Canitz et al (2016) and Simanovsky et al (2020). In the first study, the most likely ancestral chromosome number for the family was identified as n = 24 or n = 25.…”