First cytogenetic analysis of Ichthyoelephas humeralis (Günther, 1860) by conventional and molecular methods with comments on the karyotypic evolution in Prochilodontidae

Abstract

We used conventional cytogenetic techniques (Giemsa, C-banding, Ag-NOR), and fluorescent in situ hybridization (FISH) with 5S and 18S rDNA probes to investigate the karyotype and cytogenetic characteristics of Ichthyoelephas humeralis (Günther, 1860) from Ecuador. The specimens studied have a karyotype with 2n=54 biarmed chromosomes (32 M + 22 SM) and C-positive heterochromatin located on the centromeric, pericentromeric, interstitial, and terminal regions of some chromosomes. The nucleolus organizer regions occurred terminally on the long arm of chromosome pair 2. FISH confirmed the presence of only one 18S rDNA cluster with nonsyntenic localization with the 5S rDNA. Cytogenetic data allow us to refute the earlier morphological hypothesis of a sister relationship between Semaprochilodus Fowler, 1941 and Ichthyoelephas Posada Arango, 1909 and support the molecular proposal that Ichthyoelephas is a sister group to the monophyletic clade containing Prochilodus Agassiz, 1829 and Semaprochilodus.

B chromosome dynamics in Prochilodus costatus (Teleostei, Characiformes) and comparisons with supernumerary chromosome system in other Prochilodus species

“…Cytogenetic data show a conservative trend within the family Prochilodontidae, with a diploid number of 2n=54 biarmed chromosomes (Pauls and Bertollo 1983, 1990, Feldberg et al 1987, Voltolin et al 2013, Nirchio-Tursellino et al 2016). However, Semaprochilodus Fowler, 1941 and Prochilodus genera exhibit the presence of sex related and supernumerary chromosomes, respectively, different from Ichthyoelephas Posada Arango, 1909, in which these elements are absent.…”

“…Accordingly, recent molecular phylogenetic analyses (Melo et al 2016) proposed the monophily of Prochilodontidae and placed Ichthyoelephas as a sister group of the Semaprochilodus + Prochilodus clade. The absence of sex related and supernumerary chromosomes, together with differences on the location of the repetitive 5S and 18S rDNA, suggest a plesiomorphic position of the Ichthyoelephas karyotype (Nirchio-Tursellino et al 2016), involving at least two rearrangements events in the common ancestor of the Prochilodontidae.…”