Abstract
The karyotypes and chromosomal characteristics of three Acestrorhynchus Eigenmann et Kennedy, 1903 species were examined using conventional and molecular protocols. These species had invariably a diploid chromosome number 2n = 50. Acestrorhynchus falcatus (Block, 1794) and Acestrorhynchus falcirostris (Cuvier, 1819) had the karyotype composed of 16 metacentric (m) + 28 submetacentric (sm) + 6 subtelocentric (st) chromosomes while Acestrorhynchus microlepis (Schomburgk, 1841) had the karyotype composed of 14m+30sm+6st elements. In this species, differences of the conventional and molecular markers between the populations of Catalão Lake (AM) and of Apeu Stream (PA) were found. Thus the individuals of Pará (Apeu) were named Acestrorhynchus prope microlepis. The distribution of the constitutive heterochromatin blocks was species-specific, with C-positive bands in the centromeric and telomeric regions of a number of different chromosomes, as well as in interstitial sites and completely heterochromatic arms. The phenotypes of nucleolus organizer region (NOR) were simple, i. e. in a terminal position on the p arm of pair No. 23 except in A. microlepis, in which it was located on the q arm. Fluorescence in situ hybridization (FISH) revealed 18S rDNA sites on one chromosome pair in karyotype of A. falcirostris and A. prope microlepis (pair No. 23) and three pairs (Nos. 12, 23, 24) in A. falcatus and (Nos. 8, 23, 24) in A. microlepis; 5S rDNA sites were detected in one chromosome pair in all three species. The mapping of the telomeric sequences revealed terminal sequences in all the chromosomes, as well as the presence of interstitial telomeric sequences (ITSs) in a number of chromosome pairs. The cytogenetic data recorded in the present study indicate that A. prope microlepis may be an unnamed species.
Highlights
The family Acestrorhynchidae is a group of South American fishes, for which contradictory views on its phylogenetic position within Characiformes are debated
The species of the genus Acestrorhynchus are widespread in South America, where most of its diversity is concentrated in the Amazon and Orinoco river basins, and the rivers of the Guyanas (Nelson et al 2016)
The nucleolus organizer region (NOR) were located in a distal position on the p arms of pair No 23 in all the species, except for A. microlepis, in which the NORs were located on the q arms of pair No 23 (Fig. 2c, f, i, l)
Summary
The family Acestrorhynchidae is a group of South American fishes, for which contradictory views on its phylogenetic position within Characiformes are debated. Based on morphological data, Lucena and Menezes (1998) suggested that the Acestrorhynchidae (Acestrorhynchus Eigenmann & Kennedy, 1903) was a sister group of the family Cynodontidae, consisting of the subfamilies Roestinae (Roestes Günther, 1864 and Gilbertolus Eigenmann & Ogle, 1907) and Cynodontinae (Cynodon Spix & Agassiz, 1829, Hydrolycus Müller & Troschel, 1844, and Rhaphiodon Spix & Agassiz, 1829), and proposed five synapomorphic characters for Acestrorhynchus, supporting the monophyly of this genus, as proposed by Menezes (1969b) and Menezes and Géry (1983). Nelson et al (2016) offered a new classification, based on the most recent data adopted for the determination and classification of characiform taxa, in which the Roestinae was placed as a subfamily of the Acestrorhynchidae, with two genera and six species. The species of the genus Acestrorhynchus are widespread in South America, where most of its diversity is concentrated in the Amazon and Orinoco river basins, and the rivers of the Guyanas (Nelson et al 2016)
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