Karyotype description and comparative chromosomal mapping of rDNA and U2 snDNA sequences in Eigenmannialimbata and E.microstoma (Teleostei, Gymnotiformes, Sternopygidae)

Mapping of Repetitive Sequences in Brachyhypopomus brevirostris (Hypopomidae, Gymnotiformes) from the Brazilian Amazon

Brachyhypopomus (Hypopomidae, Gymnotiformes) is a monophyletic genus consisting of 28 formally described species. Karyotypic data are available for 12 species. The same karyotype is described for two species (B. brevirostris and B. hamiltoni), as well as different karyotypes for the same species from distinct locations (B. brevirostris). In this context, B. brevirostris may constitute a cryptic species complex. Thus, in the present study, we analyzed the karyotype of B. brevirostris, from Santarém, Pará, and Tefé, Amazonas, using classical cytogenetics (conventional staining and C-banding) and molecular techniques (fluorescence in situ hybridization using 18S rDNA, 5S rDNA, U2 snRNA, and telomeric probes). The results show that samples from both locations present 2n = 38, with all chromosomes being acrocentric (FC = 38a). In both populations, 18S rDNA sequences are present on only one pair of homologous chromosomes and telomeric sequences occur only at the ends of the chromosomes. In the Tefé sample, the 5S rDNA occurs in two pairs, and the U2 snRNA in three pairs. These results are the first descriptions of these sequences for B. brevirostris samples from the Tefé locality, as well as the first karyotypic description for the Santarém locality. Future cytotaxonomic studies of this genus can benefit from these results.

Cytogenetic and Molecular Characterization of Eigenmannia aff. desantanai (Gymnotiformes: Sternopygidae): A First Report of System of Sex Chromosomes ZW1W2/ZZ in Gymnotiformes

Gymnotiformes a monophyletic group of fish endemic to the Neotropics, represent an important component of the freshwater ichthyofauna that presents relevant taxonomic problems. Thus, in view of the morphological complexity involving Eigenmannia (Gymnotiformes) fish species, this study aimed to characterize Eigenmannia aff. desantanai of the upper Paraguay River basin through cytogenetic and molecular analyses, to help in the correct identification and delimitation of species. This study reports a multiple sex system of the type ZW₁W₂/ZZ, with 2n = 31 for females and 2n = 30 for males. A single pair of chromosomes carrying the nucleolar organizing regions (NORs) was detected. The heterochromatin was colocated in NOR sites and mainly located in the centromeric regions of chromosomes. Besides that, individual sequences COI from the specimens of E. aff. desantanai were obtained, totalizing three haplotypes. The distance p between the haplotypes in E. aff. desantanai, ranged from 0.2% to 7.1%. Species delimitation tests indicated the existence of two possible operational taxonomic units of E. aff. desantanai. Thus, this study reports a new multiple sex system in Gymnotiformes and these specimens previously identified as E. aff. desantanai may belong to two distinct species.