The electric eel, Electrophorus electricus, the only species of its genus, has a wide distribution in the Amazon and Orinoco drainages. There is little previous information regarding the population variation in E. electricus, with only basic karyotype data from two populations (Amazon and Araguaia Rivers). Karyotypic description and analysis of CO1 barcode sequences were performed for E. electricus from three localities (Caripetuba, Irituia, and Maicuru Rivers). All samples share the 2n=52 (42 m-sm [meta-submetacentric] +10 st-a [subtelo-acrocentric]) with previously studied material. However, the Maicuru River samples differ from the other populations, as they have B chromosomes. The distribution of noncentromeric constitutive heterochromatin between samples is relatively divergent. All samples analyzed present the Nucleolar Organizer Region (NOR) located in a single chromosome pair. In the samples from Caripetuba, NORs were colocalized with a heterochromatin block, whereas the NOR was flanked by heterochromatin in Maicuru River samples and pericentromeric heterochromatin adjacent NOR was found in Irituia River samples. Alignment of CO1 barcode sequences indicated no significant differentiation between the samples analyzed. Results suggest that karyotypic differences between samples from the Caripetuba, Irituia, and Amazon Rivers represent chromosome polymorphisms. However, differences between the samples from the Maicuru and Araguaia Rivers and the remaining populations could represent interpopulation differentiation, which has not had time to accrue divergence at the CO1 gene level.
Cytogenetic studies were carried out on samples of Parapteronotus hasemani, Sternarchogiton preto and Sternarchorhamphus muelleri (Apteronotidae, Gymnotiformes) from the Amazon basin. The first two species exhibited both a 2n = 52 karyotype, but differed in their karyotypic formulae, distribution of constitutive heterochromatin, and chromosomal location of the NOR. The third species, Sternarchorhamphus muelleri, was found to have a 2n = 32 karyotype. In all three species the DAPI and chromomycin A3 staining results were consistent with the C-banding results and nucleolar organizer region (NOR) localization. The 18S rDNA probe confirmed that there was only one pair of ribosomal DNA cistron bearers per species. The telomeric probe did not reveal interstitial telomeric sequences (ITS). The karyotypic differences among these species can be used for taxonomic identification. These data will be useful in future studies of these fishes and help understanding the phylogenetic relationships and chromosomal evolution of the Apteronotidae.
O gênero Gymnotus é um dos gêneros mais especiosos (40 espécies) e mais amplamente distribuídos nas águas continentais da América do Sul, com maior ocorrência na bacia amazônica, onde varias espécies ocorrem em simpatria. A citogenética tem sido uma importante ferramenta na citotaxinomia e na elucidação dos processos evolutivos no gênero, que possui uma diversidade cariotípica elevada, sendo os cariótipos espécie-específicos, mesmo nas espécies que possuem o mesmo número diploide e são um fator de isolamento reprodutivo pré-zigotico nas espécies em simpatria. Cromossomos metafásicos de Gymnotus arapaima (GAR, 2n=44, FC: 24 m/sm + 20 st/a) foram hibridizados com sondas de cromossomo total de Gymnotus carapo (GCA, 2n=42). A sonda da região 1 (GCA 20) hibridizou um único cromossomo, o GAR 19, portador da NOR. As sondas da região 2 (GCA 1, 2, 3 e 16) hibridizaram os cromossomos de GAR: 1, 2, 14 e 18 e 21, o braço curto do par 4 e braço curto e uma parte do braço longo do par 13. As da região 3 (GCA 4, 5, 6, 7, 8, 17, 18, 19) hibridizaram os cromossomos de GAR 3, 5, 6, 5, 16, 20 e 22, o braço longo do par 4 e parte do braço longo do par 13. As da região 4 (GCA 9 – 15 e 21) hibridizaram os cromossomo 7-12 e 17. Até a presente análise pelo menos 3 pares (GAR 1, 16 e 20) se encontram conservados em relação ao cariótipo de GCA, enquanto outros encontram-se fragmentados e/ou fusionados. A pintura cromossômica em Gymnotus é uma ferramenta recente que, como observado no cariotipo analisado e em trabalhos anteriores, vem mostrando que os rearranjos cromossômicos no gênero são numerosos e o processo evolutivo dinâmico.
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