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physical mapping of the 18s and 5s ribosomal genes in nine characidae species (teleostei, characiformes)


作者單位:Departamento de Genética e Evolução, Universidade Federal de São Carlos, São Carlos, SP, Brazil

【摘要】  characidae is one of the largest fish families of the neotropical region, and presenting a pronounced morphological variability, certainly does not constitute a monophyletic group. the cytogenetical data also show a large chromosomal variation and can provide important information for a better understanding of the relationships between the species of this group. 18s and 5s rdna probes were used in the present study for the chromosomal mapping in different characidae species from the são francisco river ( astyanax lacustris, astyanax scabripinnis, hasemania nana, piabina argentea, orthospinus franciscensis, serrapinnus heterodon, serrapinnus piaba and myleus micans ) and alto paraná ( astyanax altiparanae ) basins. species with a single pair of chromosomes bearing the nucleolar organizing regions (nors) were identified, as well as species with multiple nors, up to a maximum of seven 18s rdna sites. the number of 5s rdna site was also not constant, varying from two to eight. the mapping of the ribosomal genes was useful for the characterization and differentiation of the analyzed species.

【關鍵詞】  cytogenetics neotropical fish fluorescent in situ hybridization (fish) rdna probe


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the ribosomal genes are organized into two distinct multigenic families in eukaryotic organisms, one family comprising the 45s rdna and the other the 5s rdna. the repetitive 45s rdna units are separated by non-transcribed external spacers and are composed of the 18s, 5.8s and 28s genes, which constitute the nucleolar organizing regions (nors). nors that were active in the preceding interphase are commonly detected by the silver nitrate staining (ag-nors). in lower vertebrates, nors can also be evidenced by gc-specific fluorochromes such as chromomycin a 3 and mithramycin, independent of their activity, due to their gc-rich nature (schmid, 1980; schmid and guttenbach, 1988). nevertheless, the use of fluorochromes may not be conclusive for nor studies since gc-rich heterochromatic regions not associated with nors may be visualized (souza et al., 2001) and at the same time the few 45s rdna sites may not appear clearly (mandrioli et al., 2001; souza et al., 2001). thus, the fluorescent in situ hybridization (fish) becomes an important alternative to the study of nors due to the higher specificity of this methodology.

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