Comparative cytogenetics and molecular phylogeography in the group astyanax altiparanae--Astyanax aff. bimaculatus (Teleostei, Characidae).

The genus Astyanax comprises small characin fish of the neotropical region. The so-called 'yellow-tailed characins' compose one of the most widely distributed Astyanax groups. A. altiparanae and A. aff. bimaculatus, are evolutionarily closely related and commonly found in several Brazilian hydrographic basins. In the present work, chromosomal data of specimens of A. altiparanae and A. aff. bimaculatus from 4 hydrographic basins in the states of São Paulo (Upper Tietê, Paranapanema, Ribeira de Iguape) and Rio de Janeiro (Guapimirim) are shown. All the populations showed 50 chromosomes, with different karyotypic formula. Although only a single Ag-NOR bearing chromosome pair was observed, all populations possess multiple cistrons of 18S rDNA. FISH with the 5S rDNA probe showed single signals at the interstitial position of one metacentric chromosome pair. C-bands are distributed in the terminal and interstitial regions of several chromosomes. However, the As-51 satDNA are frugally located in a few chromosomes of fishes from Upper Tietê, Paranapanema and Guapimirim Rivers, being absent in individuals of A. aff. bimaculatus from Ribeira de Iguape River basin. Beside these 4 populations, molecular phylogeography studies were also performed in individuals from Middle and Lower Tietê River basin and from 2 additional collection sites in the Paranapanema and Ribeira de Iguape River basins. The phylogeographic analysis using 2 mtDNA regions (totalizing 1.314 bp of ND2 and ATPase6/8 genes) of 8 populations of the group of 'yellow-tailed characins' from 3 major hydrographic basins showed structuring of populations, suggesting a correlation between chromosomal (nuclear) and molecular (mitochondrial) data.

Chromosome polymorphism in Astyanax fasciatus (Teleostei, Characidae). 1. Karyotype analysis, Ag-NORs and mapping of the 18S and 5S ribosomal genes in sympatric karyotypes and their possible hybrid forms.

Astyanax fasciatus may be characterized as a chromosomally diversified 'species' presenting distinct cytotypes, each with its specific variants. The sympatric and syntopic occurrence of different cytotypes reinforces the hypothesis in which A. fasciatus may represent a group of species currently placed under a single common designation. Specimens from three collection points spread along the Mogi-Guaçu River in southeast Brazil were examined in the present work: (1) near its headwaters (Ouro Fino--MG), (2) in the middle region of the river (Cachoeira de Emas, Pirassunun ga--SP) and (3) close to its confluence with the Pardo River (Barrinha--SP). The 2n = 48 chromosomes cytotype was found in all sampling points, while cytotype 2n = 46 was only encountered in Barrinha and Cachoeira de Emas. In the latter locality, cytotype 2n = 46 predominated; nevertheless, other karyotype forms with 2n = 45 and 47 chromosomes also occurred, besides a structural variant of cytotype 2n = 46. One specimen with 2n = 47 chromosomes was also found in Ouro Fino. The Ag-NOR analysis, as well as the location of the 18S and 5S ribosomal genes, were conserved in all cytotypes. The data indicate that the variant karyotypes are a consequence of interbreeding between the standard cytotypes (2n = 46 and 48) and/or its descendants. This suggests a karyotype plasticity for this species, where at least a few variant karyotypes would not have deleterious effects on their bearers.

Karyotypic diversity and evolution of Loricariidae (Pisces, Siluriformes).

We present cytogenetic analyses of four fish species, belonging to four Loricariidae subfamilies: Neoplecostomus microps (Neoplecostominae) with 2n=54 chromosomes, Harttia loricariformis (Loricariinae) with 2n=56 chromosomes, Hypostomus affinis (Hypostominae) with 2n=66 chromosomes and Upsilodus sp. (Upsilodinae), with 2n=96 chromosomes. In addition to karyotypes, data on the location of 18s rDNA sites are presented, derived from indirect (silver nitrate impregnation) and direct (FISH) methods. There is only one pair of nucleolar organizing regions (NORs) per species, except in H. affinis. Diversity and NOR macrokaryotypic evolution in the species analyzed are discussed in relation to the evolution of the Loricariidae as a whole. In addition, a revision of the cytogenetic data available for this family is presented.

Gene mapping of 5S rDNA sites in eight fish species from the Paraíba do Sul river basin, Brazil.

The use of improved cytogenetic techniques such as fluorescent in situ hybridization (FISH) has offered important methodologies for cytotaxonomic and evolutionary studies. In particular, the mapping of 5S rDNA sites has proved to be an excellent marker in the study of different organisms and, more recently, in fish. In the present work, the FISH technique was used to map the 5S rDNA sites in the chromosomes of eight neotropical fish species from the Paraíba do Sul river basin, four of these belonging to the order Characiformes, family Characidae, genus Astyanax (A. scabripinnis, A. parahybae, A. giton and A. intermedius) and four to the order Siluriformes, family Loricariidae (Neoplecostomus microps, Harttia loricariformis, Hypostomus affinis and Upsilodus sp.). Karyotype evolution aspects of the analyzed groups are discussed.