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Transposons and retrotransposons represent a great portion of the eukaryotic genome. These repeated elements can be defined as genetic entities capable of self-inserting into different positions within the genome modifying the functions of the genes with which they are associated. The purpose of the present work was to isolate, identify, and map the sequences of Rex1 and Rex3 transposable elements present in the genomes of fishes of the family Loricariidae. The retrotransposable elements were identified through amplification with Rex1 and Rex3 primers, and the nucleotide composition of the retrotransposons was determined by automatic sequencing. In all the species analyzed it was possible to isolate Rex1 partial sequences from 520 to 563 bp in size and Rex3 partial sequences from 407 to 454 bp in size. Comparisons with the available literature showed that Loricariidae retrotransposons Rex1 and Rex3 have a high sequence similarity to putative homologous genes in different fish orders. Fluorescence in situ hybridization experiments conducted in 3 species, Hisonotus leucofrenatus, Pseudotocinclus tietensis, and Parotocinclus maculicauda, using the retrotransposable elements isolated for each species as probes, revealed that Rex1 and Rex3 share a similar dispersion pattern and are both found on all chromosomes of the 3 species organized in small clusters and dispersed on all the chromosomes, including euchromatic and heterochromatic regions, although a larger concentration was observed in the heterochromatic regions of H. leucofrenatus. The data found in the present study contradict much of the literature that substantiates that the retrotranspon elements are preferentially accumulated in the heterochromatin regions in fishes.

The analysis of the distribution patterns presented by examples of freshwater fishes restricted to headwater habitat: the anostomid Leporinus octomaculatus, the characins Jubiaba acanthogaster, Oligosarcus perdido, Moenkhausia cosmops, Knodus chapadae, Planaltina sp., the loricariid Hypostomus cochliodon, and the auchenipterid Centromochlus sp. provided evidences of a relatively recent shared history between the highlands of the upper rio Paraguay and adjoining upland drainage basins. Restricted to headwater of the uplands in the upper rio Paraguay and adjoining basins, these species provide biological evidence of the former extension of the central Brazilian plateau before the origin of the Pantanal Wetland. Disjunction took place due to an ecological barrier to these rheophilic taxa represented tectonic subsidence related to the origin of the Pantanal Wetland. Molecular analysis of Jubiaba acanthogaster revealed that the sample from the upper rio Xingu basin are the sister-group of a clade that includes samples from the upper rio Arinos (upper rio Tapajós) plus the upper rio Paraguay basin, supporting the assumption that the origin of the upper rio Paraguay basin causing vicariance between this basin and the upper rio Tapajós is the least vicariant event in the evolutionary history of the group.A análise do padrão de distribuição apresentado por exemplos de peixes de água doce neotropicais restritos à habitats reofílicos de cabeceira: o anostomídeo Leporinus octomaculatus, os caracídeos Jubiaba acanthogaster, Oligosarcus perdido, Moenkhausia cosmops, Knodus chapadae, Planaltina sp., o loricariídeo Hypostomus cochliodon e o auquenipterídeo Centromochlus sp. fornecem evidências de uma história compartilhada relativamente recente entre as terras altas do alto rio Paraguai e bacias de terras altas vizinhas. Restritas às cabeceiras das áreas altas nas bacias do alto rio Paraguai e bacias vizinhas, estas espécies fornecem evidências biológicas da pretérita extensão do Planalto Central Brasileiro anteriormente a origem tectônica da planície alagável do Pantanal. Disjunções ocorreram devido a barreira ecológica para estes taxons reofílicos representada pela subsidência tectônica associada à origem do Pantanal. A análise molecular de Jupiaba acanthogaster revelou que amostras da bacia do alto rio Xingu consistem no grupo-irmão de um clado mais inclusivo que inclui amostras do alto rio Arinos (alto rio Tapajós) mais o alto rio Paraguai, suportando a suposição de que a origem do alto rio Paraguai causou vicariância entre esta bacia e o alto rio Tapajós no último evento vicariante na história evolutiva do grupo.

Species of Neacomys are small cricetid rodents that occur in forested habitats of Central and South America, from eastern Panama to central Bolivia and central/western Brazil. In order to assess species diversity of this poorly known genus, we obtained cytochrome b gene sequences from the most comprehensive taxonomic and geographic sampling analyzed to date. We also conducted morphological analyses on a large series of specimens housed in 15 museums, including types of 10 out of 14 nominal taxa. Our analyses of the genetic data recovered 17

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