Gymnotus paraguensis, a Good Example of Phenotypic Plasticity in the Pantanal Biome, Brazil.

Gymnotus is the most studied genus of the order Gymnotiformes, but the morphological similarities of the different species make it difficult to identify taxa reliably. The present study is a continuation of the ongoing research into the taxonomic diversity of the stocks of Gymnotus sold as live bait in the Pantanal, Brazil. These studies have been based on cytogenetic analyses, DNA barcoding, and the analysis of coloration patterns. The results of the cytogenetic analysis confirmed the presence of three distinct strains, recognized as Gymnotus paraguensis, G. sylvius, and G. pantanal. However, the results revealed that the molecular operational taxonomic units identified as G. paraguensis actually include a relatively diverse set of fish, separated by considerable genetic distances. As the G. paraguensis specimens also presented considerable variation in coloration patterns, further genetic diversity analyses were conducted on these individuals, to test the hypothesis that more than one species is present in this cytotaxonomic unit. The haplotype network revealed a regional pattern in the distribution of this species. The results indicate that the observed variation in coloration patterns is associated with a high degree of phenotypic plasticity in G. paraguensis. These findings emphasize the importance of using an integrative approach for a more accurate diagnosis of Gymnotus, in particular, the species marketed as live bait for the fisheries of the upper Paraguay River basin in the Brazilian Pantanal.

New Comparative Cytogenetic Data on Three Genera of Armored Catfishes of Ancistrini (Loricariidae: Hypostominae).

The karyotypes and other chromosomal markers of 4 catfish species, namely Lasiancistrus schomburgkii, Lasiancistrus sp., Araichthysloro, and Megalancistrus sp., members of a taxonomically complex and speciose tribe of catfishes Ancistrini, Hypostominae, were examined using conventional (Giemsa staining, Ag-NOR, and C-banding) and molecular cytogenetic protocols (FISH) and DNA barcoding. In L. schomburgkii, Lasiancistrus sp., and A.loro a diploid number 2n = 54 was observed, with karyotypes composed of 28m + 16sm + 10st, 36m + 12sm + 6st chromosomes, while Megalancistrus sp. had 2n = 52, with the karyotype composed of 28m + 16sm + 8st chromosomes. The Ag-NOR phenotypes were simple in all 4 species, which was confirmed by FISH with an 18S rDNA probe. However, the positive 5S rDNA sites varied among species: 2 chromosome pairs in L. schomburgkii, Lasiancistrus sp., and A. loro, and only 1 pair in Megalancistrus sp. The blocks of constitutive heterochromatin were poorly visible in the pericentromeric and telomeric regions of most chromosomes of the examined species by C-banding. The genetic distance analysis, based on mtDNA COI gene sequences (DNA barcoding), confirmed the species as 4 taxonomic units. Ours and other published data indicate that karyotype differentiation among Ancistrini is complex and divergent and indicates the occurrence of common chromosomal rearrangements, such as pericentric inversions conserving the diploid number, and other rearrangements that are more frequent in some genera, such as centric fusions in Ancistrus.

Molecular discrimination of Ancistrus lineages (Siluriformes: Loricariidae) using barcode DNA tool.

Although several species of Ancistrus have been described from the Amazon and Paraguay river basins in the states of Amazonas and Mato Grosso, Brazil, the taxonomic status of most specimens from these regions remains doubtful. In the present work, cytogenetic and molecular data were used to discriminate and isolate unexpected Ancistrus lineages from the Amazon and Paraguay basins. For that, it was used DNA barcoding based on mitochondrial Cytochrome Oxidase Subunit I (COI) gene and cytogenic data to perform such molecular discrimination. The analyzed sequences had 669 bp, of which 171 bp were conserved and 491 bp were variable. The Neighbor-joining and Bayesian analysis revealed 21 distinct groups in topology. The genetic distances within each group was 0.4%, 21 times smaller than the mean distance observed among groups, which was 8.4%. These values showed seven distinct lineages of Ancistrus from the studied points of the Amazon basin and eight lineages from the Paraguay basin points. Our results illustrate the efficiency of this technique for the discrimination of the Ancistrus lineages once it indicates the occurrence of cryptic species in these regions, which cannot yet be identified either with just chromosomal or morphological analyzes.

Molecular Clock as Insight to Estimate the Evolutionary History and Times of Divergence for 10 Nominal Astyanax Species (Characiformes, Characidae): An Evolutionary Approach in Species with 2n = 36, 46, 48, and 50 Chromosomes.

Astyanax is a genus with a wide distribution ranging from the south United States to north of Patagonia (Argentina). The available cytogenetic data on Astyanax indicate a high karyotypic diversity, with diploid number of 36-52 chromosomes, presence of B chromosomes, heterochromatin polymorphism, and variations with respect to the number and localization of nucleolar organizer regions (NORs) and 18S and 5S ribosomal DNA sites. In the present study, we estimated the evolutionary history and times of divergence for 10 nominal Astyanax species from the South and Central/North American (Cna) continents, which present distinct chromosomal characteristics, based on molecular clocks inferred from mitochondrial DNA sequence. The molecular clock results indicate the origin of three distinct clades (Humeral dark spot [Hds]; Diffuse humeral spot [Dhs]; Cna group) during the late Miocene about 11.2 million years ago (Mya). Thus, Astyanax mexicanus (Cna) represent a species that diverged a long time ago (∼8.6 Mya) from the Hds group, and Astyanax schubarti is the oldest species (∼6.5 Mya) among the Dhs species.

Comparative cytogenetics of Astyanax (Teleostei: Characidae) from the upper Paraguay basin

Astyanax is one of the most abundant and diverse taxa of fishes in the Neotropical region. In order to increase the amount of cytogenetic information for Astyanax as well as to exhibit data to subsidize future taxonomic studies, this work analyzed three species of Astyanax: two species are cryptic, and are here reported to live in syntopy (A. abramis and A. lacustris); the first karyotype description for A. pirapuan is also presented. Cytogenetic analyzes reveal a diploid number of 2n=50 chromosomes for three species, yet with differences in their karyotype morphology. The physical mapping of 18S rDNA showed up to thirteen sites in A. pirapuan and two in A. abramis and A. lacustris. The physical mapping of 5S rDNA has proven to be an effective marker for the characterization of species of Astyanax studied in this work.(AU)

Astyanax é um dos táxons mais representados e diversos na região Neotropical. Com o intuito de aumentar as informações citogenéticas para Astyanax e apresentar dados que possam subsidiar estudos taxonômicos futuros, este trabalho traz uma análise citogenética de três espécies de Astyanax: duas espécies consideradas crípticas, aqui reportadas em sintopia (A. abramis e A. lacustris) e a primeira descrição cariotípica de A. pirapuan. As análises citogenéticas revelaram 2n=50 cromossomos para as três espécies, com diferença na morfologia cariotípica de cada uma. Foram observados apenas dois sítios de rDNA 18S em A. abramis e A. lacustris e até 13 para A. pirapuan. O mapeamento físico do rDNA 5S se mostrou como um marcador efetivo para a caracterização das espécies de Astyanax abordadas neste estudo.(AU)

Chromosome mapping of a Tc1-like transposon in species of the catfish Ancistrus.

The Tc1 mariner element is widely distributed among organisms and have been already described in different species of fish. The genus Ancistrus (Kner, 1854) has 68 nominal species and is part of an interesting taxonomic and cytogenetic group, as well as presenting a variation of chromosome number, ranging from 2n=34 to 54 chromosomes, and the existence of simple and multiple sex chromosome system and the occurrence of chromosomal polymorphisms involving chromosomes that carry the nucleolus organizer region. In this study, a repetitive element by restriction enzyme, from Ancistrus sp.1 "Flecha" was isolated, which showed similarity with a transposable element Tc1-mariner. Its chromosomal location is distributed in heterochromatic regions and along the chromosomal arms of all specimens covered in this study, confirming the pattern dispersed of this element found in other studies carried out with other species. Thus, this result reinforces the hypothesis that the sequence AnDraI is really a dispersed element isolated. As this isolated sequence showed the same pattern in all species which have different sex chromosomes systems, including in all sex chromosomes, we could know that it is not involved in sex chromosome differentiation.

Distinct classical and molecular cytogenetics of Astyanax marionae and A. fasciatus (Characiformes: Characidae): a comparative study of the organization of heterochromatin and repetitive genes.

Genus Astyanax is well distributed in Neotropical freshwater environments and its taxonomic position is uncertain, as is the case with other Characidae genera allocated in the group incertae sedis. This study aimed to analyse the karyotype of different populations of Astyanax fasciatus (Corumbataí River basin) using Giemsa staining, C-band technique, and fluorescence in situ hybridization for the H3 histone and 5S rRNA genes, in addition we describe for the first time the chromosomal organization of H3 histone and 5S rRNAgenes in A. marionae (ParaguayRiver basin). Chromosomes of three A. fasciatus populations were analysed (two with 2n = 50 and one with 2n = 48) and the heterochromatin was organized in two forms (blocks with blurred boundaries and distinct blocks). H3 histone and 5S rRNA genes were observed in all the three populations of A. fasciatus on two chromosome pairs (one metacentric chromosome showing H3 histone and 5S rRNA gene clusters). In A. marionae (2n = 48), H3 histone and 5S rRNA genes were observed in one acrocentric chromosome pair (different pairs). Further, differences between karyotypes and heterochromatin, as well as the chromosomal organization of H3 histone and 5S rRNA genes in Astyanax species, focussing on chromosome evolution in the group are discussed.

Karyotype Diversity in Doradidae (Siluriformes, Doradoidea) and Presence of the Heteromorphic ZZ/ZW Sex Chromosome System in the Family.

Karyotypes and other chromosomal markers as revealed by conventional and molecular cytogenetic protocols in four species of the catfish family Doradidae from the Araguaia-Tocantins river basin, namely Hassar wilderi, Leptodoras cataniae, Tenellus leporhinus and Tenellus trimaculatus were examined. All species had diploid chromosome number 2n = 58 and karyotypes dominated by biarmed chromosomes, simple NOR phenotype, that is, one chromosome pair bearing this site in terminal position, but some differences in karyotypes and distribution of constitutive heterochromatin, position of rDNA sites. Such characteristics appeared species-specific. A ZZ/ZW sex chromosome system was found in Tenellus trimaculatus, resulting likely from the amplification of the heterochromatin, followed by a paracentric inversion. Our results confirmed low karyotype differentiation observed until now among representatives of this endemic catfish family.

Highly Similar Morphologies Between Chromosomes Bearing U2 snRNA Gene Clusters in the Group Astyanax Baird and Girard, 1854 (Characiformes, Characidae): An Evolutionary Approach in Species with 2n = 36, 46, 48, and 50.

Repetitive sequences and their chromosomal locations have been widely studied in species of the Astyanax genus. However, the chromosomal organization of U2 snDNA remains largely unknown. The aims of this study were to examine the chromosomal contexts of U2 snRNA and 5S rRNA genes in Astyanax species and determine the degree of chromosome morphological similarity between species with different diploid numbers. Clusters of U2 snDNA and 5S rDNA were determined in nine species of Astyanax, including two karyomorphs of Astyanax fasciatus Cuvier, 1819. All species exhibited U2 snDNA clusters on two chromosome pairs, except Astyanax mexicanus De Filippi, 1853 (one pair). The 5S rDNA clusters were located on one chromosome pair in Astyanax altiparanae Garutti and Britski, 2000, and Astyanax marionae Eigenmann, 1911, two pairs in Astyanax abramis Jenyns, 1842, Astyanax asuncionensis Géry, 1972, Astyanax bockmanni Vari and Castro, 2007, Astyanax eigenmanniorum Cope, 1894, A. fasciatus (karyomorphs I and II), and Astyanax schubarti Britski, 1964, and four pairs in A. mexicanus. The relationships between the repetitive sequences in different species suggest that A. schubarti and A. mexicanus exhibit an unusual U2 snDNA chromosomal format as a result of events occurring in the evolutionary history of the Astyanax group.

Microstructural chromosome reorganization in the genus Trichomycterus (Siluriformes: Trichomycteridae)

Trichomycterus is a specious fish genus within Trichomycterinae and displays remarkable karyotype diversity. However, knowledge about their genomic structure and location of repetitive sequence is still limited. In order to better understand the karyotype diversification, we analyzed nine species of Trichomycterus using classical and molecular cytogenetic techniques. Results revealed a conserved diploid chromosome number of 2n=54 chromosomes in all analyzed species, although remarkable differences on the constitutive heterochromatin distribution were observed. In addition, while the 18S rDNA showed a conserved distribution pattern, the 5S rDNA sites showed a quite diverse location considering the analyzed species. Remarkably, both ribosomal genes were co-located in all species, except in T . iheringi , suggesting that co-localization is probably an ancestral condition in Trichomycterus . Finally, three analyzed species showed heterochromatic B chromosomes, reinforcing the intense genomic reorganization occurring in Trichomycterus . Our results showed that chromosomal variations are not restricted to differences in karyotype formula as previously proposed, but also to modifications on the microstructural level of resolution.

Trichomycterus é um especioso gênero dentro de Trichomycterinae e exibe marcante diversidade cariotípica. No entanto, o conhecimento sobre sua estrutura genômica e localização de seqüências repetitivas ainda é restrita. Para um melhor conhecimento sobre a sua diversificação cariotípica, nós analisamos nove especies de Trichomycterus usando técnicas de citogenética clássica e molecular. Os resultados revelaram um conservado número diploide de 2n = 54 cromossomos em todas as espécies analisadas, embora diferentes marcações na distribuição da heterocromatina constitutiva tenham sido observadas. Além disso, enquanto o DNAr 18S mostrou um padrão de distribuição conservado, os sítios de DNAr 5S mostraram uma localização bastante diversa, considerando as espécies analisadas. Ambos os genes ribossomais foram co-localizados em todas as espécies, exceto em T. iheringi , sugerindo que a co-localização é provavelmente uma condição ancestral em Trichomycterus . Finalmente, três espécies analisadas mostraram cromossomos B heterocromáticos, reforçando uma intensa reoganização genômica ocorrendo em Trichomycterus . Nossos resultados mostraram que variações cromossômicas não estão restritas à diferenças na fórmula cariotípica, como proposto anteriormente, mas também às alterações a níveis de resolução estrutural.

Diversity and chromosomal evolution in the genus Ancistrus Kner, 1854 (Loricariidae: Ancistrini) from three hydrographic basins of Mato Grosso State, Brazil

Cytogenetic analyses were carried out in 117 specimens of seven species of the genus Ancistrus from three hydrographic in Mato Grosso State: Paraguay, Araguaia-Tocantins and Amazon basins. Conventional cytogenetic techniques were used to obtain mitotic chromosomes. C-banding was performed to detect heterochromatic regions and silver nitrate staining was used to identify nucleolar organizer regions (Ag-NORs). The counted and paired chromosomes revealed diploid numbers ranging from 2n = 40 to 2n = 54 with karyotype formulae varying from FN = 80 to FN = 86. Single marks in distinct chromosomes identified the nucleolar organizer regions. The constitutive heterochromatin was scarce in the diploid number from 2n = 50 to 2n = 54 and conspicuous blocks were observed in a single species with 2n = 40 chromosomes. These data corroborate the hypotheses of reduction of diploid number in species with derived features such as presence of sex chromosomes and polymorphisms, besides allowing inferences about the evolutionary mechanisms and the ancestor karyotype that favored the diversification of this important genus in the tribe Ancistrini.

Foram realizadas análises citogenéticas de 117 espécimes do gênero Ancistrus de três bacias hidrográficas do estado de Mato Grosso: Paraguai, Araguaia-Tocantins e Amazônica, utilizando as técnicas de citogenética convencional para obtenção de cromossomos mitóticos, visualização de regiões heterocromáticas e regiões organizadoras de nucléolos. Os cromossomos pareados revelaram uma variação no número diploide de 2n = 40 a 2n = 54 e número fundamental de NF = 80 a NF = 86. As regiões organizadoras de nucléolos foram evidenciadas em um único par de cromossomos para todas as espécies e a heterocromatina é escassa nas espécies com números diploides elevados (2n = 50 a 2n = 54). Os blocos heterocromáticos mais evidentes foram observados nos pares portadores das AgRONs e em cromossomos da espécie com 2n = 40. Estes dados contribuem para a hipótese de redução do número diploide nas espécies que apresentam polimorfismos cromossômicos e cromossomos sexuais, além de contribuir para inferências sobre os mecanismos de evolução cariotípica que favoreceu a diversificação do gênero Ancistrus, o mais representativo na tribo Ancistrini.

Genetic structure of the ornamental tetra fish species Piabucus melanostomus Holmberg, 1891 (CHARACIDAE, IGUANODECTINAE) in the Brazilian Pantanal wetlands inferred by mitochondrial DNA sequences / Estrutura genética da espécie de peixe ornamental tetra Piabucus melanostomus Holmberg, 1891 (CHARACIDAE, IGUANODECTINAE) do Pantanal brasileiro inferida por sequências de DNA mitocondrial

The subfamily Iguanodectinae comprises a group of small Neotropical fishes composed by two genera and 11 nominal species widely distributed in the Atlantic drainages of South America. Piabucus is the only genus of Iguanodectinae found in the Paraguay River basin, especially in the Pantanal of Mato Grosso State, where it is represented by Piabucus melanostomus. Given the wide distribution and the low dispersion capacity of this species, due the ecological constraints, it is possible that many interesting genetic features could be found in different populations. In this way, the aim of his work was to perform the phylogeographic pattern of P. melanostomus populations using mitochondrial DNA sequences. A total of 13 individuals from three rivers belonging the Mato Grosso wetland were sampled. The ATP sintetase (subunits 6 and 8) gene was completely sequenced, the mean of nucleotide base composition in the sequences was 31.2% (T), 30.2% (C), 26.9% (A) and 11.9% (G), with no gene saturation. The population analysis in the TCS program generated a network with six haplotypes (A to F), where the ancestral haplotype (A) has a frequency of 25% and is composed by individuals from Cuiabá and Paraguay Rivers. The phylogenetic analysis showed the occurrence of two mtDNA lineages (1 and 2), the distance observed between the two lineages was 0.6%. The phylogenetic and phylogeographic results as well as the negative values of Fst for some populations, indicate a possible occurrence of gene flow among the analyzed populations. These results highlights the importance of flood pulse existent on wetland as a vehicle that permits a temporary connection among isolated population maintaining the species genetic variability.

A subfamília Iguanodectinae compreende um grupo de pequenos peixes neotropicais composta de dois gêneros e 11 espécies nominais amplamente distribuídas nas drenagens do Atlântico da América do Sul. Piabucus é o único gênero de Iguanodectinae encontrado na bacia do rio Paraguai, especialmente no Pantanal de Mato Grosso, onde é representada por Piabucus melanostomus. Dada a ampla distribuição e a baixa capacidade de dispersão desta espécie, devido às limitações ecológicas, é possível que características genéticas interessantes possam ser encontradas em diferentes populações. Deste modo, o objetivo deste trabalho foi estabelecer os padrões filogeográficos de populações de P. melanostomus utilizando sequências de DNA mitocondrial. Foram amostrados 13 indivíduos de três rios pertencentes ao Pantanal do Mato Grosso. O gene ATP sintetase (subunidades 6 e 8) foi completamente sequenciado, a média da composição de base de nucleotídeos nas sequências foi de 31,2% (T), 30,2% (C), 26,9% (A) e 11,9% (G), não havendo saturação. A análise populacional no programa TCS gerou uma rede com seis haplótipos (A a F), onde o haplótipo ancestral (A) tem uma freqüência de 25% e é composto por indivíduos dos rios Cuiabá e Paraguai. A análise filogenética mostrou a ocorrência de duas linhagens de DNA (1 e 2), a distância observada entre as duas linhagens foi de 0,6%. Os resultados filogenéticos e filogeográficos, bem como os valores negativos de FST para algumas populações, indicam uma possível ocorrência de fluxo de genes entre as populações analisadas. Estes resultados destacam a importância do pulso de inundação existente em zonas úmidas como um veículo que permite uma conexão temporária entre a população isolada, mantendo a variabilidade genética das espécies.

Cytogenetic analysis in Thoracocharax stellatus (Kner, 1858) (Characiformes, Gasteropelecidae) from Paraguay River Basin, Mato Grosso, Brazil.

Thoracocharax stellatus (Characiformes, Gasteropelecidae) is a small Neotropical species of fish, widely distributed in several rivers of South America. Evidence for karyotype heteromorphysm in populations from different geographical regions has been reported for this species. In this way, populations of Thoracocharax stellatus from the Paraguay River basin were cytogenetically characterized and the results were compared with other studies performed in the same species but from different basins. The results showed a diploid number of 2n = 54 for Thoracocharax stellatus, with chromosomes arranged in 6 metacentric (m), 6 submetacentric (sm), 2 subtelocentric (st) and 40 acrocentric (a), for both sexes, with a simple Nucleolus Organiser Region (NOR) system reported by the techniques of silver nitrate impregnation and fluorescent in situ hybridisation (FISH) using 18S rDNA sequences as probe. The distribution of constitutive heterochromatin, observed by the C-band technique and Chromomycin A3 staining showed great similarity among the analyzed populations and consists mainly of discrete blocks in the pericentromeric and telomeric regions of most chromosomes. The presence of female heterogamety was also observed indicating a ZZ/ZW system with W chromosome almost totally heterochromatic. The results also show cytogenetic diversity of the group and are useful to understand the mechanisms of karyotype evolution of the family.

Chromosomal diversification in ribosomal DNA sites in Ancistrus Kner, 1854 (Loricariidae, Ancistrini) from three hydrographic basins of Mato Grosso, Brazil.

Populations of seven Ancistrus species were analyzed from streams and rivers of three hydrographic Brazilian basins. All populations showed different diploid numbers (2n), fundamental numbers (FNs), and karyotypes. Some representatives of Loricariidae have 2n = 54 chromosomes, which is very likely an ancestral cytotaxonomic characteristic, but many other representatives show extensive karyotype diversification. In the Ancistrus species studied, extensive karyotypic differentiation, which is generally associated with chromosome number reduction and rearrangement of the ribosomal RNA gene (rDNA) sites, was verified. Chromosomal locations of 18S and 5S rDNA were jointly detected using fluorescence in situ hybridization (FISH). In all the Ancistrus species analyzed, 18S rDNA sites were detected only on one chromosome pair, though this differed among species. 5S rDNA was located on 1-3 chromosome pairs either separately or in synteny with 18S rDNA in four of the seven species/populations. Hence the karyotype differentiation in Ancistrus species could be associated with a morphological speciation process, suggesting that chromosome fusions, inversions, deletions, duplications, and heterochromatination could contribute to the karyotype evolution of these neotropical armored catfishes.

Análise morfométrica em Thoracocharax stellatus (Kner, 1858) (Characiformes, Gasteropelecidae) proveniente de diferentes bacias hidrográficas Sul-americanas / Morphometrics analysis inThoracocharax stellatus (Kner, 1858) (Characiformes, Gasteropelecidae) from different South American river basins

Foram utilizadas análises de variáveis canônicas livres de tamanho com o objetivo de investigar os padrões de variação morfológica entre populações de Thoracocharax stellatus em bacias hidrográficas Sul-Americanas: rios das bacias Araguaia-Tocantins e Paraguai do Brasil e Orinoco na Venezuela. Distinções entre as amostras do Araguaia-Tocantins e Orinoco foram observadas, com leve sobreposição dessas populações com as amostras da bacia do Paraguai. Os caracteres morfométricos que são os principais responsáveis por esta diversificação são comprimento da cabeça e comprimento da nadadeira dorsal. Os mecanismos que podem estar atuando nesta variação geográfica entre populações de T. stellatus são discutidos.

A free-size canonical variable analysis was made to investigate the morphological pattern of variation among different Thoracocharax stellatus populations in South American river basins: Araguaia-Tocantins and Paraguay Basins Rivers from Brazil, and Orinoco's basin in Venezuela. Distinction among the samples from Araguaia-Tocantins and Orinoco were observed, with overlap of these populations with the Paraguay's basin samples. The primarily characters responsible for this discrimination are head length and dorsal fin length. The mechanisms that can act in this geographic variation among T. stellatus populations are discussed.

Chromosome polymorphism in Ancistrus cuiabae Knaack, 1999 (Siluriformes: Loricariidae: Ancistrini)

Cytogenetic and FISH analyses were performed in 30 Ancistrus cuiabae specimens from a bay near the town of Poconé, in the Pantanal of Mato Grosso, Brazil. The observed diploid number was 2n = 34 chromosomes for both sexes and three distinct katyotypic formulae were found, namely cytotype A (20m, 8sm, 6st, Fundamental Number/FN = 68; 6 males and 11 females), cytotype B (19m, 8sm, 6st, 1a, FN = 67; 8 males and 4 females) and cytotype C (18m, 8sm, 6st, 2a, FN = 66; a single male). NORs's analyses showed that these regions were located in distinct sites on the NOR-bearing chromosome pair, according to cytotypes. Thus, in cytotype A, NORs were located in the terminal region of the short arm of the second metacentric chromosome pair; in cytotype B, they were detected in the short arm of the metacentric chromosome and interstitially on the acrocentric chromosome and, in cytotype C, NORs were observed in the interstitial region of the acrocentric chromosome pair. C-positive heterochromatic bands were adjacent to the rDNA sites in the corresponding chromosomes. Thus, the chromosomal polymorphism of A. cuiabae was probably originated through a pericentric inversion in chromosome pair nº 2 involving the NOR sites, which represents a novelty in the Ancistrini tribe. The results also broaden the knowledge of the chromosomal evolution in Ancistrus, the most derived genus of the Ancistrini tribe.(AU)

Foram analisados, com técnicas convencionais de citogenética e FISH, 30 exemplares da espécie Ancistrus cuiabae da baía Arrombado, próximo a Poconé, Pantanal do Mato Grosso. Foram observadas metáfases com número diploide 2n = 34 cromossomos para ambos os sexos e três fórmulas cariotípicas distintas, aqui denominadas de citótipo A, verificado em 06 machos e 11 fêmeas (20m, 8sm, 6st, Número Fundamental, NF = 68); citótipo B, em 08 machos e 04 fêmeas (19m, 8sm, 6st, 1a, NF = 67) e citótipo C em apenas 01 macho (18m, 8sm, 6st, 2a, NF = 66). As NORs confirmaram os distintos citótipos verificados, além de evidenciar que os cromossomos portadores de rDNA são os que representam o polimorfismo na espécie Ancistrus cuiabae. No citótipo A, as NORs foram verificadas na região terminal do braço curto do segundo par de cromossomos metacêntricos; no citótipo B, foram evidenciadas no segundo par, heteromórfico, no braço curto do cromossomo metacêntrico e intersticial no seu homólogo acrocêntrico; no citótipo C as NORs foram observadas na região intersticial num par de cromossomos acrocêntricos. A análise da heterocromatina constitutiva evidenciou blocos discretos adjacentes ao rDNA no segundo par de cromossomos de ambos os citótipos. Uma provável inversão pericêntrica é a hipótese proposta para a origem deste polimorfismo na espécie Ancistrus cuiabae. Estes resultados ampliam o conhecimento sobre o gênero Ancistrus, o mais derivado da tribo, contribuem para o conhecimento sobre este grupo de peixes e para inferir sobre a evolução cromossômica dos Ancistrini(AU)

Chromosomal differentiation between populations of Oligosarcus hepsetus (Teleostei, Characidae) from small tributaries at opposite margins of the Paraíba do Sul river (Brazil)

Cytogenetic analyses were carried out in two populations of Oligosarcus hepsetus from tributaries at opposite margins of the Paraíba do Sul river. The same diploid number was observed in both populations (2n=50), but they showed remarkable differences related to karyotype formula and distribution of rDNA sites as revealed by silver nitrate staining and in situ hybridization with 18S probes. The results suggested that the main channel of the Paraíba do Sul river acted as barrier to gene flow between populations.

Análises citogenéticas foram realizadas em duas populações de Oligosarcus hepsetus provenientes de riachos de margens opostas do rio Paraíba do Sul. O número diplóide foi o mesmo para ambas as populações (2n=50), porém diferenças significativas foram observadas com relação à fórmula cariotípica e distribuição dos sítios de DNAr detectados pela impregnação por prata e pela hibridação "in situ" com sonda 18S. Estes resultados sugerem que a calha principal do rio Paraíba do Sul estaria atuando como uma barreira para o fluxo gênico entre essas populações.

Chromosomal divergence and maintenance of sympatric Characidium fish species (Crenuchidae, Characidiinae).

Cytogenetic studies were performed in two syntopic species of Characidium, C. lauroi and Characidium sp. cf. C. alipioi, from Ribeirão Grande, Paraíba do Sul river basin. Both species have diploid number 2n=50 chromosomes, but differ in chromosome shape, C-banding pattern and location of nucleolar organizing regions. In Characidium sp. cf. C. alipioi a new type of ZW sex chromosome system composed of equal sized metacentric chromosomes is reported for the first time in the genus Characidium. Species of Characidium with a sex chromosome system form a monophyletic group. Variations in this system are interpreted as resulting from geographic isolation among allopatric species.