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.

Four populations of Astyanax hastatus Myers 1928 from the Guapimirim River basin (Rio de Janeiro State) were analyzed and three distinct cytotypes identified. These cytotypes presented 2n = 50 chromosomes, with 4M+8SM+10ST+28A (Cytotype A), 8M+10SM+14ST+18A (Cytotype B), 6M+8SM+4ST+32A (Cytotype C) and scanty heterochromatin, mainly located throughout pericentromeric regions of several chromosomal pairs. No homologies with the As-51 satellite DNA were observed in the three cytotypes, although all of them presented multiple 18S rDNA sites, as detected by both silver nitrate staining and FISH (fluorescent in situ hybridization). The application of the term “species complex” in Astyanax is discussed from a cytotaxonomic viewpoint.

The genus Hypostomus has a broad geographic distribution in Brazilian rivers and comprises armored catfishes with a very complicated taxonomy due to the absence of morphological autapomorphies. The existence of nearly 10 allopatric populations with different karyotypes suggests that Hypostomusancistroides represents a species complex in the Upper Paraná River basin. In this paper, an unusual karyotype of an isolated H. aff. ancistroides population was investigated. All specimens of this sample have 2n = 66 chromosomes except for 1 male with 2n = 67, most likely due to a supernumerary chromosome. In this population, the sexes are dimorphic, the males are heterogametic, and an XX/XY sex chromosome system is present. Phylogenetic analysis using mitochondrial and nuclear DNAs indicated that this population forms a monophyletic group separate from the other populations of H.ancistroides and may represent an incipient species.

No abstract

Several cytogenetic markers show chromosomal diversity in the fish such as “armoured catfish”. Although studies have characterized many species in the major genera representing these Siluridae, particularly in the genus Hypostomus Lacépède, 1803, trends in chromosome evolution of this group remain unclear. The Paraíba do Sul river basin contains the armoured catfish Hypostomus affinis Steindachner, 1877, which is unique because of its distribution of repetitive DNAs, the 5S and 18S rDNA. Identified samples and registered collections in Brazilian museums were identified as the same typological species, while we observed wide variations in the physical location of this gene in the karyotype based on fluorescent in situ hybridization results. In this study, we propose that these species can represent evolutionarily independent units, as these fish frequently undergo processes such as dispersion and vicariance and that the rDNA is associated with DNA that spreads in the genome, such as transposons. Additionally, the absence of gene flow due to the distance of the sample location could intensify evolutionary processes. The phenotypes found for the 18S rDNA showed minor changes in relation to the number of sites between the lower and upper drainage regions of Paraíba do Sul. The large difference in the number of sites found for the 5S rDNA entered the same region (upper drainage of the basin) and the literature data could represent a population dynamics where an expansion of the 5S rDNA sites provides an extinct or non-sampled cytotype in this work.

We present a concept that summarizes the pattern of occurrence of widely distributed organisms with large chromosomal diversity, low molecular divergence, and the absence of morphological identity. Our model is based on cytogenetic and molecular data of four populations of the siluriform Hypostomus ancistroides presented in this study in comparison with those of 15 other previously described populations but is applicable to any group of sister species, chronospecies, or cryptic species. Through the evaluation of karyotype macrostructure and physical location of genes, in addition to phylogenetic reconstructions from mitochondrial and nuclear genes, we observed morphologically indistinguishable individuals presenting different locally fixed karyomorphs with phylogeographic discontinuity. The process behind this pattern seen in many groups of cryptic species, in which variation lies mainly in the organization of their genomes but not necessarily in the ecosystems they inhabit or in their external morphology, is similar to the processes behind other events observed in the distribution of lineages. In this work we explain this pattern through the hypothesis of the existence of a process analogous to ecological-evolutionary radiation, the chromosomal radiation, which can be adaptive or non-adaptive, and explore the possibility of applying the concept of chromosomal radiation to different groups of organisms.

<p class="western"><em>Ancistrus </em>&eacute; considerado um g&ecirc;nero diversificado na tribo Ancistrini e corresponde a peixes conhecidos popularmente por &ldquo;cascudos&rdquo;. &nbsp;Os DNAs riboss&ocirc;micos mais estudados s&atilde;o o rDNA 18S, que participa da forma&ccedil;&atilde;o de Regi&otilde;es Organizadoras de Nucl&eacute;olo,&nbsp; e o rDNA 5S. O objetivo deste trabalho &eacute; determinar a localiza&ccedil;&atilde;o f&iacute;sica dos s&iacute;tios de DNAr 18S e 5S em <em>Ancistrus </em>sp. da regi&atilde;o de Angra dos Reis-RJ, bacia dos rios Costeiros. Atrav&eacute;s de Hibrida&ccedil;&atilde;o Fluorescente <em>in </em>situ (FISH) foram identificados 4 s&iacute;tios de rDNA 18S localizados em quatro diferentes cromossomos (pares 3 e 14) e 2 s&iacute;tios de rDNA 5S, localizados em dois cromossomos (par 13), todos em regi&atilde;o terminal. Os resultados deste estudo podem ser aplicados em abordagens conservacionistas, podendo auxiliar no entendimento da evolu&ccedil;&atilde;o cromoss&ocirc;mica desta esp&eacute;cie e da tribo Ancistrini.</p>

<p>O g&ecirc;nero <em>Ancistrus</em> &eacute; considerado o mais diversificado dentro da tribo Ancistrini (Loricariidae) e a an&aacute;lise de suas caracter&iacute;sticas cromoss&ocirc;micas constitui o objetivo deste trabalho. Os nove exemplares &ndash; cinco machos e quatro f&ecirc;meas &ndash; apresentaram n&uacute;mero dipl&oacute;ide 2n=52 cromossomos para ambos os sexos e f&oacute;rmula cariot&iacute;pica 6M+5SM+15ST/A. O bandeamento C evidenciou dois pares de cromossomos com blocos heterocrom&aacute;ticos na regi&atilde;o subtelom&eacute;rica e cinco pares com regi&atilde;o pericentrom&eacute;rica marcada. As Ag-RONs foram evidenciadas na mesma regi&atilde;o que as regi&otilde;es ricas em G-C. Apesar da pouca informa&ccedil;&atilde;o citogen&eacute;tica a cerca da tribo Ancistrini, os cromossomos s&atilde;o considerados pouco conservados em rela&ccedil;&atilde;o &agrave; estrutura cariot&iacute;pica. O n&uacute;mero de estudos cariot&iacute;picos est&aacute; aumentando, mas ainda n&atilde;o s&atilde;o suficientes para compreender a diversidade do grupo, bem como o padr&atilde;o de relacionamento entre os membros deste grupo.</p>