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.

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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.

The Hypostomini tribe comprises a single genus, Hypostomus, which possibly contains several monophyletic groups because of significant morphological variation and a variety of diploid numbers and karyotype formulas. The objective of this study was to infer evolutionary relationships among some species of Hypostomus found in the Paraná River basin and subsequently to identify chromosomal synapomorphies in the groupings formed. Two nuclear genes, rag1 and rag2, and two mitochondrial genes, mt-co1 and mt-cyb, were used to establish evolutionary relationships. Phylogenetic trees were inferred using the maximum likelihood (ML) method for mt-co1 and Bayesian analysis (BA) for all genes concatenated. Both phylogenetic trees showed two large monophyletic clades within Hypostomus. These clades are based on chromosome number, where haplogroup I contains individuals with 66–68 chromosomes, and haplogroup II contains species with 72–80 chromosomes. A third monophyletic haplogroup was also observed using ML, formed by H. faveolus and H. cochliodon, which present 2n = 64, reinforcing the separation of groups in Hypostomus by diploid number. Robertsonian rearrangements were responsible for forming the different diploid numbers and for the diversity of karyotype formulas. The groups based on traditional morphological taxonomy are considered artificial in this study; the staining pattern, which separates the two large groups morphologically and is supported by little chromosomal evidence, was instead determined to show homoplasy. Ag-NORs are predominantly multiple and located on st/a chromosomes, along with 18S rDNA sites; 5S rDNA sites are often seen in an interstitial position, following the trend already described for vertebrates.

<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>

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>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>