Ancistrus Kner, 1854 is a diverse catfish genus, currently comprising 66 valid species, but karyotype data were recorded for 33 species, although only ten have their taxonomic status defined. Considerable karyotype diversity has been found within this genus, with 2n varying from 34 to 54 and structural variability including heteromorphic sex chromosomes. In many cases, uncertainty on the taxonomic status of the study populations hampers reliable interpretation of the complex chromosomal evolutionary history of the group. This study aims to present the first karyotype data for a population of the Ancistrus sp. collected in Criminoso stream (tributary of the Paraguay River Basin, Mato Grosso do Sul, Brazil) in which a combination of different chromosomal markers was used and results integrated in broad discussion on karyotype evolution in the genus. The specimens presented 2n=42 with 18m+16sm+8st and a single NOR revealed by silver nitrate and fluorescence in situ hybridization (FISH) with 18S rDNA probe, located in pair No. 10. Clusters of 5S rDNA were located in the pericentromeric region of three chromosomes: pair No. 1 (metacentric) and one of the homologues of the nucleolar pair No. 10. Heterogeneity in the molecular composition of the heterochromatin was confirmed by the association of C-banding and fluorochrome CMA3/DAPI-staining. Exploring the differential composition of constitutive heterochromatin in Ancistrus may provide an important perspective to understand genome organization and evolution within this group. Our data reinforce the chromosomal diversity present in Ancistrus genus and we discuss the potential sources these variation. The karyotype structure of Ancistrus sp. “Criminoso stream” appears to be consistent with the existence of a new candidate species.

Only one species of armored catfish, Ancistrus cirrhosus Valenciennes 1836, has been historically described in the basin of the Paraná River, from Misiones (Argentina). However, the ample variation found in the morphology and coloration of the populations sampled in the tributaries of the Brazilian state of Paraná makes it difficult to establish the real taxonomic status and evolutionary history of the Ancistrus specimens, suggesting that A. cirrhosus is not the only species found in this basin. By combining data on mitochondrial DNA (COI gene) and chromosomal markers from different Ancistrus populations, totaling 144 specimens, in the tributaries of the Paraná, and specimens from Misiones (type-locality of A. cirrhosus), we detected five distinct evolutionary lineages. All the specimens were 2n = 50, but had four distinct karyotype formulae. The results of the Generalized Mixed Yule Coalescent (GYMC) and the genetic distances (uncorrected P-values) between lineages ranged from 3 to 5%. Clusters of 18S rDNA were observed in a single chromosome pair in seven populations of Ancistrus, but at different positions, in some cases, in synteny with the 5S rDNA sites. Multiple 5S sites were observed in all populations. Overall, the cytogenetic data reinforce the genetic evidence of the diversification of lineages, and indicate the existence of candidate species in the study region. The evidence indicates that at least four candidate species of the Ancistrus may coexist in the Paraná basin besides A. cirrhosus. Overall, our results provide a comprehensive scenario for the genetic variation among Ancistrus populations and reinforce the conclusion that the true diversity of the freshwater fish of the Neotropical regions has been underestimated.

The repetitive DNAs are the expressive substrate to genomic evolution and directly related to chromosomal diversification in eukaryote, including fishes. Ancistrus is an interesting group for studies about interplay between repetitive DNA and karyotype evolution, given its extensive chromosomal variation. In this study, we aimed to understand the evolutionary dynamics in genome of the distinct Ancistrus populations of the Paraná basin to the contribution of three classes of repetitive DNA sequences. Nucleotide sequence was isolated, characterized the nonlong terminal repeat (non-LTR) retrotransposable Rex-3, and evaluated the chromosomal organization in the Ancistrus populations. In addition, we also mapped microsatellite repeats on chromosomes. A high conserved level of the Rex-3 element was presented in Ancistrus genome sequences to record in other fish genomes. We recognized also five domains conserved in the amino acid sequence presumed from nucleotide sequence of the reverse transcriptase fragment, which indicates that it is potentially active in the genome. The physical mapping using the Rex-3 as probe revealed signals scattered throughout the chromosomes of all the Ancistrus specimens, while the microsatellite probes hybridized preferentially in the subterminal and interstitial regions. Physical mapping also reveals interplay between these two classes of repetitive DNA in some chromosome pairs. Besides, the spreading of Rex-3 signals in adjacencies of the 5S recombinant DNA (rDNA) sites could reflect their role in the dispersion of these regions. Our findings provide important insights into the mechanisms of karyotype diversification in the genus Ancistrus, which involve these repetitive sequences.

ABSTRACT. The Iguaçu River basin is a tributary to the upper Paraná River in southern Brazil, and is considered an important aquatic ecoregion that, although having few species of fish, 51-71% of these are apparently endemic. Ancistrus abilhoai is one of three recently described species for this basin and is currently considered endemic to the basin. In this study, we present the chromosomal structure of two populations of Ancistrus abilhoai one collected in the Iguaçu River, in Paraná State, and another collected in the Timbó River, a tributary of the Iguaçu River, in the State of Santa Catarina. Karyotype analyzes were performed in 11 specimens from the Iguaçu River (four females and seven males) and 12 specimens (all males) from Timbó River, revealing 2n = 48 chromosomes with a karyotype formula of 22m + 14sm + 6st + 6a in both populations. Analysis of active nucleolar organizer regions (Ag-NORs) and fluorescent in situ hybridization (FISH) with 18S rDNA probes revealed the submetacentric pair 13 bearing marks at terminal positions on the short arms. Considered as plesiomorphic chromosomal markers in Loricariidae, asynteny 18S and 5S rDNA, and small amounts of heterochromatin were observed. In this study, the first chromosomal data of A. abilhoai are presented with comments on karyotypic characteristics of the genus.

Hypostomus (Loricariidae) destaca-se na família pela grande diversidade cariotípica numérica (2n=64-84) e estrutural, envolvendo diferenças nas fórmulas cromossômicas até mesmo dentro da mesma espécie. Esta variabilidade pode ser melhor avaliada com a utilização de diferentes marcadores cromossômicos, os quais revelam uma diversidade críptica neste grupo. Estudos prévios em uma população de H. regani coletada no rio Taquari (bacia do rio Paraguai, Coxim, MS) revelaram 2n=72 cromossomos, porém, dois cariomorfos, sendo cariomorfo A com 12m+14sm+18st+28a e no cariomorfo B, 13m+14sm+17st+28a. O cariomorfo B é caracterizado por um intrigantante heteromorfismo de tamanho envolvendo o par cromossômico 19, constituído por um homólogo subtelocêntrico e por um cromossomo metacêntrico grande, o maior do complemento. Essa condição é observada em 43,74% de nossa amostra e independe do sexo do indivíduo analisado. O metacentrico apresentou o braço curto totalmente heterocromático (banda C positivo) e sequências ricas em CG, reveladas por CMA3. Portanto, o presente estudo tem como objetivo, complementar os dados sobre a natureza molecular da heterocromatina nos cromossomos de H. regani, com ênfase  no par heteromórfico, através do FISH com sequências de microssatélites (GA)15 e (CA)15. Foram evidenciados clusters de repetições (CA)15 principalmente nas regiões subterminais e pericentroméricas na maioria dos cromossomos do complemento. Adicionalmente, detectamos sinais discretos de hibridação na região insterticial de ambos os braços e marcações nas regiões subterminais no par de cromossomos que contém o cromossomo heteromorfo. Clusters de (GA)15  foram evidenciados nas regiões subterminais e pericentricas de vários cromossomos, destacando-se marcações subterminais, e no par 19 com o cromossomo heteromórfico, observou-se clusters  em ambas regiões subterminais e uma discreta marcação intersticial no braço longo do heteromorfo. Estes resultados contribuem com dados sobre a  composição molecular heterogênea da heterocromatina no genoma desta espécie e revela importante papel dos elementos repetitivos na evolução cariotípica da mesma.Apoio financeiro: CAPES-PBC/UEM