Abstract Among the Neotropical fish fauna, suckermouth armored catfishes (Hypostomus - Loricariidae) stands out as one of the most difficult groups to diagnose morphologically. So the use of different molecular markers, as is the case of cytogenetics, has been fundamental for a precise identification of some species. In the present study, we characterize the karyotypes of two allopatric Hypostomus spiniger populations, using classical and molecular cytogenetic methods. This species was described by Hensel (1870) but later synonymized with Hypostomus commersoni, and recently it was again recognized as a valid species. Taking to account this taxonomic problematic, the aim of this study is to determine chromosomal characters that may be useful to validate the taxonomic status of H. spiniger and to complement its diagnosis in relation to H. commersoni populations. The karyotype of H. spiniger is composed by 66 chromosomes (10m+16sm+14st+26a), few heterochromatin and a multiple nucleolar organizer regions (NORs) system. Despite, the currently geographic isolation among the samples collected in Forquetinha River (Patos Lagoon basin) and Quadros Lagoon (Tramandaí River basin), both shared the same karyotypic structure, this cytogenetic evidence, suggest that both populations belongs to the same species. Additionally, our results clearly distinguish H. spiniger from H. commersoni populations collected along the Paraná River basin, that exhibited 2n=68 chromosomes and several divergences in heterochromatin and NORs pattern. In sum, the present study reinforces the valid status of H. spiniger and demonstrated the importance of basic cytogenetic analysis to understand conflictuous taxonomic matters.

RESUMO O presente trabalho apresenta a primeira descrição citogenética de Steindachneridion doceanum, grande bagre que se encontra atualmente em alto risco de extinção e listado como ameaçado na lista vermelha do Ministério do Meio Ambiente, também sugere relações cariotípicas com outras espécies do mesmo gênero, endêmicas de outras bacias hidrográficas. Os resultados revelaram um número diplóide de 56 cromossomos e o cariótipo composto por 18 elementos metacêntricos, 20 submetacêntricos, 10 subtelocêntricos e 8 acrocêntricos (NF = 104). As marcações AgNORs e CMA3 foram coincidentes ocupando o braço curto de um par de cromossomos acrocêntricos (par 25), em uma constrição secundária. Os genes 5S rDNA foram detectados nos braços curtos de um par subtelocêntrico. A banda C revelou blocos terminais nos braços curtos em vários cromossomos, bem como blocos terminais nas duas extremidades de um par submetacêntrico. A banda C também evidenciou um grande bloco heterocromático na constrição secundária (par 25) coincidente com os sítios AgNORs e as bandas CMA3 positivas. Apesar de S. doceanum representar um táxon endêmico, suas características citogenéticas mostram semelhanças com outras espécies do gênero das quais se encontra geograficamente isolado.

ABSTRACT The present report represents the first cytogenetic description of Steindachneridion doceanum, great catfish which is currently at high extinction risk and it is listed as threatened on the red list of the Brazilian Ministry of the Environment, also are suggested karyotype relationships with other species of the same genus endemic from other river basins. The results revealed a diploid number of 2n = 56 and the karyotype composed of 18 metacentric, 20 submetacentric, 10 subtelocentric and 8 acrocentric chromosomes (NF = 104). The AgNORs and CMA3 signals were coincident in location occupying the short arm of an acrocentric chromosome pair (25th), in a secondary constriction. The 5S rDNA genes were localized on the short arms of one subtelocentric pair. C-banding revealed terminal blocks on the short arms on many chromosomes as well as terminal positive bands at the both ends of a submetacentric pair. C banding also revealed a large heterochromatic block in the secondary constriction (25th) region that was coincident with the AgNORs sites and CMA3+ bright bands. In spite S. doceanum represent an endemic taxon, in spite their geographic isolation their cytogenetic characteristics show similarities with other species of the genus.