Abstract
The family Loricariidae encompasses approximately 800 species distributed in six subfamilies. The subfamily Hypostominae consists of five tribes; of them, the tribe Ancistrini is relatively diverse, but it is not well known from the cytogenetic point of view. Genus Scobinancistrus Isbrücker et Nijssen, 1989, which is part of the tribe Ancistrini, has two species that occur in sympatry in the Xingu River, Brazil. In this work, we performed the first karyotypic characterizations of these two species and sought to identify the processes involved in their karyotypic evolution. Chromosomal preparations were subjected to Giemsa staining, silver nitrate impregnation, C-banding, CMA3 staining, DAPI staining, and FISH (fluorescence in situ hybridization) with 18S rDNA and telomeric probes. We found that Scobinancistrus aureatus Burgess, 1994 and Scobinancistrus pariolispos Isbrücker et Nijssen, 1989 shared the diploid number, 2n=52, but differed in their karyotypic formulae (KFs), distribution of constitutive heterochromatin (CH), and the localizations of their nucleolus organizer regions (NORs), which were found on the interstitial and distal regions of the long arm of chromosome pair 3 in Scobinancistrus aureatus and Scobinancistrus pariolispos respectively. We suggest that these interspecific variations may have arisen via paracentric inversion or transposition of the NOR. The karyotypic differences found between these two Scobinancistrus species can be used to identify them taxonomically, and may have functioned as a mechanism of post-zygotic reproductive isolation during the speciation process.
Highlights
The fishes of the family Loricariidae are an important component of the ichthyofauna in the Neotropical region, where they are widely distributed and occupy a great variety of freshwater environments (Isbrücker 1980)
The specimens of S. aureatus and S. pariolispos obtained from the Xingu River both had diploid numbers 2n=52 chromosomes, but they differed in their karyotypic formulas (KFs), which were 22m-20sm-10st and 24m-18sm-10st, respectively (Fig. 2A, B)
In S. aureatus, the nucleolus organizer regions (NORs) was located in the interstitial region of the long arms of pair 3, flanked by constitutive heterochromatin (CH) (Fig. 2E)
Summary
The fishes of the family Loricariidae are an important component of the ichthyofauna in the Neotropical region, where they are widely distributed and occupy a great variety of freshwater environments (Isbrücker 1980). The Ancistrini include numerous species with several yet-unsolved taxonomic problems, making it difficult to recognise them (Alves et al 2003). Cytogenetic information is incipient, given the great diversity of Ancistrini species, and has been efficient in distinguishing species of this tribe (Alves et al 2003, Souza et al 2009). The numbers and localizations of the nucleolus organizer regions (NORs) are rather variable; some species have simple NORs while others have multiple NORs (Artoni and Bertollo 2001, Alves et al 2003, 2006, Souza et al 2004, 2009, De Oliveira et al 2006, 2007, 2008, 2009)
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