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Over the last two decades, several classification changes were proposed for the family Loricariidae and their phylogenetic relationships could be only partly resolved so far. Two major problematic groups are the subfamilies Hypoptopomatinae and Neoplecostominae. Therefore, we analysed four species of this family, being three Neoplecostominae (Neoplecostomus yapo, Kronichthys lacerta and Isbrueckerichthys duseni), and one Hypoptopomatinae representative (Parotocinclus maculicauda), in order to compare the chromosomal data to other Loricariidae subfamilies and to the outgroup Trichomycterus sp. To accomplish that, conventional (diploid numbers, karyotype formulae, fundamental number and Ag-NORs -nucleolar organizer regions) and molecular chromosomal markers (dual color FISH -fluorescence in situ hybridization -with 18S and 5S rDNA probes) were used. A similar karyotype structure was reported amongst the studied species. Inferences about putative plesiomorphic chromosomal markers were established and/or corroborated in Loricariidae, such as: 2n = 54 chromosomes, small amounts of heterochromatin, a single NOR-bearing pair and synteny 18S and 5S rDNA at interstitial region. Thus, the present chromosomal analysis hypothesizes primitive chromosomal characters and contributes to a better understanding of chromosomal evolutionary processes among loricariids.

ABSTRACT. We examined chromosomes of three species of the genus Hypostomus, in order to contribute to the understanding of the karyotype evolution of this group. Specimens of H. ancistroides and H. nigromaculatus displayed differences in karyotype formulas, distribution and location of heterochromatin and nucleolus organizer regions when compared to other populations of the same species. We made the first cytogenetic characterization of H. tapijara, an endemic species in the Ribeira de Iguape River. These specimens had 2n = 66 chromosomes, while H. ancistroides showed 2n = 68 and H. nigromaculatus 2n = 76 chromosomes. Physical mapping of 18S and 5S rDNA sites of the three species showed simple, multiple and syntenic clusters. Synteny of ribosomal sites was found in H. ancistroides and H. tapijara, and an interspersed pattern between these sites in all chromosomes bearing the synteny was observed. We conclude that the genus Hypostomus has a high chromosome complexity that is accompanied by great morphological variation. It is evident that this group comprises an interesting model for understanding the chromosome evolution of Neotropical ichthyofauna.

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