Cytogenetic characterization of four species of the genus Hypostomus Lacépède, 1803 (Siluriformes, Loricariidae) with comments on its chromosomal diversity

Cytogenetic analyses were performed on fishes of the genus Hypostomus (H. ancistroides (Ihering, 1911), H. strigaticeps (Regan, 1908), H. regani (Ihering, 1905), and H. paulinus (Ihering, 1905)) from the seven tributaries of the Paranapanema River Basin (Brazil) by means of different staining techniques (C-, Ag-, CMA3- and DAPI-banding) and fluorescence in situ hybridization (FISH) to detect 18S rDNA sites. All species showed different diploid numbers: 2n=68 (10m+26sm+32st-a) in H. ancistroides, 2n=72 (10m+16sm+46st-a) in H. strigaticeps, 2n=72 (10m+18sm+44st-a) in H. regani and 2n=76 (6m+16sm+54st-a) in H. paulinus. Ag-staining and FISH revealed various numbers and locations of NORs in the group. NORs were usually located terminally on the subtelocentric/acrocentric chromosomes: on the long arm in H. strigaticeps (2 to 4) and H. paulinus (2); and on the short arm in H. ancistroides (2 to 8) and H. regani (2 to 4). Conspicuous differences in heterochromatin distribution and composition were found among the species, terminally located in some st-a chromosomes in H. ancistroides, H. strigaticeps, and H. paulinus, and interstitially dispersed in most st-a chromosomes, in H. regani. The fluorochrome staining indicated that different classes of GC and/or AT-rich repetitive DNA evolved in this group. Our results indicate that chromosomal rearrangements and heterochromatin base-pair composition were significant events during the course of differentiation of this group. These features emerge as an excellent cytotaxonomic marker, providing a better understanding of the evolutionary mechanisms underlying the chromosomal diversity in Hypostomus species.