Chromosomal Diversification in Pseudacanthicus Species (Loricariidae, Hypostominae) Revealed by Comparative Mapping of Repetitive Sequences

Abstract

Pseudacanthicus is a genus of Neotropical fish with eight valid species, in addition to numerous lineages not formally identified. It occurs along the Amazon and Tocantins River basins, in Suriname and in the Guiana shield. There are no karyotypic data in the literature for species of this genus. Here, the karyotypes of three Pseudacanthicus species (P. spinosus, P. leopardus and Pseudacanthicus sp.) were comparatively analyzed by classical cytogenetics and fluorescence in situ hybridization using 18S and 5S rDNA probes, U2 snDNA and telomeric sequences. The analyzed species presented 52 chromosomes and KF = 18 m + 34 sm. Constitutive heterochromatin occurred in blocks on a few chromosomes. The 18S rDNA occurred in a single pair; interestingly, P. leopardus presented only one locus of this sequence in its diploid genome. The 5S rDNA sequence occurred in only one pair in P. leopardus, and in multiple sites in Pseudacanthicus sp. and P. spinosus. The snDNA U2 occurred in only one pair in all analyzed species. Telomeric sequences did not show interstitial sites. Although Pseudacanthicus species share the same 2n and KF, repetitive sequence analysis revealed karyotypic diversity among these species. The occurrence of DNA double-strand breaks related to fragile sites, unequal crossing over and transpositions is proposed as the mechanism of karyotypic diversification, suggesting that the conservation of the karyotypic macrostructure is only apparent in this group of fish.

Mechanisms of Karyotypic Diversification in Ancistrus (Siluriformes, Loricariidae): Inferences from Repetitive Sequence Analysis

“…Therefore, in addition to involvement in chromosomal rearrangements, we consider that the participation of 5S rDNA in transposition events and non-homologous recombination may represent plausible mechanisms to explain the dispersion of these sequences to new chromosomal sites in different populations of Ancistrus sp. 1 (2n = 38, XX/XY) and Ancistrus cirrhosus (2n = 34), similarly to what has already been described in other fish groups [ 9 , 12 , 13 , 14 , 15 , 26 , 28 ].…”

“…The occurrence of ITS is suggestive of fusions [ 12 ]; however, in the analysis carried out previously [ 9 ] and in the present study, ITS was not found in any karyotype of the analyzed samples. Alternatively, the occurrence of these rearrangements has been corroborated to the detriment of the absence of ITS due to the accumulation of other groups of repetitive sequences in unstable sites prone to DBS, which may explain the occurrence of these rearrangements in these karyotypes [ 9 , 13 , 26 , 28 , 37 ]. Furthermore, the absence of ITS in rearranged karyotypes is commonly related to the loss of these sequences during fission/fusion events or the degeneration of these interstitial sites throughout the evolution of karyotypes [ 38 ].…”

“…18S and 5S rDNA genes are part of gene families extensively used in the comparative cytogenetic analysis in fish, showing the occurrence of chromosomal differentiation [ 9 , 14 , 24 , 26 , 28 , 37 , 38 ]. In Loricariidae, a syntenic organization is considered a plesiomorphic condition [ 11 , 39 ].…”

“…In Loricariidae, a syntenic and single-site organization between the 18S and 5S rDNA genes represents a plesiomorphic condition [ 11 ], while the synteny break and occurrence of multiple sites of these genes is frequent in Ancistrini, showing mechanisms of chromosomal differentiation involving this group of sequences [ 9 , 13 , 26 ]. However, information about the chromosomal organization of U snDNA sequences is limited to a few species of Loricariidae [ 26 , 28 , 29 , 30 ].…”

“…Comparative analyses of in situ localization and molecular characterization demonstrate that, unlike other groups of genes encoding non-coding RNAs, U snRNA genes show remarkable similarity with orthologous copies [ 27 ]. These genes may present distinct chromosomal organizations in different groups of species, occurring in single, multiple sites or clusters formed by different gene families (e.g., snRNA and rRNA genes) [ 26 , 27 , 28 , 29 , 32 , 33 , 34 , 35 ]. Furthermore, genomic data demonstrate that pseudogenes and non-functional copies of U snDNA occur in fish genomes [ 27 , 29 ], indicating the participation of these repetitive sequences in chromosomal reorganization, including transposition events.…”