Evolutionary breakpoint regions and chromosomal remodeling in Harttia (Siluriformes: Loricariidae) species diversification

Repetitive DNAs and chromosome evolution in Megaleporinus obtusidens and M. reinhardti (Characiformes: Anostomidae)

The high dynamism of repetitive DNAs is a major driver of chromosome evolution. In particular, the accumulation of repetitive DNA sequences has been reported as part of the differentiation of sex-specific chromosomes. In turn, the fish species of the genus Megaleporinus are a monophyletic clade in which the presence of differentiated ZZ/ZW sex chromosomes represents a synapomorphic condition, thus serving as a suitable model to evaluate the dynamic evolution of repetitive DNA classes. Therefore, transposable elements (TEs) and in tandem repeats were isolated and located on chromosomes of Megaleporinus obtusidens and M. reinhardti to infer their role in chromosome differentiation with emphasis on sex chromosome systems. Despite the conserved karyotype features of both species, the location of repetitive sequences - Rex 1, Rex 3, (TTAGGG)n, (GATA)n, (GA)n, (CA)n, and (A)n - varied both intra and interspecifically, being mainly accumulated in Z and W chromosomes. The physical mapping of repetitive sequences confirmed the remarkable dynamics of repetitive DNA classes on sex chromosomes that might have promoted chromosome diversification and reproductive isolation in Megaleporinus species.

Karyotype Description and Comparative Chromosomal Mapping of 5S rDNA in 42 Species

This study was conducted to evaluate the 5S rDNA site number, position, and origin of signal pattern diversity in 42 plant species using fluorescence in situ hybridization. The species were selected based on the discovery of karyotype rearrangement, or because 5S rDNA had not yet been explored the species. The chromosome number varied from 14 to 160, and the chromosome length ranged from 0.63 to 6.88 μm, with 21 species having small chromosomes (<3 μm). The chromosome numbers of three species and the 5S rDNA loci of nineteen species are reported for the first time. Six 5S rDNA signal pattern types were identified. The 5S rDNA varied and was abundant in signal site numbers (2-18), positions (distal, proximal, outside of chromosome arms), and even in signal intensity. Variation in the numbers and locations of 5S rDNA was observed in 20 species, whereas an extensive stable number and location of 5S rDNA was found in 22 species. The potential origin of the signal pattern diversity was proposed and discussed. These data characterized the variability of 5S rDNA within the karyotypes of the 42 species that exhibited chromosomal rearrangements and provided anchor points for genetic physical maps.

Homeology of sex chromosomes in Amazonian Harttia armored catfishes supports the X-fission hypothesis for the X1X2Y sex chromosome system origin

The Neotropical monophyletic catfish genus Harttia represents an excellent model to study karyotype and sex chromosome evolution in teleosts. Its species split into three phylogenetic clades distributed along the Brazilian territory and they differ widely in karyotype traits, including the presence of standard or multiple sex chromosome systems in some members. Here, we investigate the chromosomal rearrangements and associated synteny blocks involved in the origin of a multiple X1X2Y sex chromosome system present in three out of six sampled Amazonian-clade species. Using 5S and 18S ribosomal DNA fluorescence in situ hybridization and whole chromosome painting with probes corresponding to X1 and X2 chromosomes of X1X2Y system from H. punctata, we confirm previous assumptions that X1X2Y sex chromosome systems of H. punctata, H. duriventris and H. villasboas represent the same linkage groups which also form the putative XY sex chromosomes of H. rondoni. The shared homeology between X1X2Y sex chromosomes suggests they might have originated once in the common ancestor of these closely related species. A joint arrangement of mapped H. punctata X1 and X2 sex chromosomes in early diverging species of different Harttia clades suggests that the X1X2Y sex chromosome system may have formed through an X chromosome fission rather than previously proposed Y-autosome fusion.

Turnover of multiple sex chromosomes in Harttia catfish (Siluriformes, Loricariidae): a glimpse from whole chromosome painting

The remarkable fish biodiversity encompasses also great sex chromosome variability. Harttia catfish belong to Neotropical models for karyotype and sex chromosome research. Some species possess one of the three male-heterogametic sex chromosome systems, XY, X1X2Y or XY1Y2, while other members of the genus have yet uncharacterized modes of sex determination. Particularly the XY1Y2 multiple sex chromosome system shows a relatively low incidence among vertebrates, and it has not been yet thoroughly investigated. Previous research suggested two independent X-autosome fusions in Harttia which led to the emergence of XY1Y2 sex chromosome system in three of its species. In this study, we investigated evolutionary trajectories of synteny blocks involved in this XY1Y2 system by probing six Harttia species with whole chromosome painting (WCP) probes derived from the X (HCA-X) and the chromosome 9 (HCA-9) of H. carvalhoi. We found that both painting probes hybridize to two distinct chromosome pairs in Amazonian species, whereas the HCA-9 probe paints three chromosome pairs in H. guianensis, endemic to Guyanese drainages. These findings demonstrate distinct evolutionary fates of mapped synteny blocks and thereby elevated karyotype dynamics in Harttia among the three evolutionary clades.

Robertsonian Fusion Site in Rineloricaria pentamaculata (Siluriformes: Loricariidae): Involvement of 5S Ribosomal DNA and Satellite Sequences

Cytogenetic studies demonstrated that unstable chromosomal sites in armored catfishes (Loricariidae) triggered intense karyotypic diversification, mainly derived from Robertsonian rearrangements. In Loricariinae, the presence of ribosomal DNA (rDNA) clusters and their flanking repeated regions (such as microsatellites or partial transposable element sequences) was proposed to facilitate chromosomal rearrangements. Hence, this study aimed to characterize the numerical chromosomal polymorphism observed in Rineloricaria pentamaculata and evaluate the chromosomal rearrangements which originated diploid chromosome number (2n) variation, from 56 to 54. Our data indicate a centric fusion event between acrocentric chromosomes of pairs 15 and 18, bearing 5S rDNA sites on their short (p) arms. This chromosome fusion established the numerical polymorphism, decreasing the 2n from original 56 (karyomorph A) to 55 in karyomorph B and 54 in karyomorph C. Although vestiges of telomeric sequences were evidenced at the fusion point, no 5S rDNA was detected in this region. The acrocentric chromosomes involved in the origin of the fusion were enriched with (CA)n and (GA)n microsatellites. Repetitive sequences in the acrocentric chromosomes subtelomeres have facilitated the rearrangement. Our study thus reinforces the view on the important role of particular repetitive DNA classes in promoting chromosome fusions which frequently drive Rineloricaria karyotype evolution.

Chromosomal rearrangements and the first indication of an ♀X1 X1 X2 X2 /♂X1 X2 Y sex chromosome system in Rineloricaria fishes (Teleostei: Siluriformes)

Rineloricaria is the most diverse genus within the freshwater fish subfamily Loricariinae, and it is widely distributed in the Neotropical region. Despite limited cytogenetic data, records from southern and south-eastern Brazil suggest a high rate of chromosomal rearrangements in this genus, mirrored in remarkable inter- and intraspecific karyotype variability. In the present work, we investigated the karyotype features of Rineloricaria teffeana, an endemic representative from northern Brazil, using both conventional and molecular cytogenetic techniques. We revealed different diploid chromosome numbers (2n) between sexes (33♂/34♀), which suggests the presence of an ♀X₁ X₁ X₂ X₂ /♂X₁ X₂ Y multiple sex chromosome system. The male-limited Y chromosome was the largest and the only biarmed element in the karyotype, implying Y-autosome fusion as the most probable mechanism behind its origination. C-banding revealed low amounts of constitutive heterochromatin, mostly confined to the (peri)centromeric regions of most chromosomes (including the X₂ and the Y) but also occupying the distal regions of a few chromosomal pairs. The chromosomal localization of the 18S ribosomal DNA (rDNA) clusters revealed a single site on chromosome pair 4, which was adjacent to the 5S rDNA cluster. Additional 5S rDNA loci were present on the autosome pair 8, X₁ chromosome, and in the presumed fusion point on the Y chromosome. The probe for telomeric repeat motif (TTAGGG)_n revealed signals of variable intensities at the ends of all chromosomes except for the Y chromosome, where no detectable signals were evidenced. Male-to-female comparative genomic hybridization revealed no sex-specific or sex-biased repetitive DNA accumulations, suggesting a presumably low level of neo-Y chromosome differentiation. We provide evidence that rDNA sites might have played a role in the formation of this putative multiple sex chromosome system and that chromosome fusions originate through different mechanisms among different Rineloricaria species.

Comparative Cytogenetic and Sequence Analysis of U Small Nuclear RNA Genes in Three Ancistrus Species (Siluriformes: Loricariidae)

Ancistrus presents a wide karyotypic diversity, resulting from numeric and structural chromosomal rearrangements. It has been proposed that some genome-specific regions containing repetitive units could organize prone-to-break DNA sites in Loricariidae, triggering chromosomal rearrangements such as Robertsonian fusions (Rb fusions), centric fissions, translocations, and inversions. The tandemly repeats of the small nuclear RNAs (snRNAs) gene families are considered good cytogenetic markers for understanding chromosomal remodeling events among closely related species, but these snRNAs have been scarcely analyzed in Ancistrus. This study presented the nucleotide sequencing and comparative in situ location of U snRNA sequences from Ancistrus aguaboensis, Ancistrus cf. multispinis, and Ancistrus sp. (2n = 50, 52, and 50, respectively), aiming to provide information about snRNA clusters in the genome and chromosome evolution in Ancistrus. U snRNA nucleotide sequences of Ancistrus presented identity to orthologous copies and folded their secondary structures correctly. In situ localization and karyotyping of the three Ancistrus species revealed clustered copies of U2 and U5 snRNA gene families to a single chromosome site, one chromosome pair bearing U1 snRNA sequence, and one main locus of U4 snRNA sequence, besides scattered signals along the chromosomes. Previous studies related the participation of the rRNA gene families in centric fusion events, contributing to chromosome rearrangements and karyotype plasticity present in Loricariidae. In this study, homeologies in U snRNA loci chromosomal locations were detected, indicating the occurrence of conserved sites of these gene families in these three Ancistrus species with 2n = 50 or 52 chromosomes.