Chromosomal organization of four classes of repetitive DNA sequences in killifish Orestias ascotanensis Parenti, 1984 (Cyprinodontiformes, Cyprinodontidae)

Abstract

Orestias Valenciennes, 1839 is a genus of freshwater fish endemic to the South American Altiplano. Cytogenetic studies of these species have focused on conventional karyotyping. The aim of this study was to use classical and molecular cytogenetic methods to identify the constitutive heterochromatin distribution and chromosome organization of four classes of repetitive DNA sequences (histone H3 DNA, U2 snRNA, 18S rDNA and 5S rDNA) in the chromosomes of O. ascotanensis Parenti, 1984, an endemic species restricted to the Salar de Ascotán in the Chilean Altiplano. All individuals analyzed had a diploid number of 48 chromosomes. C-banding identified constitutive heterochromatin mainly in the pericentromeric region of most chromosomes, especially a GC-rich heterochromatic block of the short arm of pair 3. FISH assay with an 18S probe confirmed the location of the NOR in pair 3 and revealed that the minor rDNA cluster occurs interstitially on the long arm of pair 2. Dual FISH identified a single block of U2 snDNA sequences in the pericentromeric regions of a subtelocentric chromosome pair, while histone H3 sites were observed as small signals scattered in throughout the all chromosomes. This work represents the first effort to document the physical organization of the repetitive fraction of the Orestias genome. These data will improve our understanding of the chromosomal evolution of a genus facing serious conservation problems.

Cytogenetic characterization, rDNA mapping and quantification of the nuclear DNA content in Seriolella violacea Guichenot, 1848 (Perciformes, Centrolophidae)

“…In this sense, S. violacea presents exactly the cytogenetic pattern described for marine perciforms, highlighting the association of ribosomal clusters with heterochromatin blocks rich in CG bases in specific chromosome pairs. An association between 18S and 28S rDNA sequences and heterochromatin has been found in other fish, such as salmonids ( Pendás et al 1994 ; Fujiwara et al 1998 ), species of the genera Epinephelus Bloch, 1793 ( Sola et al 2000 ), Imparfinis Eigenmann & Norris, 1900 and Pimelodella Eigenmann & Eigenmann, 1888 ( Gouveia et al 2013 ), Orestias Valenciennes, 1839 ( Araya-Jaime et al 2017 ) and sturgeon species ( Fontana et al 2003 ). This suggests that the repeated HC sequences play an important role and exercise diverse functions in the eukaryotic genome ( Grewal and Jia 2007 ).…”

Cytogenetic Tools to Study the Biodiversity of Neotropical Fish: From the Classic to the Advent of Cell Culture

“…In addition, the technique of FISH showed the U2 snRNA marked in a chromosome linked to sex in the species G. pantanal [102]. In other Neotropical fishes, these two configurations of the location of U2 snRNA gene can be found [83,102,103,113,114].…”

“…In fishes, there are still a few studies that investigated the location and organization of these sequences, but in some of these studies histones are associated with ribosomal genes [85,98,99], and the genes H1, H3 and H4 are grouped in species of Astyanax [100,101], as well as in the case of Synbranchus, where H3 and H4 are associated and spread throughout the genomes, likely to transposable elements [102]. This conformation was also observed in Orestias ascotanensis [103], where these sequences are organized into small copies. In Characidium alipioi [104], the H3 and H4 genes were mapped in a single chromosomal pair, which seems to be a conservative characteristic of the group [105].…”

“…Several studies attempted to understand the origin, evolution and maintenance of the sexlinked chromosomes [103], and fishes have become excellent models of studies because they have a wide and varied sexual system [122]. The sex chromosomes have been described in more than 7% of the fish karyotypes [132], and with the FISH technique, many satellite DNA sequences have been isolated and mapped in different species [133][134][135][136][137].…”

Evolutionary insights in Amazonian turtles (Testudines, Podocnemididae): co-location of 5S rDNA and U2 snRNA and wide distribution of Tc1/Mariner

“…Although the U2 snRNA genes show some sequence conservation, in situ mapping reveals that their distribution patterns can be widely diverse among the karyotypes of some groups. For example, the U2 snRNAs can be: i) organized into a single or small number of chromosomal clusters, as reported in fish (Araya-Jaime et al, 2017;Piscor et al, 2018) and some invertebrates (Almeida et al, 2017;Anjos et al, 2018); ii) arranged in multiple clusters, as observed in some fish (Xu et al, 2017); iii) dispersed in small copies throughout the genome, as in fish of the family Batrachoididae (Úbeda-Manzanaro et al, 2010); iv) allocated on supernumerary chromosomes, as noted in the grasshopper, Abracris flavolineata (Bueno et al, 2013); and v) or in sex chromosomes, as described in grasshoppers of the subfamily Melanoplinae (Palacios-Gimenez et al, 2013). This broad heterogeneity of chromosomal location observed for U2 snRNA genes may be related to the evolutionary history of the snRNA U family, whose Biology Open • Accepted manuscript by guest on September 16, 2020 http://bio.biologists.org/ Downloaded from members can behave as mobile elements and exhibit very little conserved synteny (Marz et al, 2008).…”