Extensive polymorphism and chromosomal characteristics of ribosomal DNA in the characid fish Triportheus venezuelensis (Characiformes, Characidae)

Abstract

The karyotype and chromosomal characteristics of the characid fish Triportheus venezuelensis were investigated using differential staining techniques (C-banding, Ag-NOR staining) and fluorescent in situ hybridization (FISH) with an 18S rDNA probe. The diploid chromosome number (2n = 52), karyotype composition and sex chromosome determination system of the ZZ/ZW type were the same as previously described in other species of the genus Triportheus. However, extensive variation regarding nucleolus organizer regions (NOR) different from other species was observed. 18S rDNA sequences were distributed on nine chromosome pairs, but the number of chromosomes with Ag-NORs was usually lower, reaching a maximum of four chromosomes. When sequential staining experiments were performed, it was demonstrated that: 1.) active NORs usually corresponded to segments with 18S rDNA genes identified in FISH experiments; 2.) several 18S rDNA sequences were not silver-stained, suggesting that they do not correspond to active NORs; and 3.) some chromosomes with silver-stained regions did not display any 18S rDNA signals. These findings characterize an extensive polymorphism associated with the NOR-bearing chromosomes of T. venezuelensis and emphasize the importance of combining traditional and molecular techniques in chromosome studies.

Highly conserved Z and molecularly diverged W chromosomes in the fish genus Triportheus (Characiformes, Triportheidae)

“…CF Yano et al Z chromosome was reported in T. venezuelensis and T. angulatus (Nirchio et al, 2007;Marquioni et al, 2013) that represent two of the most recently derived species, as their lineage originated at 5.2 ± 2.3 and 2.6 ± 1.4 Myr, respectively (Mariguela et al, 2016). Considering the fact that most species, including those originated from older Triportheus lineages, do not show an 18S rDNA cluster on the Z chromosome, it is more plausible to assume that these sequences were firstly translocated onto W and latter transposed from the W to the Z chromosome in T. venezuelensis and T. angulatus in independent events.…”

“…In fact, both techniques were critical to demonstrate the sequences that are still shared by both sex chromosomes or, otherwise, that those are more exclusive to one of them. Noteworthy, it was evidenced that the end of the Wq has a high concentration of female-specific sequences (Figures 2-4), where an 18S rDNA cluster is also located in all Triportheus species so far analyzed (Artoni and Evolution of sex chromosomes in congeneric species CF Yano et al Bertollo, 2002;Nirchio et al, 2007;Diniz et al, 2009;Marquioni et al, 2013;Yano et al, 2014). Sex chromosomes carrying 18S rDNA sequences have already been reported in several other vertebrates, such as Characidium fishes (Scacchetti et al, 2015), cane toad Bufo marinus (Abramyan et al, 2009), Chinese softshell turtle Pelodiscus sinensis (Kawai et al, 2007) and tiger snake Notechis scutatus (O'Meally et al, 2010).…”

“…The Z is a metacentric chromosome, the largest one in the karyotype, whereas the W is always smaller than the Z, and varies in size and morphology among species (Bertollo and Cavallaro, 1992;Sánchez and Jorge, 1999;Artoni et al, 2001;Artoni and Bertollo, 2002;Nirchio et al, 2007;Diniz et al, 2008;Yano et al, 2014Yano et al, , 2016. Besides, the W chromosome is rich in heterochromatin and carries an 18S ribosomal DNA (rDNA) site on its long arms (Artoni and Bertollo, 2002;Nirchio et al, 2007;Diniz et al, 2009;Marquioni et al, 2013;Yano et al, 2014;Schmid et al, 2016). Recently, a molecular phylogeny of the Triportheidae family was introduced based on the 16S rRNA and cytochrome b (CytB) mitochondrial genes, and on the recombination activating gene 1 (Rag1), recombination activating gene 2 (Rag2) and myosin heavy chain 6 cardiac muscle-α (Myh6) nuclear genes (Mariguela et al, 2016).…”

Karyotype diversity of four species of the incertae sedis group (Characidae) from different hydrographic basins: analysis of AgNORs, CMA3 and 18S rDNA

“…The majority of these studies only refer to the diploid number, but have demonstrated karyotype variability, such as the occurrence of different cytotypes in the genus Bryconamericus (Paintner-Marques et al, 2002;Capistano et al, 2008), presence of B chromosomes in the genus Moenkhausia (Foresti et al, 1989;PortelaCastro et al, 2002), occurrence of sex chromosomes in the genus Triportheus (Nirchio et. al., 2007;Diniz et al, 2008), and different patterns of heterochromatin distribution in Oligosarcus (Rubert and Margarido, 2007;Hattori et al, 2007), among other examples.…”

Interspecific chromosomal divergences in the genus Characidium (Teleostei: Characiformes: Crenuchidae)

“…Cistrons of ribosomal genes 5.8S, 18S, and 28S that form the nucleolar organizer regions in animals and are usually stained with silver nitrate (Ag-NORs), were observed in sex chromosomes, including insects as Drosophila (Pelegrini et al, 1977), beetles (Juan et al, 1993) and flies (Parise-Maltempi & Avancini, 2001); mammals (YonenagaYassuda et al, 1983;Oshida et al, 1999), and also plant chromosomes (Nakayama et al, 2001). Reports on the presence of Ag-NORs in fish sex chromosomes are restricted to Fundulus diaphanus (Howell & Black, 1979), Salvelinus alpinus (Reed & Phillips, 1997), Hoplias malabaricus (Born & Bertollo, 2000), Triportheus guentheri (Artoni & Bertollo, 2002), Triportheus venezuelensis (Nirchio et al, 2007) and Characidium lanei (Noleto et al, 2009). In the rainbow trout Oncorhynchus mykiss, which presents a sex determination system of the XX/XY type, the 5S ribosomal DNA was also observed on the X chromosome (Moran et al, 1996).…”