Genomic Organization of Repetitive DNA Elements and Extensive Karyotype Diversity of Silurid Catfishes (Teleostei: Siluriformes): A Comparative Cytogenetic Approach

Abstract

The catfish family Siluridae contains 107 described species distributed in Asia, but with some distributed in Europe. In this study, karyotypes and other chromosomal characteristics of 15 species from eight genera were examined using conventional and molecular cytogenetic protocols. Our results showed the diploid number (2n) to be highly divergent among species, ranging from 2n = 40 to 92, with the modal frequency comprising 56 to 64 chromosomes. Accordingly, the ratio of uni- and bi-armed chromosomes is also highly variable, thus suggesting extensive chromosomal rearrangements. Only one chromosome pair bearing major rDNA sites occurs in most species, except for Wallago micropogon, Ompok siluroides, and Kryptoterus giminus with two; and Silurichthys phaiosoma with five such pairs. In contrast, chromosomes bearing 5S rDNA sites range from one to as high as nine pairs among the species. Comparative genomic hybridization (CGH) experiments evidenced large genomic divergence, even between congeneric species. As a whole, we conclude that karyotype features and chromosomal diversity of the silurid catfishes are unusually extensive, but parallel some other catfish lineages and primary freshwater fish groups, thus making silurids an important model for investigating the evolutionary dynamics of fish chromosomes.

Comparative chromosomal mapping of microsatellite repeats reveals divergent patterns of accumulation in 12 Siluridae (Teleostei: Siluriformes) species

“…A similar distribution is also present in the chromosomes of the zebrafish, Danio rerio, showing (CA) n and (GT) n repeats clustered in the centromeric and telomeric regions (Shimoda et al, 1999;Supiwong et al, 2014) (CA) 15 and (GA) 15 , showed strong hybridization signals at subtelomeric and heterochromatic regions of several autosomes, in addition to a strong accumulation on the sex chromosomes (Cioffi et al, 2011, Supiwong et al, 2014. In fact, for most of these species, the 18S rDNA repeats are found in the short arms of a single chromosome pair (Ditcharoen et al, 2019), and this region matches the position of the (CGG)n marks found in our experiments. Similarities of both microsatellite and ribosomal DNA location do not seem to be a rare event among fishes, as they are also found in other species, such as Lebiasina bimaculata (Sassi et al, 2019) and Hepsetus odoe (Carvalho et al, 2017), for example.…”

“…rDNA site is found in the telomeric region of the long arms of the 18 th chromosome pair (Ditcharoen et al, 2019), the same one that harbors a conspicuous (CAC)n site.…”

“…It is also known that Phalacronotus is the only genus that maintains the diploid number conservation with 2n = 64 in all analyzed species, while other genera in this family display a substantial variation (Ditcharoen et al, 2019). On the mapping of highly repetitive sequences, the high 2n variation also appears to be followed by a large variation of ribosomal DNAs loci among silurid species (Ditcharoen et al, 2019). Considering the extensive genomic reorganization, as revealed by CGH (comparative genomic hybridization), it is evident that silurid genomes have experienced an enormous diversification throughout their evolutionary history (Ditcharoen et al, 2019).…”

“…On the mapping of highly repetitive sequences, the high 2n variation also appears to be followed by a large variation of ribosomal DNAs loci among silurid species (Ditcharoen et al, 2019). Considering the extensive genomic reorganization, as revealed by CGH (comparative genomic hybridization), it is evident that silurid genomes have experienced an enormous diversification throughout their evolutionary history (Ditcharoen et al, 2019).…”

“…Additionally, repetitive DNAs have an important role in speciation, differentiation of sex-specific regions, and promotion of biodiversity (Vicari et al, 2005;Cioffi et al, 2009;Sember et al, 2018). Therefore, here we analyzed the chromosomal location several microsatellites repeats to explore the intergenomic divergence at the chromosomal level in 12 Silurid species; the sampling resembles the one previously analyzed by Ditcharoen et al (2019) with different cytogenetic methods. Indeed, our recent result provided new insights into the karyotype differentiation of this fish group, with a better understanding of the chromosomal organization of repetitive DNAs and uncovering chromosome homologies and differences among the studied species.…”