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Genomic Organization of Repetitive DNAs Highlights Chromosomal Evolution in the Genus Clarias (Clariidae, Siluriformes)

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Genomic Organization of Repetitive DNAs Highlights Chromosomal Evolution in the Genus Clarias (Clariidae, Siluriformes)

Nuntiya Maneechot et al. Mol Cytogenet.

Abstract

Background: The genus Clarias (Clariidae, Siluriformes) contains at least 61 species naturally spread over vast regions of Asia, India and Africa. However, Clarias species have also been introduced in many different countries and represent the most widespread catfishes in the world. These fishes are also known as "walking catfishes" due to their ability to move over land. A large degree of chromosomal variation has been previously found in this family, mainly using conventional cytogenetic investigations, with diploid chromosome numbers ranging between 48 and 100. In this study, we analyzed the karyotype structure and distribution of four repetitive DNA sequences (5S and 18S rDNAs and (CA)15 and (GA)15 microsatellites) in three Clarias species (C. batrachus, C. gariepinus, C. macrocephalus), as well as in a probable natural hybrid of the two latter species from different Thailand river basins.

Results: Clarias gariepinus and C. macrocephalus had 2n = 56 and 2n = 54, respectively, as well as karyotypes composed mainly by metacentric and submetacentric chromosomes. Their karyotypes differed in the number and location of 5S and 18S rDNA sites and in the degree of microsatellite accumulation. An intermediate chromosomal pattern incorporating those of the parental species was found in the probable hybrid, confirming its interspecific origin. Clarias batrachus had 2n = 104 chromosomes and its karyotype was dominated by mainly acrocentric elements, indicating that unusual multiple centric fissions were involved in its karyotype differentiation. The karyotype of this species presented an unexpected dispersion of ribosomal DNAs, possessing 54 and 12 sites of 5S and 18S rDNAs, respectively, as well as a high accumulation and differential distribution of both microsatellite repeats, representing 'hot spots' for chromosomal rearrangement.

Conclusion: Both conventional and molecular cytogenetic markers were useful tools for demonstrating remarkable evolutionary dynamism and highlighting multiple chromosomal rearrangements and hybridization events correlated with the notable karyotypic diversity of these walking catfishes.

Keywords: Centric fission; Chromosomal rearrangements; FISH; Karyotype evolution; Molecular cytogenetics.

Figures

Fig. 1
Fig. 1
Karyotypes arranged from metaphase chromosomes of Clarias gariepinus (2n = 56), Clarias macrocephalus (2n = 54) and the natural hybrid of these species (2n = 55) after Giemsa staining and FISH with 18S rDNA (red) and 5S rDNA (green) probes. Note the intermediate level of distribution of the ribosomal sites in the hybrid specimen compared to the parental species. Scale bar = 5 μm
Fig. 2
Fig. 2
Karyotypes arranged from metaphase chromosomes of Clarias batrachus (2n = 104) after Giemsa staining and FISH with 18S rDNA (red), 5S rDNA (green), and (GA)15 and (CA)15 microsatellite probes. Note the high dispersion of 5S rDNA sites in the karyotype. Scale bar = 5 μm
Fig. 3
Fig. 3
Karyotypes arranged from metaphase chromosomes of Clarias gariepinus (2n = 56), Clarias macrocephalus (2n = 54) and the natural hybrid of these species (2n = 55) after FISH with (GA)15 and (CA)15 microsatellite probes. Note the weak distribution of both microsatellites in C. gariepinus, their strong accumulation in C. macrocephalus and the intermediate distribution pattern in the hybrid specimen. Scale bar = 5 μm
Fig. 4
Fig. 4
Collection sites of Clarias species from Thailand analyzed in the present study

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