title: Genomic organization of repetitive DNAs highlights chromosomal evolution in the genus Clarias (Clariidae, Siluriformes).
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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Department of Biology, Faculty of Science, Khon Kaen University, Muang District Khon Kaen, Thailand.
authors.forename: Nuntiya
authors.initials: N
authors.lastname: Maneechot
authors.affiliation: Departamento de Genética e Evolução, Universidade Federal de São Carlos, São Carlos, São Paulo Brazil.
authors.forename: Cassia Fernanda
authors.initials: CF
authors.lastname: Yano
authors.affiliation: Departamento de Genética e Evolução, Universidade Federal de São Carlos, São Carlos, São Paulo Brazil.
authors.forename: Luiz Antonio Carlos
authors.initials: LA
authors.lastname: Bertollo
authors.affiliation: Department of Biology, Faculty of Science, Khon Kaen University, Muang District Khon Kaen, Thailand.
authors.forename: Nuntaporn
authors.initials: N
authors.lastname: Getlekha
authors.affiliation: Departamento de Biologia Celular e Genética, Centro de Biociências, Universidade Federal do Rio Grande do Norte, Natal, RN Brazil.
authors.forename: Wagner Franco
authors.initials: WF
authors.lastname: Molina
authors.affiliation: Department of Biology, Faculty of Science, Khon Kaen University, Muang District Khon Kaen, Thailand.
authors.forename: Sukhonthip
authors.initials: S
authors.lastname: Ditcharoen
authors.affiliation: Department of Veterinary Clinical Medicine, Faculty of Veterinary Medicine, Muang, Khon Kaen 40002 Thailand ; Toxic Substances in Livestock and Aquatic Animals Research Group, Khon Kaen University, Muang, Khon Kaen 40002 Thailand.
authors.forename: Bundit
authors.initials: B
authors.lastname: Tengjaroenkul
authors.affiliation: Faculty of Applied Science and Engineering, Khon Kaen University, Nong Khai Campus, Muang, Nong Khai 43000 Thailand.
authors.forename: Weerayuth
authors.initials: W
authors.lastname: Supiwong
authors.affiliation: Department of Biology, Faculty of Science, Khon Kaen University, Muang District Khon Kaen, Thailand ; Toxic Substances in Livestock and Aquatic Animals Research Group, Khon Kaen University, Muang, Khon Kaen 40002 Thailand.
authors.forename: Alongklod
authors.initials: A
authors.lastname: Tanomtong
authors.affiliation: Departamento de Genética e Evolução, Universidade Federal de São Carlos, São Carlos, São Paulo Brazil.
authors.forename: Marcelo
authors.initials: M
authors.lastname: de Bello Cioffi
keywords: Centric fission; Chromosomal rearrangements; FISH; Karyotype evolution; Molecular cytogenetics