E-Mail karger@karger.com Original Article Cytogenet Genome Res DOI: 10.1159/000366301 Unusual Dispersion of Histone Repeats on the Whole Chromosomal Complement and Their Colocalization with Ribosomal Genes in Rachycentron canadum (Rachycentridae, Perciformes) Gideão W.W.F. Costa a Marcelo B. Cioffi b Luiz A.C. Bertollo b Wagner F. Molina a a Department of Cellular Biology and Genetics, Biosciences Center, Federal University of Rio Grande do Norte, Natal, and b Department of Genetics and Evolution, Federal University of São Carlos, São Carlos, Brazil was demonstrated for the first time in fishes. The implica- tions of differential histone gene extension and its function- ality in the karyotype of R. canadum remain unknown. © 2014 S. Karger AG, Basel Rachycentron canadum (cobia), the only representa- tive member of the monotypic family Rachycentridae (Perciformes), is found in tropical seas worldwide. The high commercial value of this species, the facility of its spawning in captivity, its marked growth rate, good saline tolerance, and adaptability to confinement favor its in- tensive cultivation [Faulk and Holt, 2006; Liao and Leaño, 2007; Benetti et al., 2008]. Although farming methodol- ogy is constantly being improved, data available on the genome and the chromosomes of this species are still scarce [Jacobina et al., 2011]. Previous cytogenetic studies of R. canadum supplied information on karyotype structure, replication band patterns, and physical mapping of telomeric and ribo- somal sequences [Jacobina et al., 2011]. However, the composition of heterochromatic portions of the genome and the possible presence of genes associated with these regions were hitherto unknown. Histones and ribosom- al genes are vital multigene families in eukaryotes [Jor- dan et al., 2002; Roehrdanz et al., 2010]. Functionally, Key Words Fish cytogenetics · Heterochromatinization · Karyotype evolution · Multigene families · Repetitive sequence expansion Abstract Rachycentron canadum, the only representative of the family Rachycentridae, has been the focus of biotechnological in- terest due to its significant potential in marine fish farming. The chromosome set of this species has been widely investi- gated with respect to the location of genes and multigene families. A FISH analysis was performed using 4 multigene families as probes, represented by 5S and 18S ribosomal genes and histones H2B-H2A and H3. Earlier data suggested that differential replication of heterochromatin could be partially associated with functional genes. Indeed, our re- sults showed that the DNA contained in heterochromatic re- gions of R. canadum contains 5S and 18S ribosomal genes as well as the gene sequences of histones H2B-H2A and H3, which were colocalized. The distribution of H3 sequences in all heterochromatic regions, except in 13q, could indicate an important evolutionary role for this class of repetitive se- quences. Besides, the presence of chromosome regions bearing multifunctional repetitive sequences formed by H2B-H2A/H3/18S rDNA and H2B-H2A/H3/5S rDNA clusters Accepted: July 4, 2014 by M. Schmid Published online: October 17, 2014 Wagner F. Molina Department of Cellular Biology and Genetics, Biosciences Center Federal University of Rio Grande do Norte Natal, RN 59078-970 (Brazil) E-Mail molinawf  @  pq.cnpq.br © 2014 S. Karger AG, Basel 1424–8581/14/0000–0000$39.50/0 www.karger.com/cgr Downloaded by: Universidade Federal do Rio Grande do Norte 177.20.130.8 - 10/17/2014 2:10:07 PM