JFB jfb˙2550 B Dispatch: January 5, 2010 Journal: JFB CE: Narayanan Journal Name Manuscript No. Author Received: No of pages: 11 TS: Thirunavukarasu. K Journal of Fish Biology (2010) 75, 0–0 doi:10.1111/j.1095-8649.2010.02550.x, available online at www.interscience.wiley.com Variability of 18S rDNA locus among Symphysodon fishes: chromosomal rearrangements M. C. Gross*†‡, C. H. Schneider†, G. T. Valente§, C. Martins§ and E. Feldberg† *Laborat´ orio de Gen´ etica Animal, Instituto de Ciˆ encias Biol´ ogicas, Universidade Federal do Amazonas, Manaus, Amazonas, Brazil, †Laborat´ orio de Gen´ etica Animal, Instituto Nacional de Pesquisas da Amazˆ onia, Coordena¸ c˜ ao de Pesquisas em Biologia Aqu´ atica, Manaus, Amazonas, Brazil and §Laborat´ orio de Genˆ omica Integrativa, Departamento de Morfologia, Instituto de Biociˆ encias, UNESP-Universidade Estadual Paulista ‘J´ ulio de Mesquita Filho’, Botucatu, S˜ ao Paulo, Brazil (Received 9 June 2009, Accepted 13 October 2009) Three species of cichlids belonging to the genus Symphysodon have demonstrated interspecific and intraspecific variation in nucleolus organizer regions (NORs) detected with silver nitrate. In order to understand the evolution of this marker in the genus, the structural variability of these sequences in mitotic chromosomes from Symphysodon aequifasciatus, Symphysodon discus and Symphysodon haraldi was investigated using both silver nitrate impregnation and hybridization of the 18S rRNA gene probe. For the three species, the two markers were intraspecifically and interspecifically variable both in the number and in the size of the sites. This polymorphism may stem from duplications and translocations, which suggests that structural chromosome rearrangements effectively act in the karyoevolution of wild Symphysodon species and may have favoured the adaptability of these fishes to diverse aquatic environments in the Amazon. © 2010 The Authors Journal compilation © 2010 The Fisheries Society of the British Isles Key words: Amazon basin; chromosome evolution; Discus fish; fluorescent in situ hybridization; Symphysodon spp. INTRODUCTION Ribosomal RNA (rRNA) is the most abundant RNA, accounting for c. 80% of the total RNA in cells. The rRNA molecules are the major structural component of the ribosomes and they are involved in catalytic, organizational and regulation activities of protein synthesis in all cells (Doudna & Rath, 2002). The genes for these molecules are organized into two multigenic families that are repeated in tandem, that are situated in independent arrangements and that are normally in distinct chromosome regions in fish (for review, see Martins, 2007). The first class is formed by 5S rDNA, which encodes the 5S rRNA, and is synthesized by RNA Polymerase III. The second class is represented by the 45S rDNA, which codes for the 18S, 5·8S ‡Author to whom correspondence should be addressed. Tel.: +55 92 3643 3242; fax: +55 92 3643 3240; email: m.claudia.gross@gmail.com, gross@ufam.edu.br 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 1 © 2010 The Authors Journal compilation © 2010 The Fisheries Society of the British Isles