Karyotypic characterization and genomic organization of the 5S rDNA in Polypterus senegalus (Osteichthyes, Polypteridae)

Abstract

Polypteridae (Cladistia) is a family of archaic fishes, confined to African freshwaters. On account of their primitiveness in anatomical and morphological characters and mosaic relationships among lower Osteichthyans fishes, they constitute an important subject for the study of evolution in vertebrates. Very little is known about the karyological structure of these species. In this article, a cytogenetic analysis on twenty specimens of Polypterus senegalus (Cuvier, 1829) was performed using both classical and molecular techniques. Karyotype (2n=36; FN=72), chromosome location of telomeric sequences (TTAGGG)(n), (GATA)(7) repeats and ribosomal 5S and 18S rRNA genes were examined by using Ag-NOR, classical C-banding, CMA(3) staining and FISH. Staining with Ag-NOR showed the presence of two GC rich NORs on the p arm of the chromosome pair no. 1. CMA(3) marked all centromerical and some (no. 1 and no. 14) telomeric regions. FISH with 5S rDNA marked the subtelomeric region of the q arm of the chromosome pair no. 14. FISH with 18S rDNA marked the telomeric region of the p arm of the chromosome pair no. 1, previously marked by Ag-NOR. (GATA)(7) repeats marked the subtelomeric regions of all chromosome pairs, with the exclusion of the no. 1, 3 and 14. Hybridization with telomeric probes (TTAGGG)(n) showed bright signals at the end of all chromosomes. After cloning, the 5SrDNA alignment revealed an organization of sequences made up of two different classes of tandem arrays (5S type I and 5S type II) of different lengths.

Chromosomal localization of rDNA genes and genomic organization of 5S rDNA in Oreochromis mossambicus, O. urolepis hornorum and their hybrid

“…The diversity of 5S rDNA loci have been reported in the diploid genome of different plants and animals (Lomholt et al 1995;Laura et al 2003;Fujiwara et al 2007;Morescalchi et al 2008;Kwon and Kim 2009). The 5S rDNA is often located on a single chromosome pair in fish (Pendás et al 1994;Fujiwara et al 2007;Morescalchi et al 2008;Nirchio et al 2009) and in some mammals (Suzuki et al 1996;Laura et al 2003), probably representing a more ancient condition among animal groups.…”

“…The 5S rDNA is often located on a single chromosome pair in fish (Pendás et al 1994;Fujiwara et al 2007;Morescalchi et al 2008;Nirchio et al 2009) and in some mammals (Suzuki et al 1996;Laura et al 2003), probably representing a more ancient condition among animal groups. The occurrence of multiple 5S rDNA sites found in the present study, as well as in some other fish represents derived conditions in their evolutionary dynamic (Ferro et al 2001;Martins et al 2002Martins et al , 2006Zhu et al 2006).…”

“…In Tilapiini tribe Oreochromis and Tilapia species, the two classes of 5S rDNA were found to exist by analysis with the whole 5S rDNA sequences and just the NTSs sequence, but these sequences were not differentiated between the species (Alves- Costa et al 2006). These types of multi-gene families usually evolve based on homogenization processes, and were governed by molecular drive and concerted evolution, resulting in a sequence similarity of the repeating units that is greater within rather than among species (Morescalchi et al 2008). In this study, two types of 5S rDNA were observed in three types of samples.…”

“…One pair of primer (P 1 , 5 -AATACGCCCGATCTCGTCC GA-3 and P 2 , 5 -CAGGCTGGTATGGCCGTAAGC-3 ) was designed and synthesized to amplify the 5S rDNA repeats directly from genomic DNA by PCR, following the method described by Morescalchi et al (2008). The primer pair P 1 and P 2 was used to amplify the region between the 3 end of one 5S gene unit and the 5 end of the following unit.…”

“…Chromosome banding techniques and fluorescent in situ hybridization (FISH) have been demonstrated to be useful cytogenetic methods for chromosome identification, which has allowed further resolution of similar size chromosomes (Martins and Galetti 2001;Yi et al 2003;Affonso and Pedro 2005;Zhu et al 2006;Zhu and Gui 2007;Morescalchi et al 2008;Mazzuchelli and Martins 2009). In fish, the 5S rDNA multi-gene family consists of a highly conserved coding sequence of 120 bp forming arrays of hundreds to thousands of tandem copies that are separated from each other by variable non-transcribed spacers (NTSs) (Long and David 1980).…”