In this study, classical and molecular cytogenetic analyses were performed in tilapia fishes, Oreochromis mossambicus (XX/XY sex determination system), O. urolepis hornorum (WZ/ZZ sex determination system) and their hybrid by crossing O. mossambicus female × O. u. hornorum male. An identical karyotype ((2n = 44, NF (total number of chromosomal arms) = 50) was obtained from three examined tilapia samples. Genomic organization analysis of 5S rDNA revealed two different types of 5S rDNA sequences, 5S type I and 5S type II. Moreover, fluorescence in situ hybridization (FISH) with 5S rDNA probes showed six positive fluorescence signals on six chromosomes of all the analysed metaphases from the three tilapia samples. Subsequently, 45S rDNA probes were also prepared, and six positive fluorescence signals were observed on three chromosome pairs in all analysed metaphases of the three tilapia samples. The correlation between 45 rDNA localization and nucleolar organizer regions (NORs) was confirmed by silver nitrate staining in tilapia fishes. Further, different chromosomal localizations of 5S rDNA and 45S rDNA were verified by two different colour FISH probes. Briefly, the current data provide an insights for hybridization projects and breeding improvement of tilapias.
Chromosomal karyotypes of Oreochromis mossambicus and O. urolepis hornorum and their hybrid were analysed by means of Cot-1 DNA bandings through £uorescence in situ hybridization (FISH). To identify all chromosomes, Cot-1DNA^which contains highly and moderately repetitive DNA^was extracted from genomic DNA, labelled as a probe with Dig-11-dUTP, and in situ hybridized to spreads of mitotic chromosomes of the three samples. The hybridized signals were detected by means of Cy3-conjugated antidigoxigenin. The FISH results indicated that the three samples had the same diploid number (2n 5 44) of chromosomes. Speci¢c £uorescence signal bands were detected on all individual chromosome pairs. On the basis of Cot-1 DNA FISH banding patterns and chromosome morphology, the karyotypes of the three samples have been constructed; no remarkable di¡erences were detected between the karyotypes of these species using this method. These resultsŵ hich are similar to those reported previously, with respect to chromosome number, morphology and Cot-1 DNA FISH patterns^suggest chromosomal stasis during speciation and hybridization of tilapia (Oreochromis, Cichlidae). Such a molecular cytogenetic procedure, if used in conjunction with other genomic research methods, could facilitate the study of genomic structure and be adapted for chromosome studies of other animal species.
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