I n Egypt, four species of Tilapia have been described based on morphometric, meristic and cytotaxonomical characteristics. These species are Tilapia zillii, Oreochromis niloticus, Oreochromis aureus and Sarotherodon galilaeus. The accurate identification of these fishes is complicated by the high variation in these characters, similarity among species and in some cases by the size of the fish. In this paper, we examined the use of polymerase chain reaction (PCR) and restriction fragment length polymorphisms (RFLPs) analysis of the nuclear small subunit ribosomal RNA gene (srDNA) for molecular identification of Tilapia spp. in Egypt. The present study aims to evaluate such advanced molecular biological approach for identification of Tilapia spp. Genomic DNA was extracted from the four species of Tilapia. About 2000 bp 18S ribosomal DNA was amplified by PCR using specific primers. The technique of restriction fragment length polymorphisms was used to identify the specific 18S rDNA for each species. O. niloticus rDNA 'RFLPs species-specific pattern is proved by digestion with restriction endonucleases AIwNI and Aval, On the other hand, digestion of amplified 18S rDNA with endonuclease enzymes Smal, Xmal and Sstll produced speciesspecific patterns for T. zillii, O. aureus and S. galilaeus respectively. This indicates the efficiencies of these restriction endonuclease enzymes in species-specific identification of Tilapia spp. Restriction fragment length polymorphisms of the nuclear srDNA (RFLPs\PCR) proved to be a good tool for checking the relationships among species and their subspecies, even more than the morphological analysis. 466Sabry S. Ei-Serafr et ai.

Morphometric, meristic and DNA riboprinting analyses of Tilapia species and their hybrids inhabiting the River Nile were examined. Morphometric data showed striking similarities and overlapping among Tilapia species, making it impossible to differentiate these species. Meristic characteristics revealed that Tilapia species could be identified into four major groups (Oreochromis niloticus, O. aureus, Sarotherodon galilaeus and Tilapia zillii). The lateral line scales differed significantly between the four Tilapia species, while the number of fin rays in the dorsal and anal fins differed significantly, differentiating three species (but not between O. niloticus and O. aureus). Restriction fragment length polymorphisms (RFLPs) of nuclear small sub‐unit ribosomal RNA (18S srRNA) gene were used to differentiate the species. Polymerase chain reaction‐restriction fragment length polymorphisms data provided a unique pattern for each species with a specific restriction enzyme. Two hybrids of Tilapia designated H1 and H2 were detected. The endonucleases SacII and ApaI differentiated H1 and H2. This research revealed a monophylogenetic relationship among all the studied Tilapia species.

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