MORPHOLOGICAL AND MOLECULAR CHARACTERISTICS OF SOME TILAPIA POPULATIONS FROM THREE RESERVIORS IN SOUTHWESTERN NIGERIA

Abstract

The study was conducted to identify the tilapia species present in three reservoirs viz: Aiba, Asejire and Owena reservoirs located in South-Western Nigeria; to determine the morphological and genetic variation between two species (Oreochromis niloticus and Sarotherodon galilaeus) and to assess the relationship between the morphological and the molecular characteristics. The fish samples were collected from the three different reservoirs with the assistance of the artisanal fishermen in the different locations using cast nets of mesh sizes 45mm and 50mm and bamboo traps. The tilapias were identified to the species level using the keys, descriptions and Checklist of the Freshwater Fishes of Africa and West Africa Freshwater Fishes. Thirty-one morphometric characters (total length, standard length, head length, snout length, eye diameter e.t.c) and thirteen meristic traits (pelvic fin spines, anal fin rays, caudal rays, pectoral fins e.t.c) were measured to the nearest centimeter in seventy five samples of each of Oreochromis niloticus and Sarotherodon galilaeus. The total length value for the fishes ranged between 17.30-35.20cm and their weight between 110.5-860.0g. The morphological variation and relationship between the two species was investigated by multivariate analysis using the principal component analysis and cluster analysis. Principal component analysis (PCA) explained between 11-66% and 20-36% of the variance in morphometric and meristic measurements, respectively. Morphometric measures showed a low variation between the populations while in the meristic characteristics there was no pattern of variation as a result of the wide degree of overlapping observed. The morphometric differences between the populations may have been due to either genetic differences or environmental factors. Investigation of genetic variations for the two species was performed using random amplified polymorphic DNA (RAPD) technique. One milliliter of blood was collected aseptically from each fish sample into EDTA bottles using 2ml disposable syringe by gill vein puncture. The DNA of the fish samples were extracted using CTAB method. RAPD analysis was carried out using primers (OPAD-09, OPC-09, OPI-03 and OPB-15) which yielded clear and consistent DNA banding patterns. A total of seventy seven bands were scored, thirty two (41.56%) were monomorphic while forty five (58.44%) were polymorphic. The percentage of polymorphism ranged from 20-92% and the size of amplicons ranged from 150-5000bp. High degree of polymorphism observed, suggests the degree of genetic variability within the species. An un-weighted pair-group method with arithmetic mean (UPGMA) dendrogram was constructed based on genetic values to show the genetic relationship between the two populations. The dendrogram obtained from RAPD clearly showed that populations from Asejire and Aiba reservoirs are closely related to each other while populations from Owena reservoir are genetically similar. The knowledge on the genetic structure and variability of stocks should be a prerequisite of any fish breeding program to provide a greater understanding of the genetic resources and for proper management of genetic data using genetic tools to avoid inbreeding and to improve the quality of fish produced.