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Reduction of the “Ngege”, Oreochromis Esculentus (Teleostei: Cichlidae) Populations, and Resultant Population Genetic Status in the Lake Victoria Region

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Reduction of the “Ngege”, Oreochromis Esculentus (Teleostei: Cichlidae) Populations, and Resultant Population Genetic Status in the Lake Victoria Region Uganda Journal of Agricultural Sciences, 2012, 13 (2): 65-82 ISSN 1026-0919 Printed in Uganda. All rights reserved © 2012, National Agricultural Research Organisation Reduction of the “ngege”, Oreochromis esculentus (Teleostei: Cichlidae) populations, and resultant population genetic status in the Lake Victoria Region W. Waiswa Mwanja1, P.A. Fuerst2 and L. Kaufman3 1Department of Fisheries Resources, P.O. Box 4 Entebbe, Uganda 2Department of Molecular Genetics, Ohio State University, 386 Aronoff Laboratory, 318 West 12th Avenue, Columbus, OH 43210 3Boston University Department of Biology 5 Cummington Mall Boston, MA 02215 Author for correspondence: [email protected] Abstract Ngege, Oreochromis esculentus, originally formed the mainstay of the Lake Victoria Region (LVR) fisheries. Together with its indigenous congener O. variabilis, it was displaced from Lakes Victoria and Kyoga of LVR and was found to survive as isolated small populations within the peripheral minor lakes and reservoirs around the two lakes. Displacement of the two LVR indigenous tilapiines was thought to be principally driven by changed lake environment and predation by the introduced Nile perch, but also competition and genetic swamping by the closely related introduced and comparatively more ecologically versatile tilapine species. In a study carried out in the LVR between 1993 and 2003, micro satellites and RAPD markers were used to analyse the remnant populations so as to establish the population structure and extant genetic diversity of O. esculentus. Analyses indicated that the surviving O. esculentus retained a high proportion of genetic diversity with high differentiation between units an indication of genetic exchange between indigenous and introduced Nile tilapia where the two forms co-existed. While this heightened concern for genetic swamping of the remnant population units by the introduced tilapiines it was noteworthy that in a few of the satellite lakes where the O. esculentus was dominant evidence for introgression was weak. Key words: Genetic interaction, genetic swamping, Ngege, Nile tilapia Introduction 1983) locally known as mbiru, are the only endemic tilapiine species of the Victoria Oreochromis esculentus (Trewavas, and Kyoga basins - both of which 1983), formerly Tilapia esculenta constitute the major components of the (Graham, 1928), was a prized food fish Lake Victoria Region (LVR). The two endemic to the Lake Victoria and Lake species were abundant open-water Kyoga basins of East Africa (Fryer and phytoplanktivores and detrivores, Iles, 1972). Oreochromis esculentus, the respectively (Fryer and Iles, 1972). true ‘ngege’ as it was locally known Previously, ngege was the mainstay of the around Lake Victoria in Uganda together fisheries of the two lakes (Graham, 1928; with Oreochromis variabilis (Trewavas, Garrod, 1959; Welcomme, 1965; Fryer and 66 W. Waiswa Mwanja et al. Iles, 1972). This changed with the disputed this claim. Welcome (1967), Fryer introduction and establishment of non- and Iles (1972), Ogutu-Ohwayo (1988, indigenous tilapiines and a voracious 1990), Balirwa (1992), Kaufman (1992), predator, the Nile perch; Lates niloticus Kaufman et al. (1997), Bugenyi and (Linnaeus, 1758) in the 1960s (Welcomme, Balirwa (2003) highlighted the changes in 1966, 1967; Balirwa, 1992). The two LVR the ecology of the ngege with the changes native tilapiine species were found to have in lake limnology and ecology including the been extirpated from the main Lakes; introduction and establishment of the non- Victoria and Kyoga, and survived only as indigenous tilapiine species and the Nile small isolated pockets within the satellite perch. Hecky et al. (1994) attributed the lakes surrounding the two main lakes decline in ngege to the ecological changes (Balirwa, 1992; Mwanja et al., 1995; that took place from the late 1970s. Mwanja 1996, 2000). At best, the two In this study we investigated the species were a highly local component of ecological and genetic interactions the fisheries of the satellite lakes, and were between O. esculentus and Nile tilapia. faced with increasing pressure from The Nile tilapia known for its good fishing mortality, and from the effects of fisheries and aquaculture attributes, has the ecological and genetic interaction with been introduced and got established all the closely related introduced tilapiines in over the tropics and subtropics (Kocher these lakes. The satellite lakes in which et al., 1998). It is ecologically dominant the ngege still occurred include Malimbe and aggressive compared to other tilapiine and Kubegena Lakes in Tanzania (Msuku, species where they coexist, tending to 2004; Nagl et al., 2001), Lake Kanyaboli swamp its relatives both ecologically and and some reservoirs in the Siaya region in through genetic introgression (Leveque, Kenya east of Lake Victoria, and in a 1997;Liem, 1981; Lowe-McConnell, 1959, number of minor lakes in Uganda including 1987; Mwanja and Kaufman, 1995). This the Kyoga satellite lakes of Kawi, makes it urgent to evaluate what is left Nawampasa, Nyaguo and Lemwa, the of the two endemic tilapiine species of the Nabugabo Lakes (Kayanja, Kayugi and Lake Victoria and Lake Kyoga basins and Manywa), and the Kooki Lakes (Mburo, enact a policy that ensures an adequate Kijanebalola, and Kachera) of the Kagera hedge against extinction. It is anticipated basin southwest of Lake Victoria that such information may guide the (Mwanja, 1996). salvage of what is left of the original Earlier work on O.esculentus dealt highlights of the fisheries of Lakes Victoria with the taxonomy (Graham, 1928), and Kyoga. An important part of this biology and ecology (Garrod, 1959) of this study was to describe the population species and the interaction between genetic status of O. esculentus, tilapiine species as in Lowe McConnell particularly in light of Lowe-McConnell’s (1958, 1959) on the suspected (1959, 1987) early warnings concerning hybridisation between O. esculentus and the O. esculentus hybridisation with O. Nile tilapia. Previously, the Nile tilapia niloticus. We profiled the remnant was considered a sister species to O. populations of the ngege using the RAPDs esculentus (Trewavas, 1983); however, technique and analysed the current genetic recent phylogenetic studies by Nagl et al. status of the remnant O. esculentus (2001) and Klett and Meyer (2002) have populations using microsatellite markers. Reduction of the “ngege” populations, and resultant population genetic status 67 This study was carried out between 1993 extraction. For each site, up to 20 and 2000 as part of a wider international individuals were sampled for molecular project led by Ohio State University for analysis and the voucher specimens conservation of cichlid fishes of Lake archived at the National Fisheries Victoria Region. Resources Research Institute in Jinja, Uganda. Some specimens were shipped Materials and methods to Boston University, Massachusetts to confirm ID based on morphological Fishing survey and sample collection criteria. Fish samples were collected from seven lakes in the Lake Victoria basin in East Molecular analysis - RAPD analysis Africa (Figure 1, Table 2) from 1992 to DNA extraction was performed using the 1996. Fish were caught using experimental standard phenol/chloroform extraction fishing nets of varying mesh sizes mounted method (Sambrook et al., 1989). PCR into 3 fleets set starting at 2 m depths and reaction mixtures of 25 µl final volume thereafter one fleet for every 100 m containing 50 ng of genomic DNA, and offshore. Each fleet had 2 nets measuring final concentration of 25 µM of dNTP, 0.6 20 m long by 1.5m wide per mesh size of µM of primer 2.5 µl of a reaction buffer, 1.5 (38.1 mm), 2 (50.8 mm), 3 (76.2 mm), and 0.1 µl of 5 U/µl Taq polymerase 4 (101.6 mm) and 5 (127.0 mm) inches. enzyme from BRL technologies. RAPD Nets were set between 17.00 and 18.00 decamer primers (Operon Technologies, hrs at dusk and checked every 2 to 3 hrs Alameda, California) were used in a for 24-hour cycle per site of the lake Perkin-Elmer thermocycler at the studied. This allowed for getting the fish following sequence: a hot start for 3 min alive and fresh, and enabled clear at 94oC, then 45 cycles for 30 seconds at identification and taking of uncontaminated 94oC, 1 min at 35oC, and 2 min at 72oC, samples for molecular analysis. Nets were with a ten minutes extension at 72oC at set based on fishermen’s knowledge of the end of the 45 cycles. Repeatability and the occurrence of the fish and using potential contamination of reaction physical and biological (floral) conditions were checked using both a characteristics of shoreline. After retrieval positive and a negative control for every of the fish from the nets, all tilapias were reaction set for each primer. All sets of identified, sexed, weighed, and measured reactions were based on the same stock (length, body depth), and DNA samples DNA extract diluted independently to 50 (2 to 3 g of tissue from the right epaxial ng of genomic DNA for each PCR musculature placed in 95% ethanol) were reaction mixture. Amplifications were taken from specimens that were separated by 1.6% agarosesynergel unambiguously identified on site. The electrophoresis and visualized under UV tissue sample for each individual fish light after ethidium bromide staining. sampled was placed in an individual vial containing 95% ethanol. After one hour, RAPDs band scoring and data analysis the ethanol was poured off and replaced The primers used were the 10-mer from with fresh ethanol, and the vials sealed Operon Technologies, Inc. The products and labelled for shipment to the laboratory were indexed using the primer code at Ohio State University for DNA followed by a band number scored relative 68 W. Waiswa Mwanja et al. to the position of a standard DNA ladder For PCR analysis, each forward primer marker bands (123 bp Ladder DNA from was end-labelled with P32 radioisotope BRL Life technologies) electrophoresed using T4 polynucleotide kinase (GIBCO together with each reaction set.
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