Allozyme Variation in Natural and Cultured Populations in Two Tilapia Species: Oreochromis Niloticus a N D Tilapia Zulu

Heredity 76 (1996) 640—650 Received 25 September 1995

Allozyme variation in natural and cultured populations in two tilapia species: Oreochromis niloticus a n d Tilapia zulu

XAVIER ROGNON*, MARTINE ANDRIAMANGA, BRENDAN McANDREW & RENÉ GUYOMARD Laboratoire de Genetique des Poissons, INRA,78352 Jouyen Josas Cedex, France and tinstitute of Aquacu/ture, University of Stirling, Stirling, FK9 4LA, U.K.

We investigated the electrophoretic polymorphism at 30 protein loci in 27 wild and cultured populations of two tilapia species, Tilapia zulu and Oreochromis niloticus. Single and joint segregations were analysed at 12 polymorphic loci in full-sib families. All the loci showed single segregation in agreement with the Mendelian expectations, and two nonrandom joint segregations were found. In total, 26 loci were polymorphic and 12 were diagnostic between the two species. A pronounced differentiation was observed between the Ivory Coast and Nib-Sudan- ian T zulu populations (average Nei's standard genetic distance =0.13).Within the Nilo-Sudanian region, the level of variation observed in 0. niloticus and T zulu was rather low when compared with other freshwater species analysed at similar macrogeographical scales. The same pattern of geographical differentiation was found between the west and east African populations in both species suggesting that this structuring mainly reflected the same palaeogeographical events. Most of the geographical variation observed in the Nile tilapia populations analysed here was present in the cultured stocks. In contrast to fish-farmed stocks analysed in other countries, those from the Ivory Coast and Niger displayed neither evidence of loss of genetic diversity nor any trace of introgression with other cultured tilapia species, indicating that these stocks have been properly managed.

Keywords: allozyme variation,cultured stocks,Oreochromis niloticus, phylogeography, Tilapia zulu, wild populations.

Introduction niloticus stocks could have been accompanied by a rapid loss of genetic variation within populations Amongthe tilapias, Oreochromis nitoticus, the Nile through genetic drift in some cases (Brummett et at., tilapia, is the main species used in aquaculture. Its 1988; Pullin & Capili, 1988). Finally, tilapias gener- original range includes the Senegal, Gambia, Niger, ally show a strong propensity to hybridize. Intra- Volta, Lake Chad and the Nile river basins as well generic hybridization has been reported in the wild as several lakes in east Africa (Trewavas, 1983; when originally allopatric species come into contact Fig. 1), but today cultured stocks have been propa- after transplantation (Daget & Moreau, 1981) or has gated around the world. Most of these stocks origi- been done intentionally by fish-farmers in order to nated from the same sampling areas in the Nile and produce all-male populations (Jalabert et at., 1971). Volta rivers and were founded with a limited This could have led to uncontrolled cryptic and number of individuals (Pullin & Capili, 1988; undesirable introgressions of the cultured stocks by Welcomme, 1988; Lazard, 1990), regardless of the other species, as exemplified by allozyme studies on extant genetic variation within and between natural fish-farm populations (Macaranas et at., 1986). populations. Furthermore, the propagation of 0. The rational utilization of the genetic resources *Correspondence: Département des Sciences Animales, Institut and the adequate management of cultured stocks of National Agronomique Paris-Grignon, 75231 Paris Cedex 05, 0. nitoticus (as well as other tilapia species) require France. two complementary phases: (1) a description of the genetic variation within and between populations

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600 km

Fig.1 Geographical locations of Oreochromis niloticus and Tilapia zulu populations sampled: (s)wildand (A) cultured populations of 0. niloticus and of (*) T zulu. Collection sites are numbered as indicated in Table 1. Shadowed area indicates the natural distribution range of 0. niloticus.

throughout the complete geographical range of the is the level of interspecific gene introgression within species and (2) the identification of diagnostic loci these strains? which could allow the risks of interspecific introgression with related cultured species to be monitored. In this paper, we present a first evaluation of allo- Materials and methods zyme variation in natural populations of 0. niloticus throughout most of its natural range as well as in Sampling cultured stocks used in the Ivory Coast and Niger. Samples from 15 wild populations of T zulu and 0. We also describe the genetic variation in a related, niloticus and nine cultured stocks of 0. niloticus but not domesticated (at least in west Africa) were collected in west Africa (Fig. 1 and Table 1). species, T zulu, over a similar geographical area for Fish were identified by morphological characters as comparison. These results are compared with those described by Lévêque & Paugy (1984). The Lake already published on tilapia and are discussed in Manzala samples were from the F7 generation of relation to three major questions: (1) how have experimental stocks maintained at the Institute of hydrography, geographical isolation and palaeogeo- Aquaculture (University of Stirling, U.K.). The graphical events affected the population structure of initial broodstock number was 30 pairs and each these species; (2) how have cultured strains changed generation was obtained by mating a minimum of 50 in comparison to their wild progenitors and (3) what and 20 pairs for 0. niloticus and T zulu, respectively.

The Genetical Society of Great Britain, Heredity, 76, 640—650. 642 X. ROGNON ETAL.

Table 1Origin (river system and location) and size of the 27 samples of tilapia

Sample Location Origin Stock Abbreviation size number

Oreochromis niloticus Nile (Lake Manzala —Egypt) E ONnil 15 1 Lake Turkana (El Mob Bay —Kenya) E ONtur 20 2 Niger Bamako (Mali) W ONnb 11 3 Mopti (Mali) W ONnm 17 4 Niamey (Niger) W ONnn 17 5 Baoule river (Odiene —IvoryCoast) W ONbao 7 6 Sankarani river (Selingue —Mali) W ONnsa 9 7 Volta (Kou river at Bama, Burkina Faso) W ONkr 20 8 Senegal (Djoudj) W ONsg 30 9 'Bouake strain' -- Bouke Station (Ivory Coast) C ONbk 30 10 Daloa 1 (fish farm, Ivory Coast) C ONdi 24 11 Daloa 2 (fish farm, Ivory Coast) C ONd2 30 11 Gbeleban (fish farm, Ivory Coast) C ONgb 15 12 Kossou (manmade lake, Ivory Coast) C ONko 30 13 Buyo (manmade lake, Ivory Coast) C ONbu 14 14 Kouto (manmade lake, Ivory Coast) C ONkt 20 15 Kou strain (Bouake Station, Ivory Coast) C ON/CS 30 10 Niger strain (fish farm near Niamey, Niger) C ONni 27 5 Tilapia zulu Nile (Lake Manzala, Egypt) B TZnil 8 1 Sassandra (Buyo, Ivory Coast) W TZas 16 14 Bandama Kan (Bouake Station, Ivory Coast) W TZka 10 10 N'Zi (Brobo, Ivory Coast) W TZnz 11 16 Niger Bamako (Mali) W TZnb 6 3 Mopti (Mali) W TZnm 14 4 Volta Kou river (Bama, Burkina Faso) W TZkr 5 8 Red Volta (near Ouagadougou, BF) W TZrv 10 17 Senegal (Djoudj) W TZsg 4 9

W =wildpopulations, C =culturedstock; B =experimentalstocks (see text).

The Lake Turkana sample was provided by the Insti- Enzymatic electrophoresis tut für Tierzucht und Haustiergenetik (University of Electrophoresisprocedures were described by GOttingen, Germany). This stock was propagated Rognon & Guyomard (1995),exceptthat MDH was over generations by mating 35 parent pairs since its also screened following Macaranas (1991). Allele foundation with individuals collected from the lake. discrimination was improved by running 14 per cent The individuals analysed here belong to the starch gels at 6°C. The following enzyme systems F3 generation and were descended from 10 F2 were examined in each population: AAT (EC pairs. This kind of propagation should have 2.6.1.1), ADH (EC 1.1.1.1), AK (EC 2.7.4.3), CK prevented significant loss of genetic diversity (EC 2.7.3.2), EST (EC 3.1.1.-), FBP (EC 3.1.3.11), through genetic drift and these samples should be FH (EC 4.2.1.2), G3PDH (EC 1.1.1.8), GPI (EC genetically close to the wild populations from which 5.3.1.9), IDDH (EC 1.1.1.14), IDHP (EC 1.1.1.42), they originated. LDH (EC 1.1.1.27), MDH (EC 1.1.1.37), MEP (EC

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1.1.1.40), MPI (EC 5.3.1.8), PGDH (EC 1.1.1.44), standard genetic distances were estimated following PGM(EC 5.4.2.2) and SOD (EC 1.15.1.1). Locus Nei (1987). Phenograms were generated by UPGMA and allele nomenclature follow Shaklee et al. (1990). cluster analysis (Sneath & Sokal, 1973) using the Alleles were designated by their electrophoretic PHYLIP package (Felsenstein, 1993). Bootstrap values mobility relative to the most frequent allele found in were computed over 500 replications. the Bouake Station sample of 0. niloticus.

Results Inheritancestudies Fourspecific 0. niloticus families were selected Genetic interpretation according to parental phenotypes. One back-cross A conservative approach was adopted in the classifi- family between 0. macrochir and 0. niloticus [0. cation of loci and variation which resulted in 30 loci macrochir female x (0. macrochir female x 0. niloti- which could be reliably typed in these species. Single cus male)1 was also analysed. Oreochromis macrochir and joint segregations could be checked at a total of was chosen because of the high number of diag- 12 loci (Table 2). All the single segregations fitted nostic loci found between this species and 0. niloti- the expected Mendelian proportions. Two nonran- cus (Rognon & Guyomard, 1995). Gene segregation dom joint segregations were observed: AAT2*I was analysed in 62 (back-cross family) and 70 (0. MDH3*(recombination frequency, r =0.30)and niloticus families) three-month-old offspring per MPI*IEST2* (r =0.16).Significant linkages invol- cross. ving the same enzyme systems were also found in Xyphophorus spp. for these two pairs (Morizot et a!., 1991) and in salmonids for the first one (Taggart & Dataanalysis Ferguson, 1984), but locus homologies between Departuresof single and joint segregations from these species have not been established. expected Mendelian proportions in full-sib families were checked with f-tests (Taggart & Ferguson, Polymorphismand heterozygosity levels 1984). The sequential Bonferroni technique (Rice, 1989) was applied to the tables of multiple statistical Allelefrequencies at the 25 loci are presented in tests to correct for table-wide significance levels. Table 3. All samples were in agreement with the Agreement of the observed genotype frequencies Hardy—Weinberg expectations and no significant with the Hardy—Weinberg and genotypic linkage linkage disequilibrium was found. Twenty-five loci equilibrium expectations were checked for each were found to be polymorphic and 12 of them were sample with the GENEPOP package (Raymond & diagnostic of the two species. Eleven loci exhibited Rousset, 1995). Mean expected heterozygosities and variation in 0. niloticus populations. The largest

Table 2 Single (on the diagonal) and joint (off the diagonal) segregation analysis at 12 loci in specific and back-cross (italicized) families of tilapia. Numbers represent the number of families analysed. Segregation in females is indicated below the diagonal and segregation in males above the diagonal

AAT2*CK1*EST2* FH* IDDH* LDH2* MDH1* MDH3* MEP1* MEP2* MPI* SOD*

AAT2* 2 1 1 CK1* 1 1 1 1 1 1 1 EST2* 1 1 1 1 ]*** 1 FH* 1 1 IDDH* 3 1 LDH2* 1 1 1 1 1 MDH1* 1 1 1 1 MDH3* 1' 1/1 1 1 MEP1* 1 2 MEP2* 1 1 3/1 MPI* 1 1 SOD* 1 **

The Genetical Society of Great Britain, Heredity, 76, 640—650. 0 zC) z0 Table 3 Allele frequencies at polymorphic loci (common allele, *100, omitted),polymorphism (P 95) and average heterozygosity (H) rates in 27 populations of tilapia

ONnil ONtur ONnb ONnm ONnn ONbao ONnsa ONkr ONsg ONbk ONdl ONd2 ONgb ONko ONbu ONkt ONks ONni TZnil TZsas TZka TZnz TZnb TZnm TZkr TZ,v TZsg Locus Allele E E W W W W W W W C C C C C C C C C E W W W W W W W W

AAT2* *129 0.071 — — — *80 0.038 *55 0.133 — 0.409 0.591 0.500 0.500 0.286 0.700 0.533 0.400 0.187 0.317 0.367 0.383 0,393 0.625 0.533 0.500 *50 0.038 0.125 *12 0.438 — — —0.0830.071 — —0.125 A4T3* *78 0.182 CK_1* *88 1 1 1 1 1 1 I 1 1 EST2* *rns 0.017 *103 0.850 FBP1* *98 1 1 1 1 1 1 1 1 1 FBP2* *83 1 1 1 1 1 1 1 1 1 FH* *1200,8330.950 0.500 — — 0.1330.0170.0710.150— — — 0.393 G3PDHI**600 1 1 1 10.7500.308—0.063—— *400 0.2500.692 10.9371 G3PDH2**57 0.050 GPI-1 *0 — 0.525

*—150 1 1 1 1 1 1 1 1 1 GPI-2 *89 0.346 IDDH* *167 0.571 — — — 0.100 — — — 0.143 — — 0.500 *25 1 — — 1 1 i 1 1 *20 — — 0.409 0.437 0.500 0.286 0.278 0.316 0.367 0.150 0.354 0.467 0.200 0,482 0.357 0.450 0.350 0.389 *77 1 1 1 g IDHP1* *108 1 0.800 1 IDHP-2 *80 — 0.036 LDH-1" *1000 1 1 1 1 1 1 1 1 1 LDH2* *52 — — — — 0.029 — — 0.200— 0.0500.2500,2670.0330,1670.250 —0.2330.074 LDH-3 *98 — 1 1 1 1 1 1 1 1 I MDH.I *88 0.313 1 1 1 1 1 1 1 1 *45 0.687 MDH-2 *80 — 1 0.964 1 1 1 0,962 1 1 1 *64 0.036 — — — —

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—III—III—IH— allele frequency differences were found between the eastern and western populations. The cultured —1111—H—'I — stocks derived from the Bouake strain exhibited the — !(i-I l1r-— I — largest number of alleles (Mann—Whitney test; N P<0.002). MPI*107 was specific to these stocks. r..I — r — —I Ii— "• Seventeen loci were polymorphic among the Ti? zulu N populations. IDDH* was diagnostic of samples from — I — r I I I I — I I — Ivory Coast and the Nilo-Sudanian province and large allele frequency differences were found at —lII—II—Ill—,-r-'0 G3PDH1*, IDHP1*, MEP2* and SOD*. VI — — I N N All the populations of both species were polymor- I I I I — ' I — N phic (Table 3). The polymorphism rates (under the NN — I1 I© I I—I I I— c:N 0.95 criterion) ranged from 3.3 to 20 per cent in T '0— zulu, from 6.7 to 13.3 per cent in the wild Nile —lll_lII—IIl—, tilapia populations and from 13.3 to 23.3 per cent in cultured Nile tilapia stocks. The expected average h. "I— o lIlllIHllllt— heterozygosity per locus (Table 3) ranged from 1 to 6.2 per cent for T zulu, 2—5.5 per cent for wild 0. oZL) IIIHIIIllll'c— niloticus populations and 4.6—8.7 per cent in — — — sN I I cultured 0. niloticus stocks. They were significantly o I— I© I I© I I I I I I higher in cultured than in wild populations of 0. N rfl I N I I I I I niloticus (Mann—Whitney test, P.<0.002). oL)I'll - I N N o Populationstructure and phenetic relationships UPGMAphenograms derived from standard genetic o.)lIllIIHIII distances (data not shown) for wild populations plus ouIIIIIIllIII Nile River and Lake Turkana samples, and for all NN the Nile tilapia populations are given in Fig. 2a and d I I IN' I I I I I 2b, respectively. Both species were unequivocally s© m differentiated because the node at each of the ZL) —— o d© N species clusters was supported by 100 per cent boot- N strap values. Within the two species, the wild popu- pHHHII I I lations exhibited a clear geographical clustering

I I I I I I I I I pattern (Fig. 2a). In T zulu, the Ivory Coast populations (TZas,TZka,TZnz) were clearly distinctive N II I from those of the Nilo-Sudanian region (TZnil, Iq It-i I I I I I i i i o TZnb, TZnm, TZkr, TZrv, TZsg). The mean genetic IlllllHIIll distance between these two groups was 0.121 ± 0.029. d A subdivision between eastern and Sudanian popu- ''0'0 lations was found in both 1? zulu and 0. niloticus zo ici,IIIiiIllIl with a similar mean level of differentiation (0.059±0.023 and 0.064±0.012, respectively). o IHIHill11I In Nile tilapia, the average genetic distance '0 IIIIlIIIIIl'0N N between the Lake Turkana and Lake Manzala popu- o lations was quite high when compared to the — average genetic distance found within the Sudanian oz dIlllIllllIH wild populations (0.041 vs. 0.006 ± 0.005). Note that Nfl the topologies found within species were not I N I I I I I I I o supported by high bootstrap values. When we added '00 NI© cultured stocks, the topology remained the same but ** * * * bootstrap values were lower. In particular, the * N * ONtur and ONnil cluster was supported by a boot- tin — tin strap value of 36 per cent. The cultured stocks of 0.

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(a) 71

-ONtur -ONnil ONbno 88 [ONnb LI LONnsa 56 [-ONsg ONkr ONn,n 1ONnn

0.35 0.15 0.10 0.05 0 Nei's genetic distance

(b) 1

cONko Fig. 2 UPGMA trees derived from standard Nei's genetic distances between wild samples of (a) Tilapia cONkt zulu and Oreochromis niloticus or (b) wild and cultured samples of 0. niloti- ONnn cus. Bootstrap percentage values esti- .C ru mated from 500 replications are

I I I I reported when higher than 50 per cent. 0.05 The arrow indicates the position of Net's genetic distance tree (b) on tree (a). niloticus from the Ivory Coast and Niger clustered to (Cavalli-Sforza & Edwards, 1967) with slightly lower the Sudanian group (Fig. 2b). The Bouake stock was bootstrap values (results not shown). the most divergent population within this group and The partitioning of the total genetic diversity into also the closest one to the Lake Manzala and between- and within-population components (Table Turkana samples as a result of its mixed origin. The 4) showed that 63.4 per cent of the total genetic Niger fish-farm strain (ONni) and the wild popula- diversity (H1 = 9.9 per cent) was the result of tion (ONnn) did not differentiate significantly between-population differences in T zulu. In 0. nib- (P>0.98). The wild population from the Kou river ticus, 32.8 per cent of the total genetic diversity (ONkr) and its cultured derivative stock (ONk) (II = 6.9 per cent) resulted from variation between were significantly different (P

Table 4 Gene diversity analysis for Oreochromis niloticus and Tilapia zulu

No. Species populationsI-I, (%)H, (%)G, (%) 0. niloticus West African wild populations plus Nile and Lake Turkana samples 9 6.9 4.6 32.8 Cultured stocks 9 6.9 6.3 8.7 All samples 18 7 5.5 21.4 T zulu Ivory Coast 3 6.3 4.5 28.9 Nilo-Sudanian 6 6 3.2 47.1 All populations 9 9.9 3.6 63.4

H,, total gene diversity; H, average gene diversity within population; G,,, relative gene diversity, attributable to the variation between populations.

study are within the range reported for conspecific Discussion populations (Gyllensten, 1985). Our study did not include 0. niloticus populations from the lakes of the Genetic variation within and between wild Rift Valley, where several morphological subspecies populations have been recognized (Trewavas, 1983). However, Inmost cases, our interpretations of electrophore- populations from most of these lakes have been grams were consistent with those proposed in analysed by Seyoum (1989) and his results can be previous works (Basiao & Taniguchi, 1983; McAn- compared to ours because his study also included a drew & Majumdar, 1983; Seyoum, 1989; Macaranas sample from Lake Turkana. If we assume that the etal., 1995;Pouyaud & Agnèse, 1995) and were most frequent alleles found in both Lake Turkana validated by our inheritance studies for some samples are the same, the differences between the enzyme systems. The number of loci postulated for west and east African lake populations would be AAT, CK, FBP, IDHP and MDH varied between mainly the result of variations observed at one locus, authors, but in all cases, it was possible to establish out of 25 which were common to the two studies. homology between the loci and the alleles described Accordingly, the value calculated over the entire in the different works. The allele frequencies geographical range of 0. niloticusshouldnot differ reported by McAndrew & Majumdar (1983) on wild substantially from the value obtained in our study. 0. niloticus and T zulu populations from Lake According to the morphological classification of Manzala did not differ from those observed in our Trewavas (1983), the west African and Nile River samples from the same location. This supports the populations of Nile tilapia belong to the same assumption that the samples analysed here are still subspecies, 0. niloticus niloticus. The pattern of representative of original populations of the lake. differentiation which emerged from our study is not Differences were found at several loci described by congruent with this phenotypic classification because Macaranas et a!. (1995) and ourselves for the strains the population from the Nile clusters to the Lake originating from Lake Turkana. As they originated Turkana one which was described as a distinct from the same initial sampling of the lake, these morphological subspecies, 0. n. vulcani. Seyoum differences should result from the subsequent propa- (1989) also disagreed with the morphological classi- gation of the strains. fication as 0. n. cancellatusdidnot appear as a The heterozygosity levels reported here were monophyletic group in his study. In contrast, Macar- similar to the values found in other studies on wild anas et a!. (1995) observed a dichotomy between 0. 0. niloticus populations (McAndrew & Majumdar, n. vulcani and the Nile delta, Volta and Senegal 1983; Seyoum, 1989; Macaranas et al., 1995) or in River populations. Examination of additional loci other tilapia species (McAndrew & Majumdar, 1983; and populations would be necessary to ascertain the Seyoum, 1989; Pouyaud, 1994). phylogenetic relationships between Nile tilapia The G,, values observed within species in this populations.

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Genetic structure and pa/aeogeography leus (Rognon, 1993). These different patterns could Duringthe last pluvial period, i.e. from 12000 to indicate that there were several colonization events 7500 BP, lakes and rivers expanded dramatically in of the Baoule V by the Nilo-Sudanian ichthyofauna. all the Nilo-Sudanian region and connections were However, an alternative hypothesis is possible. The established between the different hydrographic rivers of the Baoule V show a gradation in ichthyo- basins investigated in this study (Roberts, 1975; fauna. Their lower courses are rain forest habitat Beadle, 1981; Grove, 1985). It is probable that these with typical Guinean forms, whereas their upper recent palaeogeographical events played a role in courses are savannah habitat and harbour Nib- the genetic structuring of wild populations of Nile Sudanian forms (Daget & Iltis, 1965; Leveque et a!., tilapia because they allowed gene flow between 1983). Thus, the aforementioned differences among areas. However, this chronology does not fit with the species could simply reflect differences in sampling divergence time which can be estimated from the locations. values of substitution rate usually postulated for proteins under the hypothesis of a molecular clock. McAndrew & Majumdar (1984) found that the Geneticcomparison between cultured stocks and substitution rate used by Gorman et a!. (1976) gave wild populations estimates of divergence time congruent with those Thecultured Nile tilapia stocks of the Ivory Coast deduced from fossil records in Tilapiines (Roberts, did not exhibit any allele of the other species at 1975) and seemed to be the most appropriate. When diagnostic loci identified in previous analyses of applied to our data, this substitution rate gave a Oreochromisspp.reared in the Ivory Coast divergence time of 12 Myr since speciation between (Rognon, 1993; Rognon & Guyomard, 1995). In 0. niloticus and T zulu, which is in agreement with contrast, introgression was commonly found in other McAndrew & Majumdar (1984), 1.1 Myr between stocks of Nile tilapia used for aquaculture or stock- the west and east African 0. niloticus populations ing (Daget & Moreau, 1981; Macaranas et a!., 1986). and 0.7 Myr between the Lake Turkana and Nile In the Ivory Coast, the Bouake Station is the only River populations. The more conservative relation- place where other Oreochromis species are found ship proposed by Nei (1987) gave coalescence times (Lazard, 1990). However, the 0. niloticus stocks of 0.3 Myr between west and east African popula- present at this station greatly outnumber the other tions and 0.2 Myr between those of Lake Turkana stocks of Oreochromis spp., rendering its contamina- and the Nile River. Despite their inaccuracy, these tion by other species improbable. Furthermore, the estimates suggest that the observed genetic distances Ivory Coast- and Niger-farmed stocks always exhib- between the east and west African populations of 0. ited heterozygosity levels as great as or greater than niloticus reflect much earlier connections and sepa- the natural populations from which they originated, rations between basins, and differentiation events. It suggesting proper management of these stocks. is noteworthy that T zilii and 0. niloticus show the Most of the Nile tilapia strains used in aquacul- same level of differentiation between east and west ture have not been chosen after an initial evaluation African populations, which suggests that these of performances of populations originating from the differentiations reflect the same palaeogeographical entire species range. Only recently has the genetic events and the same evolution rate in the two potential of 0. niloticus been studied in a systematic species. fashion (Eknath et al., 1993). Our data show that the In contrast, the three Ivory Coast populations of west African wild populations possess a rather high T zulu analysed here displayed a large genetic level of genetic variability. If these wild stocks show distance from the Nilo-Sudanian ones. The former satisfactory performances in respect of important populations originated from the Baoule V (Howes economical traits, it could be preferable to use these & Teugels, 1989), a region which harbours species local strains in fish-farming. This strategy would from both Nilo-Sudanian and Upper-Guinean prov- prevent the importation of a single exotic commer- inces. Electrophoretic studies of Nilo-Sudanian cial strain which could have a harmful effect on the species which are found in the Baoule V were equiv- genetic diversity of the species. ocal. Some species, such as Chtysichthys nigrodigita- tus (Agnese, 1989) and T dageti (Pouyaud, 1994), showed large genetic differentiation between prov- Acknowledgements inces whereas weak genetic differences were found Wewould like to thank the Office des Parcs Natio- in others such as C. auratus (Agnese, 1989), Clarias naux du Senegal, the staff of the ORSTOM Center anguullaris (Teugels et a!., 1992) or Sarotherodonga/i- at Bamako (Mali), G. Horstgen-Schwark (Institut

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