Aboubacar Toguyeni

Feeding Behaviour and Food Utilisation in Tilapia, Oreochromis Niloticus: Effect of Sex Ratio and Relationship With the Endocrine Status

The feeding behaviour of male monosex, female monosex, and mixed groups of Oreochromis niloticus was studied under conditions of self-feeding. Feeding activity was observed almost exclusively during the light period. The food intake pattern was similar whatever the sex ratio, and voluntary food intake (VFI) appeared lower in the male monosex groups than in the others. Male monosex groups displayed higher specific growth rates (SGR) and a lower food conversion ratio than female monosex and mixed groups. The SGR of males was higher in the monosex than in the mixed groups, whereas females of mixed and monosex groups displayed no significant difference in SGR. The efficiency of food utilisation was also analysed: nutrient retention ratios were higher in male monosex than in female monosex and mixed groups. Males displayed a distinctly higher metabolic capacity. Differences in sex-related hormones (11 ketotestosterone = 11-KT, 17beta-Oestradiol = 17beta-E2) and a metabolic hormone (triiodothyronine = T3) were observed between males and females. The hypothesis of an involvement of these hormones in the higher metabolic capacity of males is discussed. The observed differences in feeding behaviour between the different groups also suggest an effect of social interactions on the efficiency of food conversion and thus on the differential growth of males and females.

Influence of sexual phenotype and genotype, and sex ratio on growth performances in tilapia, Oreochromis niloticus

In tilapia, precocious sexual maturity associated with continuous and asynchronous reproductive activity often results in unwanted reproduction and overcrowding in ponds where the food supply is limited. This constitutes a major limitation in the culture of most tilapia species. Furthermore, divergent growth performances linked to sex are regularly observed. Males present better growth performances than females. The first experiment of the present study analysed the influence of sexual genotype on phenotypic male growth rates. Males with three different sexual genotypes were produced: YY100 (YY male×YY female), XY100 (YY male×XX female) and XX100 (hormonally sex-inverted females). They were compared with classic (XY male×XX female) or all female (XX male×XX female) progenies. The objectives of the second experiment were to study the influence of social interactions through the use of different sex ratios: F100, F75, F50, F25, F0 (F100=100% females in the population). These two experiments were carried out in hapas implanted in pond with “Egypt-UCS” strain of Oreochromis niloticus. At the end of the first experiment (140 days post-fertilisation), the XY genotype presented the best growth performances and YY genotype the lowest. Final body weights of neomale (XX100) were significantly (P<0.05) higher than those of YY100 males. According to our results, the involvement of a genetic component in sex-related growth difference between males and females is strongly suspected. We stressed upon the involvement of genes bound to the sexual genotype (especially XY in comparison to YY and XX) independently of the sexual phenotype. Furthermore, sexual maturity is not the only phenomenon involved in the sexual dimorphism. From 102 days post-fertilisation until the end of the second experiment, females of the F25 population presented the best growth performances. Their final body weights were significantly higher than those of all the males except F50 population males. In general, female body weight tended to increase inversely to their proportion in the population. Males of the F50 population presented better growth rate than the other males. Thus, tilapias seems to be very sensitive to the effects of social interactions. This social behaviour could strongly influence growth differences between males and females.

Spatial and temporal variation in population genetic structure of wild Nile tilapia (Oreochromis niloticus) across Africa

BackgroundReconstructing the evolutionary history of a species is challenging. It often depends not only on the past biogeographic and climatic events but also the contemporary and ecological factors, such as current connectivity and habitat heterogeneity. In fact, these factors might interact with each other and shape the current species distribution. However, to what extent the current population genetic structure reflects the past and the contemporary factors is largely unknown. Here we investigated spatio-temporal genetic structures of Nile tilapia (Oreochromis niloticus) populations, across their natural distribution in Africa. While its large biogeographic distribution can cause genetic differentiation at the paleo-biogeographic scales, its restricted dispersal capacity might induce a strong genetic structure at micro-geographic scales.ResultsUsing nine microsatellite loci and 350 samples from ten natural populations, we found the highest genetic differentiation among the three ichthyofaunal provinces and regions (Ethiopian, Nilotic and Sudano-Sahelian) (RST = 0.38 - 0.69). This result suggests the predominant effect of paleo-geographic events at macro-geographic scale. In addition, intermediate divergences were found between rivers and lakes within the regions, presumably reflecting relatively recent interruptions of gene flow between hydrographic basins (RST = 0.24 - 0.32). The lowest differentiations were observed among connected populations within a basin (RST = 0.015 in the Volta basin). Comparison of temporal sample series revealed subtle changes in the gene pools in a few generations (F = 0 - 0.053). The estimated effective population sizes were 23 - 143 and the estimated migration rate was moderate (m ~ 0.094 - 0.097) in the Volta populations.ConclusionsThis study revealed clear hierarchical patterns of the population genetic structuring of O. niloticus in Africa. The effects of paleo-geographic and climatic events were predominant at macro-geographic scale, and the significant effect of geographic connectivity was detected at micro-geographic scale. The estimated effective population size, the moderate level of dispersal and the rapid temporal change in genetic composition might reflect a potential effect of life history strategy on population dynamics. This hypothesis deserves further investigation. The dynamic pattern revealed at micro-geographic and temporal scales appears important from a genetic resource management as well as from a biodiversity conservation point of view.

Oestrogen and insulin-like growth factors during the reproduction and growth of the tilapia Oreochromis niloticus and their interactions

Oestrogens and insulin-like growth factors (Igfs) play both a central role in the regulation of reproduction and growth and can interact especially in species showing a clear-cut sex-linked growth dimorphism (SGD) like in tilapia. Aromatase is essential in ovarian differentiation and oogenesis since it controls oestrogen synthesis. During tilapia sex differentiation, aromatase cyp19a1a expression increases from 9 days post-fertilization (dpf), resulting in high oestradiol level. High temperature, exogenous androgens or aromatase inhibitors override genetic sex differentiation inducing testes development through the suppression of cyp19a1a gene expression and aromatase activity. Supplementation with 17ß-oestradiol (E2) of gonadectomized juveniles induced a sustained and higher E2 plasma level than in intact or gonadectomized controls and both sexes showed reduced growth. Juvenile and mature females treated with the aromatase inhibitor 1,4,6-androstatriene-3,17-dione had 19% lower E2 plasma level compared to controls and they showed a 32% increased growth after 28 days of treatment. Altogether, these data suggest that E2 inhibits female growth leading to the SGD. Regarding Igf-1, mRNA and peptide appeared in liver at ∼ 4 dpf and then in organs involved in growth and metabolism, indicating a role in early growth, metabolism and organogenesis. Gonad igf-1 showed an early expression and the peptide could be detected at ∼ 7 dpf in somatic cells. It appeared in germ cells at the onset of ovarian (29 dpf) and testicular (52 dpf) meiosis. In testis, Igf-1 together with steroids may regulate spermatogenesis whereas in ovary it participates in steroidogenesis regulation. Igf-1 and Igf-2 promote proliferation of follicular cells and oocyte maturation. Igf-3 expression is gonad specific and localized in the ovarian granulosa or testicular interstitial cells. In developing gonads igf-3 is up-regulated in males but down-regulated in females. In contrast, bream Gh injections increased igf-1 mRNA in male and female liver and ovaries but gonadal igf-3 was not affected. Thus, local Igf-1 and Igf-2 may play crucial roles in the formation, development and function of gonads while Igf-3 depending on the species is involved in male and female reproduction. Furthermore, precocious ethynylestradiol (EE) exposure induced lasting effects on growth, through pituitary gh inhibition, local suppression of igf-1 expression and in testis only down-regulation of igf-3 mRNA. In conclusion, SGD in tilapia may be driven through an inhibitory effect due to E2 synthesis in female and involving Igfs regulation.

Sexual dimorphism in two pure cichlid species, Oreochromis niloticus niloticus (Linnaeus 1758) and Sarotherodon melanotheron melanotheron Rüppel 1852, and their intergeneric hybrids

Growth performances and sexual growth dimorphism were compared in two pure species of tilapia, Oreochromis niloticus niloticus (OO) and Sarotherodon melanotheron melanotheron (SS), and their reciprocal intergeneric hybrids (male O. n. niloticus × female S. m. melanotheron [OS] and male S. m. melanotheron × female O. n. niloticus [SO]). Fish obtained from artificial reproduction were reared on artificial diets over 10 weeks at 25 ± 2 °C, under light regimes of 12 h light:12 h darkness. Growth was measured on a weekly basis. Social interactions were recorded with a video camera. Pure O. n. niloticus achieved the fastest growth rates (Mean Specific Growth Rate (SGR) of 2.7 ± 0.6% d–1 for males and 2.3 ± 0.4% d–1 for female) and S. m. melanotheron the slowest (1.3 ± 0.3% d–1 for males and 1.4 ± 0.3% d–1 for females). The SGR of the intergeneric hybrids fell between that of the two pure strains. OS females grew faster (1.7 ± 0.4% d–1) than SO females (1.3 ± 0.2% d–1), whereas no difference was observed between males. Aggressive behaviour emerged first among faster-growing fish (O. niloticus and SO). The role of parental components in behavioural and physiological traits of tilapia is discussed.

Temperature Preference and Sex Differentiation in African Catfish, Clarias gariepinus

The African catfish Clarias gariepinus has a genetic sex determination system in which high temperature induces masculinization. The thermosensitive period for sex differentiation is short and occurs very early (from 6 to 8 days posthatching [dph]). As young juveniles can encounter high masculinizing temperature (36.5°C) in African water points, we aimed to determine the thermal preference of sexually undifferentiated juveniles and investigate if they spontaneously move toward high masculinizing temperature. Experiments were carried out in an environmental continuum (28-28-28°C and 28-32-36.5°C) made up of three 50-L aquariums connected together. Four hundred larvae from 10 different full-sib progenies were reared successively from 2 to 14 dph in these facilities. Before and after thermal treatments, fish were reared at 28°C until sex ratio determination at 70 dph. In the control continuum, fish were nearly equally distributed in the three compartments. Conversely, in the thermal continuum, compartment occupation significantly differed with progeny and period. During the highly thermosensitive period, two of five progenies significantly preferred (54.7% and 39.8% occupation) the 36.5°C compartment. All tested progenies reared in thermal continuum and separated 36.5°C aquarium showed a skewed sex ratio toward the male phenotype (78-100%). Nevertheless, no correlation was found between 36.5°C compartment occupation and sex ratio in thermal continuum groups. As masculinization temperature could be encountered in African water points during the spawning season, we discussed the adaptive advantage for the African catfish to display a sex differentiation process controlled by a temperature effect.

Genetic characterization of Benin’s wild populations of Sarotherodon melanotheron melanotheron Rüppell, 1852

The Cichlid fish Sarotherodon melanotheron is typically found in West and Central African estuaries and lagoons. It represents a good candidate for promoting tilapia farming in brackish waters. Understanding the genetic diversity in its populations from the hydrographical basins of Southern Benin is primordial before designing selective breeding programs. For this purpose, 202 samples collected from four rivers of Southern Benin and were genotyped using 15 polymorphic microsatellite DNA markers. Each river was split up into three sampling sites. We found significant global linkage disequilibrium across the genome of natural populations of this tilapia species overall the loci. However, when the loci that display aberrant Wrights (FIS and FST) were removed from the data, a linkage disequilibrium was detected for the remaining 11 loci and became compatible with the null hypothesis. Null alleles explained at least 20.58% of FIS variation. We found a significant isolation by distance across subsamples. Effective population size averaged 210 individuals, with a range from 36 to 517 individuals. Assuming that 79% of heterozygote deficits are explained by sib mating lead to a rough estimate of rsm = 0.4 of mating rate between full sibs within S. melanotheron subpopulations. The fish size correlated positively and significantly with the observed FIS (r = 0.58; p value = 0.04806). Reproduction system (endogamy) in S. melanotheron could explain the strong heterozygote deficit observed. Our results provide technical guidance for efficient management of this tilapia species’ genetic resources for breeding programs in fresh and brackish waters.

Effects of Hydrogeographical Origin on Zootechnical Parameters of Wild Populations of Oreochromis niloticus (Linnaeus, 1758)

The effect of geographical origin on the zootechnical parameters of tilapias is very poorly documented, unlike production systems and farming methods. In this study, the growth and survival characteristics of Oreochromis niloticus individuals were analyzed in relation with their waterway of origin, sex and age at experimental fishing. Durung the rearing, the duplicate groups were constituted keeping the 2 sexes (male vs female) separately. Fingerlings were fed using a granulated commercial fish feed (35% of proteins; 3 mm in diameter). Apart from chlorophyll «a», 3 phytoplankton and 5 zooplankton species were also available in the rearing tank. The body weight, total length, standard length, average weight gain, nutritive quotient and survival rate were similar (p>0.05) between lake Toho and Couffo river but weaker at these two waterways than Oueme river (p<0.05). The ponderal and linear specific growth rates were significantly identical between lake Toho and Couffo river but more important (p<0.001) in these two waterways than Oueme river. The majority of the individuals indicated an allometric growth (b<3), which is less desirable in fish farming. In short, 2 populations were identified: the group constituted by Oueme river and the one including lake Toho and Couffo river. It appears important to lead actions (selective breeding, crossbreeding) to improve the zootechnical performances of these local populations in order to promote their aquaculture.