Tilapia Populations Research Articles

Calculus of a minimal recruitment rate in a cultivated tilapia fish population to improve farm sowing-harvest management practices

Three mathematical models are proposed to find a minimal recruitment rate which guarantees an increase in the fish population in optimal size as opposed to only using recruitment rates plus mortality. A model of difference equation and two matrix population models represented the population dynamics of fish in optimal size and weight. The models showed the cohorts existence into the tilapia populations during culture period, and showed their effect on the population for different values of the parameters considered. Two models were tested with data from tilapia farms. The sensitivity and elasticity analysis showed that the solutions of those models are more sensitive to changes in the recruitment rate (γ), followed by the proportion (β) of fish with a size close to the population mean in length and weight, while elasticity shows that the model solutions are more sensitive to proportional changes in survival (σ) and the recruitment rate (γ) for an all-male population. The above results gave the basis to build a general model for mixed populations, for the case when the masculinization ratio was lower than 95%. Different sceneries are simulated to show the sensitivity of the solutions to female proportion and to the fertility rate in the population. These results could be used as a predictive and decision-making support tool to improve the production practices in tilapia farms.

Phylogenetics and Molecular Divergence of Tilapia Fish (Oreochromis Species) Using Mitochondrial D-Loop and Cytochrome b Regions

Understanding the level of genetic diversity in any population is an important requisite towards strategizing measures for conservation and improvement of stocks. This study focused on the assessment of phylogenetics and molecular divergence of tilapia fish species obtained from two populations (Domita in South-South and Odeda in South-West, Nigeria) using the displacement loop (D-loop) and cytochrome b region of the mitochondrial deoxyribonucleic acid (mtDNA). A total of 28 samples (15 from South-South and 13 from South-West) were used for the genetic analysis. DNA was extracted from the tissue of all the samples using Quik-gDNATM miniPrep kit. The D-loop containing the hypervariable region was sequenced for all samples from the two populations, while cytochrome b (Cyt b) region of mtDNA was only sequenced for samples from South-South population. Chromatograms of the sequences were viewed and edited using Bioedit software. Multiple sequence alignment was carried out using molecular evolutionary genetic analysis (MEGA) software before subsequent genetic analyses. Phylogenetic analysis grouped the samples into two clusters based on population. Also, when the two mitochondrial regions were pooled together, they clustered into two major groups based on mitochondrial regions. Analysis of molecular variance (AMOVA) revealed 37.32% variation within population and 62.68% variation among population with a significant fixation index of 0.627 (p 0.05). The genetic distance inferred between D-loop regions of South-South and South-West populations was 0.243. Maternal lineage analysis revealed that the origin of tilapia fish from both populations could be traced to Oreochromis spirilus and Oreochromis leucostictus based on mitochondrial D-loop region. The findings of this study revealed molecular divergence among the tilapia populations and may serve as pivot information for the genetic improvement of this important species.

Comparative population genetic structure of redbelly tilapia (Coptodon zillii (Gervais, 1848)) from three different aquatic habitats in Egypt.

Recently, tilapia have become increasingly important in aquaculture and fisheries worldwide. They are one of the major protein sources in many African countries and are helping to combat malnutrition. Therefore, maintenance and conservation genetics of wild populations of tilapia are of great significance. In this study, we report the population genetic structure and genetic diversity of the redbelly tilapia (Coptodon zillii) in three different Egyptian aquatic environments: brackish (Lake Idku), marine (Al‐Max Bay), and freshwater (Lake Nasser). The habitat differences, environmental factors, and harvesting pressures are the main characteristics of the sampling sites. Three mitochondrial DNA markers (COI: cytochrome oxidase subunit I; the D‐loop; CYTB: cytochrome b) were used to assess population structure differences among the three populations. The population at Lake Nasser presented the highest genetic diversity (H d = 0.8116, H = 6), and the marine population of Al‐Max Bay the lowest (H d = 0.2391, H = 4) of the combined sequences. In addition, the phylogenetic haplotype network showed private haplotypes in each environmental habitat. Results presented here will be useful in aquaculture to introduce the appropriate broodstock for future aquaculture strategies of C. zillii. In addition, evidence of population structure may contribute to the management of tilapia fisheries in Egyptian waters.

Genetic parameters for resistance against Flavobacterium columnare in Nile tilapia Oreochromis niloticus (Linnaeus, 1758).

Columnaris disease is a major cause of mortality in tilapia hatcheries and commonly occurs during the summer season in Thailand. One way of reducing the problem is by selective breeding for increased disease resistance. The objective of this study was to estimate quantitative genetic parameters for resistance against columnaris in the Chitralada 4 strain of Nile tilapia. Data from 43 full-sib families (2,580 records) of fry (age=32±4days post-hatch) were used in the analyses. Initially, fry were subjected to bath challenge with Flavobacterium columnare (LD50 concentration=1.2×106 CFU/ml) for 14 days. Disease resistance was defined as the number of days from challenge until death (DD) or as a binary trait (dead/alive) on day 14. Linear animal and sire-dam models were used for DD, while threshold animal, threshold sire-dam, binary linear animal and binary linear sire-dam models were used for binary outcomes. Covariate effect of age, fixed effect of challenge day and random effects of the individual animals or sires and dams were included in the models. Mean survival was 32.4±11.6%, and survival rates of the best and poorest families were 70% and 8%, respectively. The highest estimate of heritability (0.30±0.025) was obtained under the threshold sire-dam model. Heritability estimates for DD (0.16±0.034 and 0.17±0.046) were comparable to those obtained from the threshold animal (0.15±0.031) and the binary linear (0.14±0.045 and 0.15±0.044) models. The linear animal and sire-dam models for DD and the threshold sire-dam models performed equally with similar values of rEBV (0.629, 0.628 and 0.627) and accuracy of selection (0.793, 0.793 and 0.791). This study reveals the potential of selective breeding to increase disease resistance to F.columnare in the studied population of Nile tilapia.

Genetic analysis of a red tilapia (Oreochromis spp.) population undergoing three generations of selection for increased body weight at harvest

Quantitative genetic analysis was performed on 10,919 data records collected over three generations from the selection programme for increased body weight at harvest in red tilapia (Oreochromis spp.). They were offspring of 224 sires and 226 dams (50 sires and 60 dams per generation, on average). Linear mixed models were used to analyse body traits (weight, length, width and depth), whereas threshold generalised models assuming probit distribution were employed to examine genetic inheritance of survival rate, sexual maturity and body colour. The estimates of heritability for traits studied (body weight, standard length, body width, body depth, body colour, early sexual maturation and survival) across statistical models were moderate to high (0.13-0.45). Genetic correlations among body traits and survival were high and positive (0.68-0.96). Body length and width exhibited negative genetic correlations with body colour (-0.47 to -0.25). Sexual maturity was genetically correlated positively with measurements of body traits (weight and length). Direct and correlated genetic responses to selection were measured as estimated breeding values in each generation and expressed in genetic standard deviation units (σG). The cumulative improvement achieved for harvest body weight was 1.72 σG after three generations or 12.5% per generation when the gain was expressed as a percentage of the base population. Selection for improved body weight also resulted in correlated increase in other body traits (length, width and depth) and survival rate (ranging from 0.25 to 0.81 genetic standard deviation units). Avoidance of black spot parent matings also improved the overall red colour of the selected population. It is concluded that the selective breeding programme for red tilapia has succeeded in achieving significant genetic improvement for a range of commercially important traits in this species, and the large genetic variation in body colour and survival also shows that there are prospects for future improvement of these traits in this population of red tilapia.

Genetic variation and correlated changes in reproductive performance of a red tilapia line selected for improved growth over three generations

The present study examines genetic variation and correlated changes in reproductive performance traits in a red tilapia (Oreochromis spp.) population selected over three generations for improved growth. A total of 328 breeding females (offspring of 111 sires and 118 dams) had measurements of body weight prior to spawning (WBS), number of fry at hatching (NFH), total fry weight (TFW) and number of dead fry (NDF) or mortality of fry including unhatched eggs at hatching (MFH). Restricted maximum likelihood (REML) analysis in a multi-trait model showed that there are heritable genetic components for all traits studied. The heritability for WBS was very high (0.80). The estimates for traits related to fecundity (NFH, TFW) and survival (NDF) were low and they were associated with high standard errors. Genetic correlations of WBS with other reproductive performance traits (NFH, TFW and NDF) were generally positive. However, NFH was negatively correlated genetically with TFW. As expected, body measurements during growth stage exhibited strong positive genetic correlations with WBS. The genetic correlations between body traits and reproductive performance (NFH, TFW, NDF) were not significant. Correlated responses in reproductive traits were measured as changes in least squares means between generations or spawning years. Except for WBS that increased with the selection programs, the phenotypic changes in other reproductive traits observed were not statistically significant (P>0.05). It is concluded that the selection program for red tilapia has resulted in very little changes in reproductive performance of the animals after three generations. However, periodic monitoring of genetic changes in fecundity and fitness related traits such as NDF or MFH should be made in selective breeding programs for red tilapia.

Hormonal sex reversal in red tilapia (Oreochromis niloticus and Oreochromis mossambicus) and inheritance of body colour in O. mossambicus and red tilapia: implications for commercial farming

The present investigation was conducted with a view to produce a female-free population and to test the heredity of body colour in tilapia. In this study, 6-day-old red tilapia were fed diets incorporated with 0, 25, 50, 75 and 100 mg/kg diet of a synthetic androgen, 17α-methyltestosterone (17α-MT), for 30 days. Administration was followed by 120 days of rearing in nylon hapas where hormone-free diet was given. At the end of the 120-day rearing, 17α-MT at 50 ppm induced 100% sex reversal, whereas 25, 75 and 100 mg/kg resulted in incomplete sex reversal. Immersion of newly hatched red tilapia fry in an aqueous solution of 17α-MT at 200 μg/L for 1 h produced 73% males; however, when given to 2-day-old fry, it yielded 78% males. The results indicate that oral administration of 17α-MT is more effective than treatment by immersion to produce male dominant red tilapia populations. Treatment of red tilapia with the androgen did not indicate any definite trend in the gonado-somatic index of both males and females. The results of progeny testing indicate that red tilapia does not belong to either XX-XY or WZ-ZZ sex determination systems, the two most common sex determination systems that exist in fish. When mating, Oreochromis mossambica (black tilapia) and red hybrid tilapia produced F1 progeny with a brown colour, the intermediate colour between the two parental body colours. When F1 hybrids were interbred, the three phenotypes got separated in the F2 generation with a 1 black/2 brown/1 red phenotypic ratio as expected of a single autosomal gene with incomplete dominant gene action. The importance of monosex male red tilapia farming and body colour inheritance is also discussed.

Morphological variation of Nile tilapia populations from major water bodies of Uganda

With increasing demand for improved farmed Nile tilapia ( Oreochromis niloticus ) fish seed, there is heightened demand for quality and proven broodstock by hatchery operators throughout the country and in the East African region. Uganda, fortunately, is endowed with vast natural Nile tilapia ( Oreochromis niloticus ) stocks in lakes Albert, Edward and George, as well as in Albert Nile River. To aquaculturally benefit from the different attributes of different stocks/strains, it is important to establish the existing strains in a manner that is phenotypically discernible where such differences are apparent. In this study, we used a suite of morphological traits to identify strains of Nile tilapia among 425 individuals sampled from the four major water bodies, lakes – Albert, Edward-George, Kyoga and Victoria. Morphological variation was investigated using multivariate approaches including principal component analysis, discriminant analysis and cluster analysis of 22 morphometric characters. These analyses indicated that there was high morphological variation among the different populations of Nile tilapia and the fish could be grouped into their respective sampled areas based on these morphological differences. Most of the variation (86.97%) was associated with the fish body size, the peduncle length and the interorbital distances. Findings imply that there are four major Nile tilapia strains in Ugandan waters that will be subjected to aquaculture performance evaluation before being used in the genetic improvement programme to produce improved Nile tilapia fish broodstock/seed for use in fish farming. Differences among populations may reflect genetic differences and/or environmental factors. Keywords: Morphometrics, Oreochromis niloticus , strain identification

Copy Number Variations in Tilapia Genomes.

Discovering the nature and pattern of genome variation is fundamental in understanding phenotypic diversity among populations. Although several millions of single nucleotide polymorphisms (SNPs) have been discovered in tilapia, the genome-wide characterization of larger structural variants, such as copy number variation (CNV) regions has not been carried out yet. We conducted a genome-wide scan for CNVs in 47 individuals from three tilapia populations. Based on 254Gb of high-quality paired-end sequencing reads, we identified 4642 distinct high-confidence CNVs. These CNVs account for 1.9% (12.411Mb) of the used Nile tilapia reference genome. A total of 1100 predicted CNVs were found overlapping with exon regions of protein genes. Further association analysis based on linear model regression found 85 CNVs ranging between 300 and 27,000 base pairs significantly associated to population types (R 2>0.9 and P>0.001). Our study sheds first insights on genome-wide CNVs in tilapia. These CNVs among and within tilapia populations may have functional effects on phenotypes and specific adaptation to particular environments.

Growth Performance and Nutrient Utilization of Hormonal Sex-reversed Male and Mixed Sex <i>Oreochromis niloticus</i> under Outdoor Rearing Condition

This study investigates the performance of all male and mixed sex tilapia ( Oreochromis niloticus ) raised in outdoor rearing conditions. All male tilapia were obtained by hormonal administration of 17 α-methyltestosterone (30 µg) into the diet of hatchlings of O. niloticus and fed them for 28 days. Both sex-reversed male and mixed sex O. niloticus were fed with coppens ® commercial diet and replicated thrice at 45% crude protein for 24 weeks Biological evaluation such as Mean Weight Gain (MWG), Specific Growth Rate (SGR) and Feed Conversion Ratio (FCR) were determined. Temperature, pH, conductivity and total dissolved solids were measured using standard methods. Data were analyzed using descriptive statistics and ANOVA at P=0.05. Result revealed that all male tilapia had significant weight gain (p<0.05) (5.93 ± 0.13 g) compared to the mixed sexes (2.19 ± 0.09 g). However, breeding activity was not noticed within the mixed sex tilapia population after a culture period of six months. Hence, poor performance could not be directly linked to spawning activity. The value of temperature, pH, conductivity and total dissolved solids were closely related and were within recommended range for the culture of tropical fishes. It was therefore hypothesized that 17 α-methyltestosterone besides achieving masculation, may have growth promoting characteristic. The detail mechanism responsible for this better performance could be the focus of future research.

Molecular characterization of sdf1 and cxcr4 in the Mozambique tilapia, Oreochromis mossambicus

Animal sexual reproduction relies on primordial germ cells (PGCs), the predecessors of the germ cell lineage, giving rise to either spermatogonia or oogonia after the completion of gonadal differentiation. There is limited information on the mechanism of PGC migration leading to the formation of the primordial gonad in Perciform fish. Oreochromis mossambicus, a tilapiine species, was investigated that is a commercially important aquaculture species in many parts of the world while in other areas it has become an invasive pest. Key components involved in PGC migration were identified, including the stromal-cell derived factor 1 (Om-sdf1a, Om-sdf1b) and the CXC receptor 4 (Om-cxcr4): both share conservation with existing model species. The spatial gene expression profiles were determined through transcript and protein analysis and displayed distinct localisation within the region of the developing gonad in larvae and within the adult gonads of certain cell populations. A recombinant Om-sdf1a was produced in Escherichia coli that activates Om-cxcr4 using a BRET-based yeast in vitro assay system, suggesting that it is structurally similar to the native Om-sdf1a and is appropriate for further structural studies. This study has improved understanding of the molecular basis of tilapia reproduction through investigation of gonad development, which may be important in the progression towards reproductive suppression methods to control tilapia populations in the wild. In addition, this research will facilitate developments in germ cell transplantation, an innovative technique that harnesses germ cell migration and allows the uptake of foreign germ cells, which differentiate to produce sperm or ova.

Genetic improvement of farmed tilapias: Response to five generations of selection for increased body weight at harvest in Oreochromis niloticus and the further impact of the project

Selection for increased body weight at harvest in Nile tilapia (Oreochromis niloticus) was carried out for five generations from 1991 to 1996, as a part of the project Genetic Improvement of Farmed Tilapias (GIFT). The base population for selection was composed of a mixture of various three- and four-way cross individuals descending from four wild African strains and four farmed Asian strains. Methods for single pair mating, separate rearing of full-sib groups and individual tagging and pedigree recording were developed. The parents for a new generation were selected based on a selection index including their own age corrected-body weight at harvest as well as that of their full- and half-sibs. Across generations, a total of 512 males were mated in a nested mating design to 941 females to produce 81,429 tagged progenies. Of those, 56,633 progeny had their body weight recorded at harvest after grow-out testing in a diverse range of farming environments. Estimates of the within generation realized response to selection across farming environments were obtained as the difference between the least squares mean performance of offspring of selected parents and of offspring of parents with average values of the selection index. The average realized response to selection per generation was 13.6% (range 9.0 to 20.1%), resulting in an accumulated response over five generations of 88% relative to the base population. A genetic trend analysis based on BLUP breeding values estimated across generations after the termination of the project suggested an accumulated response of 67%. The genetic composition of the synthetic population also changed during selection. The proportion of ancestors from three of the wild African founder strains increased from 59.7 to 76.3%. The accumulated coefficient of inbreeding was 7.1%. After the termination of the GIFT project, selection was continued in the GIFT population. The population has also been used as a genetic source in a number of similar public and private selection programs. A status review of the presently recorded dissemination of the genetic material and methodology is presented. Statement of relevanceThe GIFT project showed that five generations of repeated selection based on testing of individually tagged and pedigreed individuals from a synthetic farmed population of Nile tilapia gradually increased the body weight at harvest. The total increase was 67–88%. Reports from several descending populations shows that the selection response has continued during >10–15 additional generations.

Effects of salinity on cold tolerance of Malaysian red tilapia

This study was conducted to investigate the effect of salinity on growth, survival, osmoregulation, metabolism, hematological and biochemical parameters and T-complex protein (TCP-1) mRNA expression levels in Malaysian red tilapia at declining tank water temperatures. Five salinity concentrations (0, 5, 10, 15, 20 ‰) were evaluated during the study period. Samples were collected on a range of biochemical parameters at 16.5 (week 2), 12 (week 4) and 9.5 °C (week 6). No significant growth (body weight) was showed in the study (P > 0.05). Survival was monitored in the groups of fish until the holding tank temperature reached 5.6 °C. The lowest temperature in the study population of Malaysian red tilapia tolerated was 9 °C, although the fish were able to survive in temperature of 5.6 °C if they had been reared at 10 ‰ water salinity. The salinity and change in water temperature had significant effects on hematological and biochemical parameters and the TCP-1 gene expressions in Malaysian red tilapia (P < 0.05). The effect of temperature and salinity on superoxide dismutase (SOD), malondialdehyde (MDA) and hemoglobin (HGB) levels was significant (P < 0.05 or P < 0.01); with decreasing temperature, SOD and MDA activities increased, whereas HGB concentrations decreased. With increasing level of salinity, the SOD, MDA and blood glucose levels significantly increased (P < 0.05). The highest cholesterol (TC), triglycerides (TG), red blood cells (RBC) and HGB values were apparent in the moderate salinity groups, whereas the highest high-density lipoprotein cholesterol (HDL-C) was in the moderately high-salinity group. Rearing at low temperature, Na+–K+-ATPase (NKA) increased with increasing level of salinity at 16.5 °C, although it initially declined before rising again at water temperatures 12 and 9.5 °C. Rearing at low temperature, fish in the low-salinity groups significantly up-regulated the expression of the TCP-1 genes (P < 0.05), whereas fish in mild to moderate salinity groups significantly down-regulated the expression (P < 0.05). Taken together, these results suggest that exposure to environmental conditions of low temperature, low salinity and high salinity influence osmoregulation, metabolic parameters and the fish physiology. However, a change in the isotonic point at cold temperatures was observed by rearing in a salinity of under 10 ‰. This could potentially provide a means to increase cold tolerance of overwintering Malaysian red tilapia.

A theoretical model with time-delay, competition and transitions among cohorts to study tilapia populations

A tilapia culture system can have many cohorts mixed over a growing time and these coexist with some competitive pressure that affect the populational structure from the beginning to harvesting time. This paper presents the first model that integrates competition, transition rates among cohorts, and time-delay, which aims to know the biomass proportions corresponding to each cohort at the end of a period time in regards to the initial populational structure. Through several simulations with different value intervals, the sensitivity of model to its parameters was analyzed. It was observed that the transition rates among cohorts could define the final populational structure. The highest values provoked a fall of biomass proportions of all cohorts about zero, while that when the medium values were used the solutions could be closer to natural phenomenon. When the values of competition rates between cohorts were tested, there was an inverse relationship with the maximum point reached by the small and medium fish cohorts because the large individuals have a greater impact on the small class. Such solutions presented oscillation periods that increased with the highest values assigned to this variable. As the time-delay values were varied, in high competition conditions, the solutions approximated a periodic orbit that was far from the equilibrium. It was due to the negative effect of the large fish cohort on the other cohorts was possible in short and more frequent time periods or long and infrequent time periods. In this model the positive effect of competition towards the large fish cohort depended on the time-delay. Based on this one might suggest that the time-delay could also be incorporated into competition and growth rates of small and medium fish cohorts. Despite that the model still has the disadvantage of incorporating constant transition rates among cohorts, it was possible observe its sensitivity to the changes of these variables. The model provides final proportional biomass once the solutions have been stabilized. This permits us to predict the changes in the biomass of a population at time of harvesting.

Morphological variations of wild populations of Nile tilapia (Oreochromis niloticus) living in extreme environmental conditions in the Kenyan Rift-Valley

In this study, we use geometric morphometric method to establish morphological differences between natural populations of Nile tilapia from two extreme environmental conditions (high temperature and salinity) in Kenya, and compare them to two populations from regions experiencing less extreme conditions. To determine genetic influence on morphology, we correlated genetic data with morphological data. The study observed significant morphological differences between all studied populations, including three closely related hot spring populations (Bogoria, Chelaba and Turtle Springs), and two populations with similar genetic background inhabiting saline environments in Lake Turkana basin (Turkana and Crocodile Lake populations). In addition, allometric growth patterns of the seven populations differed significantly, demonstrating that developmental changes (plastic or adaptive) operated between these populations. Positive correlation between morphometric and genetic data confirmed the influence of genetic factors on morphology. All observed differences were attributed either to genetic and/or environmental factors, which seemed to play a major role in influencing morphology of wild Nile tilapia populations. We recommend further studies to be carried out under controlled conditions to confirm the role of temperature, pH and salinity in morphological diversification of Nile tilapia.

Reproduction Control Of Male Oreochromis Niloticus (Nile Tilapia) Using Gossypium Herbaceum (Cotton) Root Bark Meals As Fertility Inhibitor

The study was carried out to control Tilapia population using Gossypium herbaceum root bark meal (GHRBM) as reproduction inhibitor. Gossypium herbaceum (Cotton) root bark meal was fed to male O. niloticus (31.38±0.67g). The root bark was added to a basal diet (35% crude protein) at 0, 5, 10, 15 and 20 g/kg diet. A total of 75 fishes were randomly distributed in triplicate into 15 plastic tanks (1x1x1m and 1.45 inches thick) at a stocking density of 5 fishes per 1m3. Groundwater of 400 litres volume was maintained throughout the period of experiment. The pH value was between 6.90 and 7.67, temperature ranged between 25.4 and 26.60C and dissolved oxygen concentration between 2.8 and 3.9 ml/l. The fish was fed at 4% body weight/day in two instalments at 09:00-09:30 hour and 17:00-17:30 hour. In fish fed 0g of GHRBM/kg diet, histological examination showed spermatocytes released into the lumen of the seminiferous tubules while in fish fed 5 g, 10 g, 15 g and 20 g GHRBM/kg diets, the sections showed eroded connective tissue, hydropic degeneration, disintegration in seminiferous lobule, damaged connective tissue and increased in interstitial cells. Except in the control treatment, it was observed that O. niloticus spermatozoa maintained normal morphology but low sperm count and low volume. Milt volume and milt count (P&lt;0.05) decreased with increase in concentration of treatments used (from 0.5 to 0.1ml and 3.59 x109 to 0.33 x109 (Sperm/ml) respectively. Weight gained was highest in fish fed 10g GHRB/kg diet. It was observed that the feeding of Gossypium herbaceum root bark meals had no effect on water quality. Histological observations and milt analysis revealed that Gossypium herbaceum root bark may be effective as reproduction inhibitor in O. niloticus.

Magadi tilapia ecological specialization: filling the early gap in the speciation continuum.

Cichlid fish are well known for their high speciation rates, which are usually accompanied by spectacular and rapid diversification in eco-morphological and secondary sexual traits. This is best illustrated by the famous repeated explosive radiations in the African Great Lakes Tanganyika, Malawi and Victoria, each lake harbouring several hundreds of mostly endemic species. Correspondingly, cichlids diversified very rapidly in many other lakes across their range. Although the larger radiations, unparalleled in vertebrates, are certainly the most intriguing, they are also the most intricate and difficult to address because of their complex nature. This is where smaller, simpler systems may prove to be the most useful. In this issue of Molecular Ecology, Kavembe et al. (2016) report very recent genetic diversification accompanied by ecological specialization in cichlids of the small and ecologically extreme Lake Magadi, in Kenya. Combining geometric morphometrics, stable isotope analysis, population genomics using RADSeq data and coalescent-based modelling techniques, the authors characterize the eco-morphological differences between genetically distinct populations of Magadi tilapia (Alcolapia grahami), which are consistent with the different environmental conditions they experience, and infer their history of divergence. The simplicity of the focal system and the use of a multidisciplinary approach make this work particularly important for our understanding of the early stages of speciation, in both cichlids and other organisms.

Identification of wild tilapia species in the main rivers of south China using mitochondrial control region sequence and morphology

Tilapias are now the most common alien fish in the rivers of South China and the dominant species in many sites. However, wild tilapia species have not been well-characterized or studied systematically. In this study, mitochondrial DNA control regions (mtDNA CR) and morphological characteristics were used to identify invasive tilapia species. Using mtDNA CR sequence, we identified Oreochromis niloticus, Tilapia zillii, Oreochromis aureus, Sarotherodon galilaeus, and hybrids of Oreochromis mossambicus and O. niloticus in the rivers of South China, and characterized their morphological features. Of the 5 species, O. niloticus and T. zillii were the most abundant in the rivers of South China. Furthermore, the distribution of O. niloticus may be related to their large numbers in the aquaculture industry, and the explosion of T. zillii invasion may be related to their strong capacity for environmental adaptation. These findings are beneficial for identifying wild tilapia populations and for the specific control of invasive tilapia species.

Correlated changes in body shape after five generations of selection to improve growth rate in a breeding program for Nile tilapia Oreochromis niloticus in Brazil.

Body shape is a commercial trait of great interest as it impacts profit and productivity of aquaculture enterprises. In the present study, we examined correlated changes in two measures of body shape (depth to length ratio, DL-R and ellipticity of mid sagittal plane, EL-H) from a selection program for high daily weight gain in a Nile tilapia population reared in freshwater cages in Brazil. Genetic parameters for body shape and its genetic association with growth traits (body weight and daily gain) were also estimated from 8,725 individuals with growth performance recorded over five generations from 2008 to 2013. Mixed model analysis showed that the selection program resulted in substantial improvement in growth performance (about 4% genetic gain per generation or per year) and also brought about trivial changes in body shape. The heritabilities ranged from 0.470 to 0.564 for growth traits and 0.180 to 0.289 for body shape. The common family effects were low for all traits studied, accounting for only 3-11% of total phenotypic variance. The genetic correlations between body shape and growth traits were weak, i.e., -0.385 between EL-H and growth traits and 0.28 between DL-R and body weight or daily gain. Strong and negative genetic association was found between the two body shape traits (rg = --0.955). Harvest body weight and daily gain are essentially the same traits, as indicated by the close to one genetic correlations between the two characters. Our results demonstrated that the selection process to increase growth rate had small, but slowly constant effect in body shape traits; and in the long term, the fish would have become rotund.

Evaluation of the growth and carcass quality of diallel crosses of four strains of Nile tilapia (Oerochromis niloticus)

This study examined four commercial lines of Nile tilapia (Oerochromis niloticus) ultimately to implement a breeding program to produce clean fillets from an all-male fish population that is grown to 1kg. Contemporary progenies were produced to perform a complete 4×4 diallel cross. Two experimental populations were studied: (1) a set of hormonal sex-reversed (SR) progeny populations that were processed at 1kg to obtain carcass quality data and (2) a set of populations that did not receive hormone treatment (no hormone, NH) that were used to assess growth between 100g to 400g. The growth traits of 3997 NH fish from 124 families were recorded at 400g, and the processing plant traits were obtained from 2721 SR fish from 99 families. In general, all sources of variation were highly significant for most traits with the effect of linebreeding explaining the largest (from 35 to 50%) fraction of the explained variance (FEV) for all traits, except for the dressing percentage, for which the effect of the general combining ability (gca) was the most important source of variation (51%). The gca exerted the second most important effect on the body weight, fillet weight, and abdominal fat percentage, and reciprocal effects constituted the second most important influence on the head, dressing and fillet percentages, with an FEV of 20% to 35% being observed. The maternal effect was relatively important only for the specific growth rate (SGR), explaining 26% of the total variance. The effect of heterosis was highly significant for all traits but corresponded to only 5% or less of the explained variance for almost all traits, except for the fillet percentage and for days to 1000g with FEVs of 16% and 9%, respectively. In the NH population, the heterosis for all traits indicates a favorable effect of crossbreeding, particularly the heterosis for body weight (6.7%) and average daily gain (6.8%). In the SR population, the levels of heterosis demonstrated the favorable moderate effects of crossbreeding on the body wt. at 400g (4.0%), days to 1kg (−6.3%), average abdominal fat % (−7.4%), and fillet yield (1.6%). These results suggest that crossbreeding does not further improve the exploitation of genetic variation for single traits in this particular farmed tilapia population; however, the observed composite trait fillet productivity, i.e., the total biomass of fillet produced per number of farmed fish,yielded promising levels of heterosis with levels that reached 38% and 43% in specific crosses. Furthermore, four of six cases exhibited a cross that was better than the best pure line, demonstrating that profit heterosis is a relevant parameter to consider for assessing breeding strategies. Statement of relevanceTotal biomass of fillet produced, yielded promising levels of heterosis.