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Flavobacteriosis is a widespread challenge for the aquaculture sector, with infections reported in salmonids farmed in Scotland, Chile, Norway, and Canada, causing major welfare, environmental and economic concerns. A better understanding of the interactions between the host immune system and the pathogens are required for the development of new methods to control or prevent disease outbreaks. The current treatment and prevention programmes are limited to antimicrobial therapy and vaccines (ALPHA JECT® IPNV-Flavo) delivered by injection and therefore not suitable for fry, which is when the fish are most vulnerable to this pathogen. As the sector continues to move away from antibiotic treatments, a genetic breakthrough could be the key to preventing Flavobacterium psychrophilum infections in Atlantic salmon and rainbow trout. The selection of families of fish with resistance or decreased susceptibility to this pathogen offers another possibility for addressing the problems posed by F. psychrophilum. Therefore, the aim of this thesis was to study F. psychrophilum infection in rainbow trout, Oncorhynchus mykiss (Walbaum, 1792), and Atlantic salmon (Salmo salar L.) resistant and susceptible families, and to uncover the genetic underpinnings of resistance to F. psychrophilum. To achieve this, a successful immersion challenge model for F. psychrophilum in Atlantic salmon and rainbow trout, which mimics the natural infection route, was developed. Rainbow trout and Atlantic salmon were challenged using this newly developed challenge model, and a genome wide association study (GWAS) was conducted in both species, which identified genetic markers for disease resistance. In rainbow trout resistance to F. psychrophilum is a polygenic trait, while in Atlantic salmon the trait is oligogenic, which suggests that the trait is influenced by few genes with large effects (Chapter 2). A SYBR Green quantitative polymerase chain reaction (qPCR) was developed, validated, and used to quantify F. psychrophilum load in Atlantic salmon and rainbow trout head kidney tissues at different time points post challenge. However, there was not a clear relationship between fish genotype and bacterial load in head kidney tissues (Chapter 3). Since F. psychrophilum infection was not solely localised to the head kidney, and low bacterial load was determined in this tissue, the mucosal immune response of Atlantic salmon and rainbow trout was investigated by immunohistochemistry (IHC) by detecting and quantifying F. psychrophilum in different tissues (gills, head kidney, and gut), and the IHC supported the qPCR results of low bacterial load in the head kidney. Similarly, immunoglobulin T (IgT) positive cells were detected in all mucosal tissues, with the highest detection in the secondary lamellae of the gills, suggesting the importance of this immunoglobulin (Ig) in teleosts (Chapter 4). Finally, for the first time the transcriptomic response to F. psychrophilum infection in Atlantic salmon head kidney was investigated to unravel genes and pathways that take part in disease resistance. Genes and pathways up regulated in infected Atlantic salmon head kidney included those related to the immune response, cell chemotaxis and iron homeostasis; pattern recognition receptors (PRRs), complement molecules, inflammatory cytokines, chemokines, and iron-related genes, which indicated these genes participate in the response of Atlantic salmon to F. psychrophilum challenge (Chapter 5). This present study is the first to describe a F. psychrophilum immersion challenge model that can be used to study host-pathogen interactions, and molecular and immunological techniques that allow for the rapid detection of this bacterium in infected fish tissues. Moreover, the transcriptomic study provided an insight into the immune system and the defence mechanisms of Atlantic salmon against F. psychrophilum, which is an important step forward towards preventing infections by this bacterium in farmed Atlantic salmon.

Seawater-transferred rainbow trout (Oncorhynchus mykiss) has become an attractive aquaculture product in recent years. Industrial interest is mainly due to its resistance to infectious pancreatic necrosis and its adaptability to brackish water, which allows farming in otherwise unexploited locations. However, most practices for the aquaculture of this species have been imported from Atlantic salmon (Salmo salar) without evidence data supporting their suitability for the species. These include protocols to induce their preadaptation to seawater (smoltification). As a result, following seawater transfer, large numbers of fish die (around 10%) or become growth-stunted (GS; 10-60%). Therefore, species-specific rearing protocols and seawater-readiness biomarkers are needed. In the present PhD thesis, the effects of different photoperiod and temperature protocols for rearing in freshwater were assessed on the development of smoltification traits and subsequent seawater performance. This was achieved by using an array of molecular tools to measure osmoregulation-, growth- and haematopoiesis-related genes, proteins and hormones. Moreover, the discovery of potential seawater-readiness biomarkers and the study of smoltification, seawater adaptation and GS fish development were performed using several mass spectrometry proteomic and lipidomic approaches. Results suggest that winter light signals are inadvisable for the species, while all tested summer signals produced similar good results. Moreover, increased temperature protocols not only failed to improve smoltification and growth but potentially compromised the immune system of the fish. Overall, continuous light seems an advisable light regime, irrespective of temperature. Moreover, new promising potential biomarkers for seawater-readiness were identified using proteomics, while also suggesting a previously unknown role of these proteins in smoltification and seawater adaptation. Finally, GS development was shown to be related to low insulin-like growth factor 1 levels following seawater transfer. Moreover, other related factors to the phenotype were higher stress levels, possibly caused by bullying by bigger fish, and hepatic anomalies related to oxidative stress.

The African catfish, Clarias gariepinus, is the most important fish species for aquaculture in sub-Saharan Africa. Despite its long-standing history in aquaculture (since the 1950's) and current rapid expansion, little work has been done on its genetics and the genetic management/improvement of different populations globally. The industry, currently worth over USD 720 million in Nigeria, and with so much more growth potential, is faced with numerous challenges. To understand the extent of these challenges and possible areas/types of intervention, the Nigerian catfish aquaculture industry was reviewed. Inadequate supply of good quality fingerlings/broodstock and feeds were notably the most significant challenges. As a step towards addressing the former, a survey of the current practise in catfish hatcheries was conducted, to identify problems and prospects therein. Over 90% of the hatcheries surveyed use shooters (fast growers) as broodstock, use only farmed broodstock and have no broodstock management/replacement programmes. Findings from these studies informed research on the development of genetic improvement for C. gariepinus. Just as in salmon, tilapia, carp, etc., the use of molecular markers as tools for genetic management and improvement of C. gariepinus was explored. Problem-solving markers, separating C. gariepinus from its closest relative, C. anguillaris, were developed. A total of 24 diagnostic SNP markers were identified from double-digest restriction-site associated DNA sequencing (ddRADseq). Following validation using KASP assay, 8 of the 24 SNPs were tested on a total of 291 Clarias catfishes and 7 Heterobranchus longifilis (out groups). The Clarias samples were separated into 259 putative C. gariepinus and 32 putative C. anguillaris. These are the first diagnostic markers for separating these species, for which morphological features perform poorly (effectively cryptic species). A set of eight new microsatellite markers was developed from the ddRADseq data and microsatellite enrichment. These microsatellite markers, together with four others sourced from the literature were optimised, multiplexed and used to genotype populations of C. gariepinus being evaluated for suitability for aquaculture. Although incomplete (due to problems with parental DNA quality), preliminary assessment of the assignment power by simulation shows that over 90% of the offspring could be assigned to a pair of parents. The high parentage assignment power and polymorphic information content (>0.5), suggest the usability and reliability of these markers in genetic management and improvement in the Clarias catfish industry, enabling parental assignment and kinship studies, and for evaluation of practices such as the use of "shooters" as broodstock in the industry.

This thesis has successfully advanced understanding regarding potentially sub-optimal early life stage triploid culture. Further investigation of egg incubation temperature requirements elucidated that a constant temperature of 6 °C from fertilisation to first feeding would improve survival and performance of triploids compared to the diploid standard of 8 °C or any temperature alterations throughout embryogenesis. Triploids were more sensitive to post-ovulatory ageing as shown with significantly higher mortality and larger size variation compared to their diploid siblings. Quality indicators suggested that ageing resulted in increased lipid peroxidation and decreased antioxidant activity in unfertilised eggs, suggesting fertilising eggs within five days post-ovulation is beneficial for triploid culture. Nutritional programming was successfully demonstrated in Salmo salar of both ploidy. A nutritional intervention at the point of first feeding resulted in better acceptance and utilisation of vegetable-rich diets in later life. Ploidy generally responded the same, however triploids showed a higher variation in nutrient retention and LC-PUFA biosynthesis, highlighting the need to understand further the difference between nutritional requirements. Fundamentally, the impact of the second set of maternal chromosomes present in triploids is not understood. A panel of expressed microsatellite loci was identified and exploited to assess allele expression and results confirmed codominant expression of these genes in diploid fish. There was clear evidence that all three alleles present in triploids were expressed at both the offspring whole animal level and in blood samples, however poor amplification of RNA from single cells prevented further understanding. Collectively, these studies highlight differences in early life culture requirements of diploid and triploid Salmo salar.

The Atlantic salmon (Salmo salar) is an iconic species that dominates the global finfish production sector with increasing market demand. The Scottish industry and government alone aspires for expansion of the sector to 210,000 t by 2020 with 154, 000 t produced in 2013. As such, there are pressures to improve sustainable development in particular to minimise the genetic impact of escapees on wild populations and reduce sea lice infection which are required for the granting of “green licenses” in Norway. The use of triploidy has been tested in the 1980’s with little success owing to suboptimal rearing conditions leading to elevated mortalities, poorer growth and a higher prevalence of deformities, in particular of the skeleton. Collectively: recent success of triploid trout farming, expansion to the salmon production sector and potential resulting pressure on wild stocks through escapee increases have reinstated interest to implement artificially induced triploid Atlantic salmon in commercial production. As diploid Atlantic salmon have undertaken extensive domestication to achieve the high quality production and welfare standards observed to date, triploid conspecifics too require husbandry optimisation to realise potential. In particular, industrialisation requires that higher observations of deformities and inconsistent growth trajectories during seawater ongrowing be resolved through optimisation of rearing regimes and subsequent standardization of husbandry protocols. Triploids possess additional genomic material and increased cell size yet reduced frequency that reflects known differences in physiology and supports that, in effect, triploids should be considered as a new species relative to diploid conspecifics. Therefore, this doctoral thesis aimed to study nutrition and temperature effects on triploid Atlantic salmon traits throughout the production cycle from ‘egg to plate’. Nutrition trials aimed to improve growth potential and mitigate skeletal deformities both in freshwater (FW) and saltwater (SW) whilst attempts were made to define a window of smoltification to ensure optimal ongrowing performance. Finally, impacts of embryonic temperature regimes that are known to impact long term performance and deformity development in triploids, were examined in relation to DNA regulation and yolk composition in an attempt to underpin potential mechanisms for the environmental impact of temperature on developmental phenotype. One of the main restrictions to triploid Atlantic salmon implementation is the increased prevalence and severity of skeletal deformities, particularly after the maring phase. The work performed in this thesis first demonstrated that protein and/or phosphorous (P) supplementation throughout SW ongrowing not only reduced the level of severely deformed (≥ 10 deformed vertebrae observable by x-radiography) individuals by 30 % but also sustained 6.8 % faster growth and improved harvest grade compared to triploids fed a standard grower diet (chapter 2). Comparison of x-radiography and severely deformed individuals between harvest and sea transfer highlighted that protein and P supplementation arrested deformity development whereas prevalence increased in triploids fed a standard grower diet. This implied that severe deformities were of FW origin and strongly suggest requirement for improved nutrition in FW to optimise SW performance. Therefore investigation of higher dietary P inclusion in FW was investigated and results showed significantly reduced number of deformed vertebrae and no severely deformed individuals in those fed 19.7 g total P Kg-1 compared with those fed 13.0 & 16.7 g total P Kg-1 (chapter 3). Most deformities were localised in the central (vertebrae 27 – 31) and caudal (vertebrae 52 – 57) regions for all treatments. However, triploids fed lower dietary P displayed a particular increase in prevalence within the tail region (vertebrae 32- 47) which is consistent with SW ongrowing reports and results from chapter 2, further highlighting FW origin of higher vertebral deformities reported in SW ongrowing in triploids. Higher P supplementation in FW also significantly improved growth in triploid parr compared to diploids and lower supplementation. However, this effect did not transpire in later FW smolt stages where weights were significantly higher in triploids fed lower compared to higher P supplementation. Expression of target genes involved in osteogenesis and bone P homeostasis in vertebrates were then analysed and a ploidy effect of osteogenic genes alp, igf1r and opn as well as a dietary effect on P homeostasis gene fgf23 was apparent in the parr stages but not smolt. In addition, stronger ploidy-diet effects were also observed in parr stages for whole body mineral concentrations. Collectively, growth, gene expression and whole body mineral content results indicate these earlier parr life stages may be more sensitive to P supplementation. This pronounced effect may be a consequence of seasonal accelerated growth associated with this period, where higher temperatures were also observed. The potential for shorter P supplementation windows in commercial production was addressed in chapter 4 with hope to cut economic cost to raw mineral inclusion in feed and also mitigate potential anthropogenic eutrophication on the environment that may be induced by P leached through uneaten feed and faeces. Triploids were fed higher dietary P (17.4 g total P Kg-1) until either early (5 g) or later (20 g) parr stages, or smolt (83 g) and monitored for performance throughout freshwater (FW) development. During later parr development (30 g), x-radiography assessment demonstrated that increased dietary P reduced the number of deformities and severely deformed individuals with no indication that feeding P for shorter windows improved skeletal integrity. Hence, P supplementation may be required throughout FW development for optimal skeletal performance. In addition, no differences in deformities were observed between triploid treatments at smolt. An effect of dietary P supplementation on whole body mineral concentration was observed in the early and later parr stages that was not as pronounced as smolt, which is consistent with results in chapter 3. Together, these results indicate that skeletal assessment during early developmental stages may not reflect smolt performance most likely as a consequence of seasonal effects of improved linear growth in the cooler winter temperatures prior to smolt where reversible deformities observed at parr may also be alleviated. In the same study (chapter 4), the inclusion of the probiotic Pediococcus acidilactici (Bactocell™) was also tested as a means to enhance gut assimilation as suggested in previous studies and therefore reduce the levels of P supplementation. Results clearly indicate superior skeletal performance in parr (30 g) as well as significantly less deformed vertebrae and no severely deformed individuals. However, at smolt (~83g), no effects of the dietary probiotic treatment were observed which may also be attributed to seasonal effects. Overall, nutritional research clearly indicate triploids require higher dietary P for optimal growth and skeletal development, which although is not consistent between life stages, is ultimately required throughout FW for optimal skeletal development at smolt. The use of probiotics offer a promising avenue for reduced P requirement in FW feed and further research should verify results and assess long-term performance. Timing of SW transfer according to correct parr-smolt transformation (PST) is essential for survival and growth performance in ongrowing where feeding and growth rate accelerate post-transfer. So far, SW transfer regimes and in particular the smoltification ‘window’ remains loosely defined in triploid Atlantic salmon and it is crucial that this be addressed to ensure optimal ongrowing survival and performance.

Marine teleosts, unlike their freshwater counterparts, have a repressed ability to synthesise long chain highly unsaturated fatty acids (HUPA). In competent species, the A6 and A5 fatty acid desaturases are critical in the biosynthetic pathway that produces the HUFA’s arachidonic acid (20:4/z-6; AA), eicosapentaenoic acid (20:5n3; EPA) and docosahexaenoic acid (22:6/z-3; DHA) from the Ci8 polyunsaturated fatty acids (PUFA), linoleic acid (18:2/2-6) and a-linolenic acid (18:3/2-3). The deficiency in HUFA biosynthesis in marine fish is of considerable practical significance because, in consequence, farmed marine species require a dietary source of presynthesised HUFA. This is provided by processed products from “industrial” species of marine fish such as sand eel, sardine, capelin and anchovies which themselves obtain HUFA through the food chain. Indicators suggest that the wild fishery supporting the aquaculture feed industry is unsustainable at current levels of exploitation. This has consequential effects on human health as fish, especially marine fish, are the predominant dietary source of HUFA that are crucial for maintaining cell membrane integrity as well as being central to eicosanoid metabolism. Therefore, the primary aims of this project were to further our understanding of the molecular differences in HUFA biosynthesis between marine and freshwater teleosts. This was achieved by comparing the fatty acid desaturase genes of representative marine and freshwater fish. The desaturases are enzymes involved in the biosynthesis of HUFA from PUFA and have been considered as one of the steps that may be compromised in marine fish. The desaturase genes were studied with a view to relating structural, and potential functional differences with different HUFA synthesis phenotypes. During the course of this project sequences of putative desaturase genes were cloned from two freshwater (zebrafish and carp), two marine (turbot and cod) and one anadromous fish species (Atlantic salmon). Once translated, the protein sequences of all the gene products contained all the necessary domains and motifs shown to be required for efficient desaturase function including an N-terminal cytochrome bs domain, and three catalytically important histidine boxes conserved in all members of the gene family. They all included the variant third histidine box that seems typical of A5 and A6 desaturase genes described to date. All of the protein sequences from the fish species had greatest homology to the mammalian desaturases, specifically the human A6 desaturase. The cDNAs of salmon, carp and zebrafish were functionally characterised in Saccharomyces cerevisiae. Three carp transcripts were sequenced and functionally characterised. Two had no A5 or A6 desaturase activity, while the third efficiently desaturated 18:3/2-3 at the A6 position. Of the two functionally characterised salmon transcripts one had no A5 or A6 activity whereas the third efficiently desaturated 20:4/2-3 at the A5 position. The transcripts that had no desaturase activity were considered either non-functioning alleles or pseudogenes acquired as a result of a genome doubling event. It is believed that other A5/6 like desaturases probably exist for both carp and salmon as salmon is known to have high levels of A6 desaturase activity. However, neither the cod nor the turbot cDNAs were functionally characterised in yeast. The most significant result of the functional characterisation study concerned the zebrafish (Danio rerio). The 1590 bp transcript has close similarity to mammalian A6 desaturase. However, the clone encodes a novel desaturase. When expressed in yeast the zebrafish gene confers the ability to convert 18:2/2-6 and 18:3/2-3 to their corresponding A6 desaturase products, 18:3/2-6 and 18:4/2-3. In addition, it confers the ability to convert 20:3/2-6 and 20:4/2-3 to their A5 desaturase products, 20:4/2-6 and 20:5/2-3, respectively. Therefore, the zebrafish gene encodes a bi-functional enzyme having both A6 and A5 desaturase activity. This was the first report of a functionally characterised desaturase of fish, and, in particular, of a fatty acid desaturase with both A6 and A5 activity. The structure of the primary sequences of the fish desaturases were analysed in relation to function and some interesting and potentially highly significant relationships were discovered. However, it was not possible to determine which residue or residues were responsible for the differing substrate specificities between the transcripts. In summary, the results presented in this thesis indicate that (i) all the fish species used in this study possessed desaturase-like sequences (ii) the zebrafish contains a novel, unique desaturase enzyme with both A6 and A5 desaturase activity (iii) marine fish possess A5/6 desaturase-like transcripts (iv) there is some evidence that fish species that have undergone tetraploidy or recent genome duplication appear to have duplicated genes, possibly pseudogenes and/or non-functioning alleles (v) significant differences in primary structure which may have important consequences for function were observed although unequivocal identification of residues responsible for determining function or specificity was not possible. In conclusion, this study has produced results that not only further our understanding of the fatty acid genes of fish but which also furthered our knowledge of the fatty acid desaturases in general. The data will facilitate studies of how fatty acid desaturase primary structures relate to function. Information from this and other studies will lead to complete knowledge of how sequence and structure contribute to confer substrate specificity and how the fatty acid desaturase gene family has evolved.

Four species of fish from the genus Oreochromis (Cichlidae) were studied cytogenetically through meiotic prophase I using TEM. The female synaptonemal complex complement length (SCCL) in these species was around 1.4 times longer than that of the males. The SCCL of neomales (hormonally masculinized genetic females) was reduced significantly compared to control females, while the SCCL of genetic males given the standard hormonal masculinization treatment was increased slightly compared to control males. Unpaired lateral elements were observed in the heterogametic sex of O. niloticus (XY: unsynapsed sub-terminal and terminal region of bivalent 1). In O. aureus, two distinct regions of restricted pairing were present only in the heterogametic sex. The first, a subterminal region of the largest bivalent, was located near to the region of unpairing found in the closely related O. niloticus, while the second was in a small bivalent, most of which was unpaired. These results suggest that O. aureus has two pairs of sex chromosomes. In O. mossambicus, unpaired lateral elements were not observed, nevertheless, a region of bivalent 1 with diffuse chromatin and differential staining was observed in one XY male, perhaps near to the kinetochore. A similar pattern was observed in female O. niloticus x male O. mossambicus hybrids. The recombination rates between the centromere and sex loci in O. mossambicus also suggested that sex genes in this species are located close to the centromere. An autosomal influence on sex determination was also observed. During meiosis in the female O. niloticus (2n = 44) x male O. karongae (2n = 38) hybrid (2n = 41), three trivalents were observed in some spreads. In other spreads failures in chromosome synapsis were observed as aberrant pairing and multivalents. The O. karongae karyotype appears to have arisen from the typical Oreochromis one by three fusions.

The giant tiger prawn {Penaeus monodon) was collected from 11 different sites throughout its range (Kenya -> Indo china-»Philippines). Individuals were analyzed for allozyme (46 loci) and mtDNA restriction fragment length polymorphisms (RFLPs) using 5 restriction enzymes. The average heterozygosity calculated from the allozyme data H = 0.027 (0.020-0.032) was similars to other studies on decapod crustacean. In total 10 polymorphic (p 0.99) loci were observed several of these {AAT-1 , ALAT*, GPI, IDHP , MDH-1 , MPI and PGM") contributing to significant differences in allele and genotype frequencies between populations. No significant differences were observed between samples collected at the same site in different years and no significant differences in HWE were observed within any single population. Genetic difference between all populations F^,=0.384 was high but was much smaller for SE Asian populations alone F, =0.014. The allozyme data suggested that the P. monodon populations structure could be divided into 4 main groups: Kenya, Philippines, Andaman Sea, and South China Sea. The mtDNA genome was estimated to be 15.76±.57 kb. Only four enzymes could be reliable used (BamHI, EcoKV, PvuII and SacI) which gave 12 mtDNA haplotypes (Kenya population not included). Significant differences between haplotype frequencies were observed between the Andaman Sea and South China Sea populations. Nucleotide divergence data suggested two main clonal forms of mtDNA were present in SE Asian populations. These clones appear to have diverged gene 0.1-0.7 million years ago. Clone A was more common in Andaman Sea population whereas Clone B was more common in South China Sea. Both techniques suggested that P. monodon can be subdivided into at least 3 isolated groups: Kenya, Andaman Sea, and South China Sea populations. This pattern of isolation we see today may be a relic of past separation during the last glaciation. Which has been modified by recolonization of new areas as the sea levels rose and present day patterns of gene flow caused by prevailing current patterns.

The results of a study aimed at the identification of treatment optima for triploidy induction in recently fertilised Oreochromis niloticus eggs by altering the intensity, duration and timing of application of pressure, heat and cold shocks are reported. Preliminary, but not directly comparable, trials suggested the following treatments to be close to the individual agent optima. Pressure; 8,000 p.s.i. 2 mins, duration applied 9 mins, after fertilisation (a.f.); heat: 41 *C, 3.5 mins, duration applied 5 mins. a.f.; cold: 9 *C. 30 mins, duration applied 7 mins. a-f. In a directly comparable trial in which the eggs of eight different females were separately exposed to the optimum shocks listed above, individual triploid yields were more variable following cold shocks and mean triploid yields were, therefore, higher following pressure and heat shock. The effects of triploidy on sexual development and maturation in female and male fish were examined in successive age groups (4 - 10 months). Significant differences in ovary weight and OSI were found between 3n and 2n females. But 3n males showed normal testis development and some of them could produce aneuploid sperm. The testis weight and OSI of such males were not significantly different compared to 2n. Comparative performance of growth, biochemical composition and endocrine profiles of normal diploids and pressure, heat and cold shocked triploids was investigated. 3n females were not significantly different in growth rate and proximate nutrition parameters compared to the 2n controls but were found to be functionally and endocrinologically sterile. 3n males showed no significant differences in growth or proximate nutrition parameters but were genetically sterile despite showing normal 2n endocrinological profiles and secondary sexual characteristics. Evidence is presented for successful suppression of first cleavage in O. niloticus and thereby the production of mitotic gynogenetics. The optimal parameters for UV irradiation of milt were 300 - 310 pW/cm > for 2 min at 4-C. The optimal pressure shock W.S 9000 p.s.i. for 2 mins at 28 *C at 40 - SO mins a.f. and that for heat was 41 *C for 3.5 mins at 27.5 - 30 mins. a.f. Isozyme analysis of putative mitotic gynogenetic survivors at ADA' locus confirmed homozygosity, therefore, restoration of diploidy occurred by inhibition of first mitosis. Subsequently gene-centromere recombination frequency estimated at six enzyme loci further revealed no recombination between the respective gene and centromere at all the loci. In contrast, recombination frequency in meiotic gynogenetic progeny was 0 - 100* (mean y = 0.41) which suggesting that this probably a reflection of the relative position of the various loci to centromere on their respective chromosomes. Production of heterozygous clones of O. niloticm was successfully carried out by crossbreeding between viable mitotic gynogenetic female and male sibs. At the same time, homozygous clones of the fish was produced by gynogenetic reproduction (retention of 2nd polar body) using optimal pressure/heat shock treatments. The outbred (OCL) and inbred (ICL) nature of two types of clonal lines were checked and identified at ADA' marker locus. A model for the large scale production of such clonal lines is presented. The effect of inbreeding on various phenotypic characters of two types of gynogenetics (meiotic and mitotic) in comparison to full sib controls was investigated. The coefficient of variation values of growth, meristic and all reproductive train were lowest in control, intermediate in meiotic and highest in mitotic gynogenetic group. This study revealed that the expansion of variation in such performance train in both type of gynogenetics was possibly the result of phenotypic expression of unmasked homozygous recessive and deleterious genes due to increasing levels of homozygosity (F value). Mendelian mode of ted body colour inheritance was studied in Egyptian red O. niloOcus and Thai ted tilapia attains. Cross-breeding between ted a ted and ted x wild parents resulted in mostly all red coloured progenies and in some red x wild crosses progenies were also segregated into red and wild types. The F. red hybrids mated infer se and back-crossed to wild type, the progeny phenotype segregated closely into approximating the expected 3 red: 1 wild and 1 red : 1 wild ratios respectively. These results demonstrate that red body colour in two mutant attains of tilapia is controlled by a single autosomal dominant "R" gene. But both the red strains contain differing proportion of heterozygotes (Rr). In order to produce pure breeding strains, it will be important to identify heterozygotes so that the "r" gene can be culled in some way. In this case. Mendelian test-cross technique has been proposed on the light of the present study as a means of probable solution. The possible implications of above results of genetic manipulation study in the aquaculture of Oreochromis spp. are discussed.