GH Gilthead Seabream

What is the GH/IGF axis in gilthead seabream?

The growth hormone/insulin-like growth factor (GH/IGF) axis in gilthead seabream (Sparus aurata) represents a complex endocrine system that regulates somatic growth through multiple tissues and signaling pathways. Growth hormone is produced and secreted by the pituitary gland and interacts with specific receptors (GHRs) in target tissues, primarily the liver. This interaction stimulates the production of insulin-like growth factors, particularly IGF-1, which mediates many of the growth-promoting effects of GH .

The system features various components including growth hormone receptors (GHR-1 and GHR-2), different IGF-1 splice variants (IGF-1a, IGF-1b, IGF-1c), IGF-2, IGF receptors (IGF-1ra and IGF-1rb), and IGF binding proteins (IGFBP-1a, IGFBP-2a, IGFBP-4) . These components are expressed in different tissues including liver, stomach, and white muscle, with tissue-specific regulation patterns that respond to nutritional and environmental factors.

The liver serves as the primary site for IGF-1 production in response to GH stimulation, but extra-hepatic tissues also contribute to local IGF-1 production, creating both endocrine and autocrine/paracrine signaling pathways that coordinate growth processes throughout the body .

How is the GH receptor characterized in gilthead seabream?

The growth hormone receptor (GHR) in gilthead seabream has been fully cloned and characterized at the molecular level. The full-length GHR codes for a mature 609 amino acid protein containing a hydrophobic transmembrane region and all characteristic motifs typical of GHRs . Sequence analysis reveals high conservation within related species, with 96% amino acid identity to black sea bream (Acanthopagrus schlegeli) GHR and 76% identity to turbot (Scophthalmus maximus) GHR .

The gilthead seabream GHR exhibits several unique characteristics:

• Unlike some other fish species, gilthead seabream appears to lack alternative splicing at the 3' end of the GHR gene, as evidenced by Northern blot and 3' RACE analyses .

• Cross-linking assays have identified only two protein bands corresponding to glycosylated and non-glycosylated forms of the full-length GHR .

• The receptor contains all functional domains necessary for signal transduction, including box 1 and box 2 motifs that are essential for JAK-STAT signaling pathway activation.

• Two receptor subtypes, GHR-1 and GHR-2, have been identified with different tissue expression patterns and potentially different functional roles in GH signaling .

The hepatic expression of GHR is regulated in conjunction with seasonal growth patterns, with concurrent changes in GHR and IGF-I expression occurring during the summer spurt of growth rates and corresponding to elevated circulating levels of GH and IGF-I .

What are the population characteristics of gilthead seabream relevant to GH research?

Gilthead seabream (Sparus aurata) is a perciform fish belonging to the Sparidae family that inhabits the Atlantic coasts of Europe, Mediterranean, and Black Sea. It represents one of the most important marine fish species in both fishery and aquaculture, particularly in the Mediterranean region, with European aquaculture production reaching 128,943 tonnes as of 2008 .

Several biological characteristics make this species significant for GH research:

• It exhibits euryhaline and eurythermal habits, allowing it to inhabit both marine and brackishwater environments, particularly during early life stages .

• It is a protandrous hermaphrodite with asynchronous ovarian development and mass spawning behavior, which introduces complexity to growth regulation studies across different life stages .

• It demonstrates habitat diversity, being found on rocky, seaweed, and sandy bottoms, with young fish remaining at low depths (<30m) while adults can reach depths of up to 150m .

• It has primarily carnivorous feeding habits (consuming shellfish including mussels and oysters) with accessory herbivorous tendencies, which influences nutritional factors affecting the GH/IGF axis .

These characteristics make gilthead seabream an excellent model for studying growth regulation in fish, as its adaptive capacity and commercial importance drive research into optimizing growth for aquaculture applications while improving our understanding of basic GH physiology in teleosts .

How does cysteamine (CSH) affect the GH/IGF axis in gilthead seabream?

Cysteamine (CSH) has demonstrated significant effects on the GH/IGF axis in gilthead seabream through multiple mechanisms. In an 18-week trial, CSH supplementation at both 1.65 g/kg and 3.3 g/kg of feed substantially improved growth performance (26.7% and 32.3% respectively) compared to control diets . The molecular and hormonal responses reveal complex interactions:

Plasma Hormone Levels:
CSH supplementation led to a dose-dependent reduction in circulating GH levels after 18 weeks, while simultaneously increasing plasma IGF-1 levels after both 9 and 18 weeks . This inverse relationship between GH and IGF-1 suggests enhanced negative feedback in the GH/IGF axis, potentially indicating improved GH sensitivity and IGF-1 production efficiency.

Tissue-Specific Gene Expression:
CSH modulates the GH/IGF axis differently across tissues:

• Liver: CSH at 3.3 g/kg increased total igf-1 expression, with specific upregulation of the igf-1a splice variant at both doses. Interestingly, igfbp-2a (a binding protein that can inhibit IGF action) was reduced in the CSH-1.65 group .

• Stomach: Higher dose CSH (3.3 g/kg) enhanced ghr-1 transcript levels, while lower dose (1.65 g/kg) reduced igf-1ra and igf-1rb receptor expression compared to control .

• White Muscle: CSH altered the expression of several GH/IGF axis genes in muscle tissue, affecting local IGF production and GH sensing .

Direct Cellular Effects:
In vitro experiments with cultured myocytes demonstrated that CSH (at a non-toxic dose of 200 μM) directly upregulates ghr-1, igf-1b, igf-2, and igf-1rb expression, suggesting that CSH can stimulate muscle growth through direct action on muscle cells, independent of systemic GH levels . This represents a novel mechanism by which CSH might enhance growth performance.

What dose-response relationships have been observed with CSH supplementation in gilthead seabream?

The dose-response relationship of CSH supplementation in gilthead seabream demonstrates both efficacy and potential limitations that must be considered in experimental design. Research has evaluated different inclusion levels with the following outcomes:

Hormonal Response:
A clear dose-dependent relationship was observed in plasma GH levels, which decreased progressively with increasing CSH dose after 18 weeks of supplementation. Conversely, plasma IGF-1 concentrations were elevated at both doses .

Gene Expression Patterns:
Certain genes showed dose-dependent responses:

• ghr-1 expression in liver showed opposite patterns between doses, decreasing at 1.65 g/kg but increasing at 3.3 g/kg

• igf-1b in stomach exhibited higher expression at 3.3 g/kg compared to 1.65 g/kg

• Total igf-1 in liver was significantly increased only at the higher dose

Cell Viability Considerations:
In vitro testing of CSH on cultured myocytes revealed that 200 μM was the maximum non-toxic dose, with higher concentrations (400 μM and 800 μM) reducing cell viability . This highlights the importance of careful dose calibration, as the therapeutic window appears narrow.

Researchers should note that CSH has been associated with detrimental effects including digestive ulcers, oxidative stress, reduced growth, and hormonal downregulation of the GH/IGF axis at inappropriate doses, making the inclusion range relatively narrow . This necessitates species-specific and life-stage-specific optimization of CSH dosage in experimental designs.

How do in vitro and in vivo studies on GH signaling compare in gilthead seabream?

Comparing in vitro and in vivo approaches to studying GH signaling in gilthead seabream reveals both complementary insights and important methodological considerations:

Similarities in Gene Expression Patterns:
Both approaches demonstrated similar expression patterns for certain genes. For example, CSH stimulation showed the same pattern of expression for ghr-1 and ghr-2 in cultured myocytes as observed in white muscle tissue in vivo . This concordance validates the in vitro model for studying certain aspects of GH receptor regulation.

Differences in Hormonal Context:
A fundamental difference between the approaches is the absence of circulating GH in vitro, whereas in vivo studies include the complex interplay of multiple hormones. In culture conditions, myocytes are only exposed to GH traces present in fetal bovine serum supplementing the culture media . This difference allows researchers to isolate direct effects of CSH on muscle cells that are independent of systemic GH regulation.

Novel Insights from In Vitro Studies:
The in vitro approach revealed that CSH directly stimulates igf-1b and igf-2 expression in myocytes, similar to the effects observed when these cells are exposed to amino acids, suggesting a nutrient-like mechanism of action . This supports the hypothesis that CSH may act partly through its role as a precursor to taurine, a semi-essential amino acid in carnivorous fish.

Complementary Value of Both Approaches:
In vivo studies provide the physiological context necessary to understand systemic effects and tissue interactions, including how liver-produced IGF-1 affects muscle growth. In vitro studies allow for precise control of experimental conditions and isolation of direct effects on specific cell types. Together, they provide a more complete understanding of GH signaling mechanisms .

Methodological Considerations:

• In vitro studies require careful determination of non-toxic doses (200 μM maximum for CSH in gilthead seabream myocytes)

• In vivo studies must account for age/size of fish, duration of supplementation, and potential adaptation responses

• Cell culture models are valuable for initial screening but must be validated with in vivo studies to confirm physiological relevance

What techniques are most effective for measuring GH and IGF-1 levels in gilthead seabream?

Accurate quantification of GH and IGF-1 levels in gilthead seabream requires specialized techniques adapted for fish physiology. Based on current research methods, the following approaches have proven effective:

Plasma Hormone Quantification:

• Blood Sampling Technique: Blood should be collected from the caudal vein using heparinized syringes, with samples immediately centrifuged (3,000 g, 15 min, 4°C) to obtain plasma that is stored at -80°C until analysis .

• Radioimmunoassay (RIA): Species-specific RIAs have been developed for gilthead seabream using recombinant GH and IGF-1. These assays typically have detection limits of 0.15 ng/ml for GH and 0.05 ng/ml for IGF-1 with inter- and intra-assay coefficients of variation below 10% .

• Enzyme-Linked Immunosorbent Assay (ELISA): Commercial ELISA kits with validation for fish species can be used, though cross-reactivity issues may arise, making species-specific validation essential.

Gene Expression Analysis:

• RNA Extraction: Total RNA should be extracted from tissue samples using appropriate isolation methods (e.g., TRIzol reagent), with RNA quality verified by spectrophotometry (A260/280 ratio) and agarose gel electrophoresis .

• Reverse Transcription: First-strand cDNA synthesis using oligo(dT) primers and reverse transcriptase enzymes, with standardized input RNA amounts (typically 1-2 μg) .

• Real-Time PCR (qPCR): Quantitative PCR using specific primers for GH, GHR subtypes, IGF-1 variants, and reference genes (e.g., β-actin, 18S rRNA, or EF1α for normalization). Data analysis typically uses the 2^-ΔΔCt method for relative quantification .

Protein Detection:

• Western Blotting: For GHR protein detection, using specific antibodies after tissue homogenization and protein separation by SDS-PAGE.

• Cross-Linking Assays: These have been successfully employed to identify GHR protein variants, revealing glycosylated and non-glycosylated forms of the full-length GHR in gilthead seabream .

Cell Culture Methods:
For in vitro studies, established protocols for primary culture of gilthead seabream myocytes include isolation through enzymatic digestion, culture in suitable media (e.g., DMEM with fetal bovine serum supplementation), and viability assessment using methylthiazolyldiphenyl-tetrazolium bromide (MTT) assays .

What are the key experimental design considerations for studying the effects of feed additives on the GH/IGF axis?

When designing experiments to evaluate feed additives like cysteamine (CSH) on the GH/IGF axis in gilthead seabream, researchers should address several critical factors:

Feed Formulation and Manufacturing:

• Base Diet Development: Formulate experimental diets based on practical commercial formulations to ensure relevance to aquaculture applications. For gilthead seabream, diets typically contain approximately 30% fish meal and 9% fish oil to fulfill essential nutritional requirements .

• Additive Incorporation: Additives should be thoroughly mixed during feed formulation before extrusion to ensure homogeneous distribution. For CSH, tested inclusion levels range from 1.65 g/kg to 3.3 g/kg .

• Control Considerations: Ensure that control and experimental diets are isolipidic and isonitrogenous to isolate the effects of the test additive .

Experimental Duration and Phasing:

• Multiple Growth Phases: Design trials to span different growth phases (e.g., Phase 1 for fingerlings, Phase 2 for juveniles) to assess stage-specific responses .

• Duration: Sufficient experimental duration is crucial; in CSH studies, significant growth effects were observed after 9 weeks, with enhanced effects after 18 weeks .

• Sampling Timeline: Include multiple sampling points to track temporal changes in the GH/IGF axis (early, mid, and endpoint sampling) .

Comprehensive Response Assessment:

Complementary In Vitro Studies:

• Primary Cell Cultures: Establish primary cultures of relevant cell types (e.g., hepatocytes, myocytes) from the same species to investigate direct cellular effects of additives .

• Dose-Response Assessment: Determine effective and non-toxic dose ranges in vitro before in vivo application (e.g., CSH viability testing on myocytes revealed 200 μM as the maximum non-toxic dose) .

• Molecular Endpoints: Analyze the same molecular endpoints in vitro and in vivo to establish relationships between direct cellular effects and whole-organism responses .

Statistical Considerations:

• Sample Size: Adequate replication at both the tank level (typically 3-4 tanks per treatment) and individual fish level (n=14-15 per treatment for molecular analyses) .

• Statistical Methods: Apply appropriate statistical tests (ANOVA followed by Tukey's post-hoc test for multiple comparisons) .

How can contradictory findings in GH studies be reconciled across different fish species?

The growth hormone signaling pathway shows considerable species-specific variations in fish, requiring researchers to address several key considerations when reconciling apparently contradictory findings:

Evolutionary and Phylogenetic Context:
The gilthead seabream GHR shares 96% amino acid identity with black sea bream, 76% with turbot, but only 52% with goldfish GHR . This phylogenetic distance explains why mechanisms that function in one species may not operate identically in others. Researchers should explicitly place their findings within an evolutionary framework, comparing results primarily between closely related species.

GHR Heterogeneity Differences:
A striking example of species variation is the absence of alternative splicing at the 3' end of GHR in gilthead seabream, while truncated or longer GHR variants exist in turbot and black sea bream respectively . When contradictory functional results emerge, researchers should first verify whether the same receptor variants are being studied across species.

Environmental and Physiological Adaptations:
Gilthead seabream is euryhaline and eurythermal, inhabiting both marine and brackishwater environments . These adaptations may necessitate unique GH signaling mechanisms compared to strictly freshwater or marine species. Studies should account for these ecological differences when comparing results across species.

Methodological Reconciliation Strategies:

• Comparative Studies: Design experiments that directly compare multiple species under identical experimental conditions.

• Meta-analyses: Conduct systematic reviews that account for methodological variations, species differences, and environmental conditions.

• Multi-omics Approaches: Combine transcriptomics, proteomics, and metabolomics to obtain a systems-level view of GH signaling across species.

• Evolutionary Rate Analysis: Examine the evolutionary rate of GH signaling components to identify rapidly evolving elements that might explain functional divergence.

Contextual Factors to Consider:

• Life stage differences in GH responsiveness

• Seasonal variations in GH/IGF expression patterns

• Nutritional status effects on GH signaling sensitivity

• Stress responses that may modify GH/IGF axis function

By systematically addressing these factors, researchers can better understand whether contradictory findings represent genuine biological differences or result from methodological variation .

What are the current knowledge gaps in understanding GH function in gilthead seabream?

Despite significant advances in understanding the GH/IGF axis in gilthead seabream, several important knowledge gaps remain that present opportunities for future research:

Receptor Signaling Dynamics:
While the full-length GHR has been characterized in gilthead seabream, the specific signaling pathways downstream of GHR activation remain incompletely understood . Questions persist about how GHR-1 and GHR-2 subtypes differentially activate JAK-STAT, MAPK, and PI3K pathways in different tissues, and how these pathways interact to coordinate growth responses.

Transcriptional Regulation of GH/IGF Components:
The regulatory elements controlling the expression of GHRs, IGFs, and IGFBPs in gilthead seabream have not been fully characterized. Research is needed to identify transcription factors, enhancers, silencers, and epigenetic modifications that modulate gene expression in response to nutritional, environmental, and developmental cues .

Tissue-Specific IGF Production and Action:
While liver is the primary site of IGF-1 production, local IGF production in extra-hepatic tissues likely plays important roles in tissue-specific growth regulation. The relative contribution of endocrine versus autocrine/paracrine IGF signaling to tissue growth remains poorly quantified in gilthead seabream .

Integration with Other Endocrine Systems:
The cross-talk between the GH/IGF axis and other endocrine systems (thyroid, reproductive, stress) in gilthead seabream requires further investigation, particularly in the context of its protandrous hermaphroditic life history .

Nutritional Modulation Beyond Cysteamine:
While CSH has shown promise as a growth promoter, comparative studies of different feed additives on GH/IGF modulation are needed to identify optimal strategies for enhancing growth in aquaculture .

Genetic Variability in GH Responsiveness:
Population-level variation in GH/IGF axis components and their responsiveness to nutritional and environmental factors remains largely unexplored . Understanding this variation could inform selective breeding programs for improved growth performance.

Long-term Developmental Programming:
The potential for early-life nutritional interventions to program the GH/IGF axis for enhanced long-term growth performance represents an important knowledge gap with practical implications for aquaculture .

Mechanistic Basis for CSH Effects:
While CSH improves growth and modulates the GH/IGF axis, the precise molecular mechanisms by which it enhances GH sensitivity and IGF-1 production remain incompletely understood. Further research is needed to determine whether its effects stem from its role as a taurine precursor, its antioxidant properties, or other unidentified mechanisms .

Addressing these knowledge gaps would advance both basic understanding of growth regulation in teleosts and applied aspects of enhancing growth performance in aquaculture .

How should researchers interpret gene expression data for the GH/IGF axis in different tissues?

Interpretation of gene expression data for the GH/IGF axis requires careful consideration of tissue-specific patterns, temporal dynamics, and physiological context. Based on current research, the following guidelines are recommended:

Liver Expression Patterns:
The liver is the primary site of IGF-1 production in response to GH stimulation. When interpreting hepatic gene expression:

• GHR Expression: Changes in ghr-1 and ghr-2 expression should be evaluated in relation to circulating GH levels. Decreased receptor expression with elevated GH may indicate negative feedback, while increased receptor expression despite high GH may suggest enhanced GH sensitivity .

• IGF-1 Variants: The three major IGF-1 splice variants (igf-1a, igf-1b, igf-1c) show differential responses to nutritional status and growth promoters. In CSH supplementation studies, igf-1a showed the strongest response, suggesting this variant may be most sensitive to nutritional modulation .

• IGF Binding Proteins: Decreased igfbp-2a expression in response to CSH supplementation correlated with improved growth, consistent with this binding protein's role in potentially inhibiting IGF action. Changes in binding protein expression should be interpreted alongside changes in IGF expression to understand net effects on IGF bioavailability .

Muscle Expression Patterns:
White muscle is both a target of systemic IGF-1 and a site of local IGF production:

• Local vs. Systemic Effects: Increased muscle igf-1 expression without corresponding increases in circulating IGF-1 suggests enhanced autocrine/paracrine regulation that may contribute to tissue-specific growth effects .

• GHR Expression: Enhanced ghr-1 expression in muscle following CSH supplementation indicates improved GH sensing capacity in this tissue, potentially amplifying growth responses to circulating GH .

• Receptor Expression: Changes in igf-1ra and igf-1rb expression affect tissue responsiveness to both circulating and locally produced IGFs, with the balance between these receptors potentially determining specific growth responses .

Stomach Expression Patterns:
The stomach shows distinct GH/IGF axis expression patterns that may relate to digestive function regulation:

• Growth-Digestive Connections: Enhanced ghr-1 expression in stomach following CSH supplementation may indicate improved digestive function that contributes indirectly to growth enhancement through better nutrient utilization .

• Receptor Downregulation: Reduced igf-1ra and igf-1rb expression in stomach after CSH supplementation suggests tissue-specific modulation that may optimize digestive processes .

Integrated Interpretation Approach:

What significance do the different IGF-1 splice variants have in gilthead seabream research?

The multiple IGF-1 splice variants in gilthead seabream represent an important aspect of growth regulation that demands careful consideration in research design and data interpretation:

Structural and Functional Differentiation:
Gilthead seabream expresses at least three major IGF-1 splice variants (igf-1a, igf-1b, and igf-1c) that share a common mature peptide sequence but differ in their E-domains (the extension peptides cleaved during post-translational processing) . These structural differences may influence:

• Protein Stability and Half-life: Different E-domains can affect the stability of IGF-1 precursors and their processing efficiency.

• Binding Affinity: Variants may have different affinities for IGF binding proteins, affecting bioavailability.

• Local Retention: E-domain characteristics may influence whether IGF-1 remains local or enters circulation.

Differential Regulation and Expression:
The splice variants show distinct expression patterns and regulatory responses:

• Tissue Specificity: While all variants are expressed in liver, their relative abundance differs across tissues, with variant-specific roles in extra-hepatic IGF-1 production .

• Nutritional Responsiveness: In CSH supplementation studies, igf-1a was significantly upregulated in liver at both CSH doses, while other variants showed less consistent responses, suggesting igf-1a may be particularly sensitive to nutritional modulation .

• Developmental Regulation: The relative expression of variants likely changes throughout development, though this aspect requires further investigation in gilthead seabream.

Research and Methodological Implications:
Understanding IGF-1 variant significance has several important implications for research:

Physiological and Applied Significance:
The differential regulation of IGF-1 variants has potential implications for aquaculture applications:

• Biomarker Development: Specific variants might serve as more sensitive biomarkers of growth potential or nutritional status.

• Selective Breeding: Genetic variation in variant expression patterns might be exploited in selective breeding programs for enhanced growth.

• Targeted Nutritional Interventions: Feed additives might differentially affect variant expression, offering opportunities for optimized growth promotion strategies .

Further research is needed to fully elucidate the functional significance of each IGF-1 variant in gilthead seabream and to determine whether variant-specific expression patterns could be manipulated to enhance growth performance in aquaculture settings .

How can researchers effectively design studies to evaluate novel feed additives for improving growth in gilthead seabream?

Designing effective studies to evaluate novel feed additives for growth enhancement in gilthead seabream requires a comprehensive approach that addresses multiple experimental aspects:

Experimental Design Framework:

• Dose-Response Assessment:

  • Test multiple inclusion levels to establish optimal dosage

  • Include both sub-optimal and potentially excessive doses to determine safety margins

  • For reference, CSH was effective at 1.65-3.3 g/kg feed, but higher doses may have adverse effects

• Growth Stage Considerations:

  • Design multi-phase trials that span different growth stages (e.g., fingerlings to juveniles)

  • Consider age-specific responses, as demonstrated in the CSH studies where Phase 1 (fingerlings) and Phase 2 (juveniles) showed different response magnitudes

• Duration Optimization:

  • Plan sufficient experimental duration to capture full growth effects

  • Include multiple sampling points (minimum 9 weeks apart based on CSH studies)

  • Consider seasonal effects on growth patterns and GH/IGF regulation

Comprehensive Endpoint Analysis:

• Tiered Analytical Approach:

  • Tier 1: Basic growth parameters (weight gain, SGR, FCR, condition factor)

  • Tier 2: Plasma hormone levels (GH, IGF-1)

  • Tier 3: Tissue-specific gene expression (liver, muscle, stomach)

  • Tier 4: Cellular mechanisms through targeted in vitro studies

• Tissue Sampling Protocol:

  • Collect blood samples for hormone analysis using standardized procedures

  • Harvest multiple tissues for gene expression analysis (liver, white muscle, stomach)

  • Preserve samples appropriately (-80°C for RNA and protein work)

• Expression Analysis Strategy:

  • Analyze key components across the entire GH/IGF axis (receptors, ligands, binding proteins)

  • Include tissue-specific markers of growth and metabolism

  • Consider both systemic (liver) and local (muscle, stomach) regulation

Integrative Analytical Framework:

• Correlation Analysis:

  • Correlate molecular markers with growth performance

  • Establish relationships between plasma hormone levels and tissue gene expression

  • Identify potential biomarkers of growth response

• Mechanistic Investigations:

  • Complement in vivo trials with in vitro studies using primary cell cultures

  • Determine direct effects of additives on relevant cell types

  • Establish dose-response relationships at the cellular level

• Comparative Assessments:

  • Include positive control additives with established effects when possible

  • Consider parallel studies with related species to identify conserved and divergent responses

  • Compare findings with published literature using meta-analytical approaches

Practical Implementation Considerations:

• Diet Formulation:

  • Ensure experimental diets are practical and commercially relevant

  • Maintain isonitrogenous and isolipidic formulations across treatment groups

  • Verify additive stability during feed manufacturing and storage

• Statistical Design:

  • Employ adequate replication at both tank and individual levels

  • Calculate appropriate sample sizes based on expected effect magnitudes

  • Use nested designs to account for tank effects

• Long-Term Assessment:

  • Consider extended monitoring beyond the main experimental period

  • Evaluate potential compensatory growth or adaptation responses

  • Assess cost-benefit ratio for practical implementation

By following this comprehensive framework, researchers can effectively evaluate novel feed additives while generating mechanistic insights into how these additives modulate the GH/IGF axis to enhance growth in gilthead seabream .

How can the molecular characterization of the GH/IGF axis inform selective breeding programs for enhanced growth?

Molecular characterization of the GH/IGF axis provides valuable information that can strategically inform selective breeding programs for enhanced growth in gilthead seabream aquaculture:

Genetic Marker Identification:

• Polymorphism Screening:

  • Identify single nucleotide polymorphisms (SNPs) in key genes of the GH/IGF axis, including GH, GHR-1, GHR-2, IGF-1 variants, and IGFBPs

  • Focus on functional polymorphisms in promoter regions that affect expression levels or in coding regions that modify protein function

  • The full-length GHR characterization provides the sequence data necessary for such screening

• Expression Quantitative Trait Loci (eQTLs):

  • Identify genetic variants associated with differential expression of GH/IGF axis components

  • Map eQTLs controlling the expression of growth-related genes

  • Correlate eQTL patterns with growth performance traits

• Epigenetic Markers:

  • Characterize DNA methylation patterns in promoter regions of GH/IGF genes

  • Identify stable epigenetic marks associated with enhanced growth

  • Develop epigenetic selection criteria that complement genetic selection

Functional Response Assessment:

• GH Challenge Tests:

  • Evaluate individual variation in responsiveness to GH stimulation

  • Measure IGF-1 production following standardized GH administration

  • Identify high-responder genotypes for breeding programs

• Nutritional Response Profiling:

  • Assess variation in GH/IGF axis responsiveness to growth-promoting additives like CSH

  • Identify individuals with enhanced molecular responses to nutritional stimulation

  • Select for optimized feed conversion efficiency based on molecular markers

• Tissue-Specific Expression Patterns:

  • Characterize individual variation in the balance between systemic and local IGF production

  • Select for optimal patterns of tissue-specific GHR and IGF expression

  • Target enhanced muscle-specific IGF signaling for improved fillet yield

Integration with Breeding Programs:

• Marker-Assisted Selection:

  • Develop SNP panels for key GH/IGF axis components

  • Implement early selection of broodstock based on favorable genetic markers

  • Combine multiple markers into selection indices weighted by their correlation with growth traits

• Transcriptomic Selection Criteria:

  • Establish baseline expression profiles associated with superior growth

  • Develop non-lethal sampling methods for expression analysis in potential broodstock

  • Implement selection based on optimal expression patterns of GH/IGF axis components

• Genomic Selection Approaches:

  • Incorporate GH/IGF markers into broader genomic selection programs

  • Assign appropriate weighting to GH/IGF pathway components based on their contribution to growth variation

  • Develop breeding values that account for both phenotypic performance and molecular markers

Population-Level Considerations:

• Genetic Diversity Management:

  • Maintain genetic diversity within the GH/IGF axis to preserve adaptive potential

  • Monitor for unintended consequences of selection on other traits

  • Balance selection intensity with population genetic diversity

• Environment-Specific Optimization:

  • Select for GH/IGF profiles optimized for specific production environments

  • Consider temperature, salinity, and feeding regime interactions with GH/IGF genotypes

  • Develop environment-specific breeding lines based on molecular responses

• Life-Stage Appropriate Selection:

  • Account for age-specific expression patterns in selection criteria

  • Balance early growth performance with sustained growth through market size

  • Consider selection for optimal GH/IGF expression throughout the production cycle

By systematically integrating molecular characterization of the GH/IGF axis with traditional selective breeding approaches, aquaculture programs can accelerate genetic improvement for growth while maintaining genetic diversity and environmental adaptability in gilthead seabream populations .