Sex Determination in Ornamental Fish: The Impact of the Molecular Revolution

The ornamental fish industry represents a strategic pillar of aquaculture, with a market value exceeding $15 billion annually. In this highly competitive environment, precise sex identification is not merely a technical detail but a critical factor for maximizing reproductive efficiency, population management, and the commercial profitability of species. Specimens such as koi, guppies, cichlids, and bettas are coveted for their vibrant coloration and distinctive morphologies—attributes that typically manifest unevenly between males and females.

Historically, methods based on phenotypic observation (external traits) have shown severe limitations. Multiple species exhibit late sexual dimorphism or present traits conditioned by environmental variables, such as temperature. In this context, molecular tools emerge as a disruptive solution, ensuring early and accurate detection at any stage of biological development.

According to research published in METAMORFOSA: Journal of Biological Sciences by experts from Universitas Udayana, teleost fish possess unprecedented evolutionary plasticity, reinforcing the necessity of implementing these advanced diagnostics for the sector’s success.

Key Study Highlights

• Extreme biological diversity: There is no universal molecular marker; sex determination systems in fish range from chromosomal (XX/XY, ZZ/ZW) to polygenic and environmental influences.
• Accessible technologies: PCR-based methods (SCAR, RAPD, AFLP) remain the most practical and cost-effective options for routine applications in validated farms.
• Precision via NGS: Next-Generation Sequencing allows for the identification of high-resolution genetic variants (SNPs), which are crucial for complex or domesticated species.
• Impact of domestication: Intense artificial selection can destabilize sexual inheritance patterns, necessitating the validation of markers in specific strains.
• Portable future: The development of Point-of-Care diagnostic kits promises to bring genetic analysis directly to hatcheries.

The Challenge of Sexual Plasticity in Teleost Fish

Unlike mammals, teleost fish exhibit exceptional evolutionary plasticity in their sex determination mechanisms. While the human system is strictly male heterogametic ($XX/XY$), ornamental aquaculture presents a far more complex spectrum:

• Male Heterogamety ($XX/XY$): Predominant in species such as guppies and koi carp.
• Female Heterogamety ($ZZ/ZW$): Observed in specimens like the Japanese parrotfish.
• Polygenic Systems: Where the interaction of multiple genes defines the final phenotype.
• Environmental Sex Determination (ESD): External factors such as temperature or pH can “reprogram” biological sex.

This variability is heightened by domestication and intensive selective breeding. The pressure to achieve specific aesthetic traits can shift sex-determining genomic regions, causing genetic markers effective in wild populations to lose validity in commercial aquarium varieties.

Master Regulatory Genes in Gonadal Development

Scientific research has identified critical genes that orchestrate cellular differentiation toward testes or ovaries:

• dmrt1: A fundamental master gene for testicular development.
• amh (Anti-Müllerian Hormone): A key protein in male differentiation.
• sox9: Essential for Sertoli cell maturation in males.
• cyp19a1a (Aromatase): The terminal enzyme responsible for estrogen synthesis and ovarian formation.

Molecular Tools: From PCR to Whole-Genome Analysis PCR-Based Methods

Polymerase Chain Reaction (PCR) remains the gold standard due to its optimal balance between cost and diagnostic effectiveness.

• SCAR Markers: These exhibit high reliability in validated populations because of their inheritance linked to sex-determining loci.
• InDel Markers: Based on DNA insertions or deletions, they enable the development of precise and specific assays, which are fundamental in species such as the Japanese parrotfish.

The Leap Toward Next-Generation Sequencing (NGS)

For non-model species lacking a prior genetic database, NGS technologies have been disruptive, allowing for the massive discovery of sex-linked Single Nucleotide Polymorphisms (SNPs) across the entire genome. This capability is vital for the ornamental industry, where recurrent hybridization often masks phenotypic sex expression.

Epigenetics and Transcriptomics: The Future of Diagnostics

In systems where environmental control is decisive—such as in zebrafish or silverside—gene expression (RNA) and DNA methylation markers represent high-value tools. These indicators allow for the prediction of an individual’s sexual trajectory before gonads become visible, detecting critical changes induced by thermal stress or social interactions.

Industrial Impact and Sustainability: Toward Precision Aquaculture

The integration of these advanced methodologies into breeding centers and production units not only optimizes reproductive efficiency but also exerts a direct impact on sustainability and animal welfare. By implementing early sex identification, producers achieve:

• Resource Optimization: A substantial reduction in feed costs and infrastructure requirements by maintaining only those specimens aligned with market objectives or breeding programs.
• Strategic Stock Management: Enhanced control over broodstock nuclei, which is fundamental in conservation programs for species classified as endangered.
• Operational Risk Mitigation: Elimination of the human error inherent in manual selection, thereby minimizing opportunity costs and maximizing batch profitability.

Biotechnological Challenges and the Path Toward Diagnostic Portability

Despite its vast potential, the industrial implementation of these tools faces critical technical challenges. Non-specific amplification and high homology between sex-linked sequences and autosomal regions can induce false positives. Furthermore, the lack of reference genomes for “non-model” ornamental species restricts the development of standardized diagnostic panels.

Obstacles in the Hatchery Environment

Practical application in production units is conditioned by complex biological variables:

• Strain Divergence: Due to artificial selection pressure, a marker effective in one Betta genetic line may lose validity in another.
• Induced Sex Reversal: Thermal factors can cause genetically female individuals to become functional males, creating a discordance between genotype and phenotype.

The Future: Point-of-Care (PoC) Diagnostics

The trend points toward technological democratization through on-site diagnostics. Innovations such as Loop-mediated Isothermal Amplification (LAMP) promise sex identification at the “pondside.” This approach eliminates dependency on centralized laboratories, facilitating access to high-precision tools for small and medium-scale aquaculturists.

Conclusion: The Future of Molecular Sex Determination in the Ornamental Industry

Molecular sex determination has established itself as an indispensable strategic tool for the ornamental industry, providing precise solutions where phenotypic inspection proves insufficient. The success of these innovations lies in the rigorous selection of markers, exhaustive validation across diverse strains, and the synergistic integration of genetic data with environmental culture variables.

Looking ahead, sustained investment in genomic resources and the establishment of standardized validation frameworks will be critical factors. These elements will enable the transformation of experimental findings into robust industrial applications, driving an aquaculture production that is more efficient, profitable, and committed to global sustainability.

Reference (open access):
Dwipayana, I. D. A. P., Febryanti, N. L. P. K., & Unique, I. G. A. N. P. (2026). A Review of Molecular Approaches for Sex Determination in Ornamental Fish. Metamorfosa: Journal of Biological Sciences, 13(1), 88–101. https://doi.org/10.24843/metamorfosa.2026.v13.i01.p8

Milthon Lujan

Editor at the digital magazine AquaHoy. He holds a degree in Aquaculture Biology from the National University of Santa (UNS) and a Master’s degree in Science and Innovation Management from the Polytechnic University of Valencia, with postgraduate diplomas in Business Innovation and Innovation Management. He possesses extensive experience in the aquaculture and fisheries sector, having led the Fisheries Innovation Unit of the National Program for Innovation in Fisheries and Aquaculture (PNIPA). He has served as a senior consultant in technology watch, an innovation project formulator and advisor, and a lecturer at UNS. He is a member of the Peruvian College of Biologists and was recognized by the World Aquaculture Society (WAS) in 2016 for his contribution to aquaculture.