An international team including researchers from Temasek Life Sciences Laboratory in Singapore, the Institute of Marine Sciences (ICM-CSIC), the University of Konstanz and the University of Würzburg has uncovered an unusual genetic mechanism in Mozambique tilapia (Oreochromis mossambicus): a gene originally associated with ovary formation has evolved into the primary regulator of male development.
The research, published in Science Advances, identifies the gene figlaY as the trigger for male sex determination in this species. Remarkably, the gene originated from a modified version of figla, a gene that normally plays a key role in ovarian development and female differentiation.
In other words, a gene associated with female development has evolved to perform the opposite function: preventing ovarian formation and promoting testis development. According to the researchers, this evolutionary pathway has not previously been observed in vertebrates.
The discovery may also have implications for aquaculture. A better understanding of how sex is determined in fish could eventually contribute to the development of more precise approaches for controlling this process.
In species such as tilapia, males generally grow faster and more uniformly than females, making predominantly male population attractive from a production perspective.
Although further research is needed any practical applications can be considered, the study provides important new insights into the genetic mechanisms controlling sex determination in fish.
The findings also highlight the remarkable flexibility of sex-determination system in fish, where the genes responsible for directing male or female development can evolve rapidly and follow very different evolutionary trajectories.
CRISPR confirms its essential role
The researchers identified the function of figlaY through a combination of genomic analysis, gene-expression studies and gene-editing experiments. They first observed that the gene was present only on the Y chromosome and was activated during the early stages of gonadal development, precisely when the pathway towards male or female differentiation is established.
They then reconstructed its evolutionary history and found that it originated from a duplicated and modified copy of the female-associated figla gene. Their analyses showed that the new version had acquired structural and regulatory changes that enabled it to perform a function different from that of the ancestral gene.
The decisive evidence came from CRISPR-Cas9 gene-editing experiments. When figlaY was inactivated in genetically male (XY) fish, the animals developed normal ovaries and became phenotypic females.
In contrast, fish retaining an active copy of the gene developed normally as males.
These results demonstrate that figlaY acts as a key regulator initiating male development in Mozambique tilapia.
The study also revealed that transposable elements, often referred to as “jumping genes”, played a central role in the origin of figlaY. These mobile DNA elements contributed to the duplication, modification and regulatory rewiring that gave rise to the new gene and helped shape its expression pattern during sexual development.
Reference
Wang, L., Sun, F., Yang, Z., Lee, M., Wong, J., Yeo, S., Wen, Y., Meyer, A., Piferrer, F., Schartl, M. and Yue, G.H. (2026). Transposons drove the evolution of a male sex-determining gene from a female gene with deep introgression across cichlids. Science Advances, 12(23), eaed6242
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