The motivation behind the work lies in an awkward truth about tilapia farming. Nile tilapia is among the most widely farmed fish in the world, prized for its rapid growth and tolerance of dense culture conditions. But the species matures early and reproduces prolifically, and in mixed-sex ponds that reproductive exuberance becomes a liability: populations balloon, food resources are stretched thin, individual growth stalls, and ponds fill with stunted, overcrowded fish. All-male stocks solve the problem neatly, because males grow faster and larger than females, and they cannot multiply. For decades, farmers have produced such stocks using 17α-methyltestosterone (MT), a synthetic androgen dissolved into the feed given to newly hatched fry. It works with remarkable efficiency, but the hormone raises well-documented concerns over worker safety, residues in farmed fish, and the environmental fate of steroid compounds released into waterways surrounding farms.

That is where pine pollen enters the picture. A traditional food and medicine in parts of Asia, pine pollen is rich in phytosterols and compounds with androgen-like biological activity, making it a candidate “phytoandrogen”—a plant-derived substance capable of mimicking the hormonal signals that steer development toward maleness. Previous studies had hinted that pine pollen could skew sex ratios in tilapia, but the underlying mechanism had remained an open question. The new research, led by Ivan Abaho of Uganda’s National Agricultural Research Organization, together with Clifford L. W. Jones of Rhodes University and Peter Akoll and Charles Masembe of Makerere University, set out to close that gap by following the entire trajectory of sex change, day by day, from undifferentiated larva to sexually mature juvenile.

The experimental design was straightforward but rigorous. Three-day-old all-female Nile tilapia fry were divided into treatment groups and fed one of three diets from 3 to 30 days post-hatch (dph), the developmental window during which tilapia gonads commit to their sexual fate. One group received a basal diet supplemented with pine pollen at a concentration of 1,280 milligrams per kilogram of feed. A second group received the basal diet containing 60 milligrams per kilogram of MT, the conventional synthetic treatment. A control group received the basal diet alone. After 30 dph, all fish in every group were switched to the same unsupplemented basal diet and reared until 120 dph, allowing the researchers to observe whether any effects of the early dietary intervention persisted into maturity.

The results unfolded as a story of competing developmental programs. In the control group, the gonads followed the expected female pathway: between 21 and 30 dph, tissue sections revealed the formation of an ovarian cavity and the appearance of oocyte-like germ cells, the unmistakable hallmarks of ovarian differentiation. These fish remained entirely female through the study period. In the pine pollen group, by contrast, the same window of development looked radically different. Gonadal tissue shifted progressively away from the undifferentiated state, and by 21 to 45 dph the fish were unmistakably on the path to maleness, their gonads becoming dominated by spermatogonia—the stem cells of the testis—and later by spermatocytes undergoing the early stages of sperm production.

The synthetic hormone group moved along the same trajectory, but faster. In the MT-treated fish, spermatogenesis was accelerated, with gonads containing advanced germ-cell types including spermatids and fully formed spermatozoa by the end of the treatment window. This histological difference proved consequential at the level of the whole population. When sex ratios were assessed at the end of the study, the MT treatment had achieved near-complete conversion, producing 97.8 percent males, with a standard error of 1.1 percent. Pine pollen produced a substantial but more moderate effect, yielding 77.8 percent males with a standard error of 2.9 percent—a dramatic skew compared with the control group, in which not a single fish became male.

Beneath those visible changes lay a coherent molecular story. Using quantitative real-time PCR, the team tracked the expression of four pivotal genes that form the regulatory axis of sex differentiation in teleost fish. Two of them, dmrt1 and amh, are the architects of maleness: dmrt1 drives testicular development and is widely regarded as a master sex-determining factor in fish, while amh governs Sertoli cell function and the pace of germ-cell maturation. The other two, cyp19a1a and foxl2, are guardians of femaleness: cyp19a1a encodes aromatase, the enzyme that converts androgens into estrogens, and foxl2 is a transcription factor that maintains ovarian identity and keeps aromatase switched on.

In both the pine pollen and MT groups, the male genes were significantly up-regulated while the female genes were significantly down-regulated, and the timing of these shifts matched the histological observations of gonadal transformation between 21 and 45 dph. In the control fish, the pattern was exactly reversed, with female transcripts predominating as ovaries formed. The result was not merely a change in gene activity for its own sake; it cascaded directly into the hormonal environment of the developing fish. Testosterone and 11-ketotestosterone—the latter being the most potent androgen in fish—were significantly elevated in both treatment groups, consistent with the growing proportion of male individuals. In the control group, concentrations of 17β-estradiol, the principal fish estrogen, were significantly higher, sustaining the female pathway.

Taken together, the data sketch a mechanistic model of how dietary pine pollen orchestrates female-to-male sex change. The pollen, or its bioactive constituents, appears to disrupt the delicate balance of the sex-biased gene network during the labile period of gonadal differentiation. By suppressing aromatase expression and the ovarian maintenance factor foxl2, it throttles estrogen production; by boosting dmrt1 and amh, it recruits the developing tissue toward testis formation and spermatogenesis. The resulting rise in androgens reinforces the male program, creating a self-consistent hormonal and transcriptional environment in which maleness prevails. It is, in essence, the same logic that underlies MT treatment—shift the androgen-to-estrogen balance, and the fish follow—but achieved through a natural dietary supplement rather than a synthetic steroid.

The researchers are careful to note the limits of the current results. At roughly 78 percent males, pine pollen falls short of the near-total masculinization that commercial hatcheries demand, and the authors explicitly call for further work to enhance its androgenic potency, whether through optimized dosing, extended treatment windows, refined extracts, or synergistic combinations with other plant-derived compounds. The dose used in this study, 1,280 milligrams per kilogram of feed, also represents a considerable quantity of material relative to the 60-milligram-per-kilogram dose of the far more potent synthetic hormone, raising practical questions about cost and supply that will need to be resolved before pine pollen can compete at industrial scale.

Even so, the study marks a significant advance for sustainable aquaculture. It is the first work to connect dietary pine pollen treatment to the full chain of causation—histological sex change, sex-biased gene expression, and measurable shifts in steroid hormone profiles—in Nile tilapia. Plant-based alternatives such as Tribulus terrestris and Mucuna pruriens have been explored in earlier research, and non-steroidal aromatase inhibitors have been tested in hybrid red tilapia, but pine pollen now stands among the most fully characterized natural masculinizing agents, with its mechanism of action laid bare at the molecular level. Because it is safe to handle, widely available and inexpensive relative to regulated hormones, it holds particular promise for smallholder farmers in developing countries, where access to and safe use of synthetic steroids is often constrained by regulation, cost and infrastructure.

The work was funded by the European Union through the Collaborative Training in Fisheries and Aquaculture in Eastern, Central, and Southern Africa (COTRA) project and carried out with the rearing facilities of Uganda’s Mukono Zonal Agricultural Research and Development Institute and the molecular and analytical laboratories of Makerere University, under animal ethics approval from Rhodes University. The authors also emphasize that the treatment was administered only during the short early window in which gonadal fate is decided; after 30 dph, all fish received ordinary feed, meaning no pollen-derived residues would accumulate in the tissue of the fish as they grew to harvest size.

As pressure mounts on the aquaculture industry to reduce its reliance on synthetic hormones, the pine pollen study offers a compelling proof of concept: that a plant product, sprinkled into the feed of three-day-old fry during a precise developmental window, can reprogram the genome-wide conversation between maleness and femaleness and redirect an entire cohort of fish along a new developmental path. Whether future refinements can close the gap between 78 percent and the 98 percent achieved by synthetic MT will determine how quickly this nature-based approach moves from the laboratory to the hatchery—and how soon one of aquaculture’s thorniest sustainability dilemmas can be solved with something as simple as pollen from a pine tree.