Avocado Trees Switch Sexes Twice A Day – This Ancient Gene Explains How

Long before the guacamole reaches the table, the tree that grew that avocado has already run through a biological performance that has puzzled scientists for nearly a century. Avocado trees switch sexual roles twice a day, every day, on a precise schedule. Now, researchers have identified the single gene most strongly linked to it, and its story stretches back about 42 million years.

A new study published in the Proceedings of the National Academy of Sciences identifies a single gene, nicknamed SDMYB, as strongly linked to the daily switch in sexual role that avocado trees perform. Growers have long known that trees come in two varieties: A-types, which act as female in the morning and male in the afternoon, and B-types, which do the reverse. This daily rhythm, which botanist A.B. Stout described in 1927 as reaching “a perfection of physiological regulation in avocados that is unapproached, as far as is now known, in any other group of plants,” appears to be largely controlled by two ancient versions of one flower gene that have been locked in a biological standoff for more than 40 million years.

Especially arresting is how old and widespread this genetic variation really is. Researchers found the same two gene variants distributed across at least 26 non-avocado species related to avocado. Both versions of the gene diverged during the Eocene epoch, tens of millions of years before our own species existed.

A Century-Old Avocado Tree Mystery, Finally Cracked

To find the gene, the research team scanned the entire genome for genetic markers associated with flowering type in 374 avocado seedlings, the offspring of a cross between an A-type variety called “Gem®” and a B-type variety called “Luna UCR®.” When they looked for regions of the genome that predicted whether a seedling would be an A-type or B-type, one location on chromosome 10 stood out clearly above all others.

Within that region, only one gene made biological sense as the culprit. SDMYB belongs to a family of proteins that act as master switches in flowers, controlling when petals open, when pollen is released, and when a flower is ready to receive pollen. Studies in other plants, including rice, had already shown that related genes can control the timing of flower opening. In avocado, researchers confirmed that SDMYB ranks among the most active genes in flower tissue, landing in the top 0.14% of all genes expressed there.

Each avocado flower actually opens twice over two consecutive days. On the first opening, it functions as female, receiving pollen. It closes, then reopens the following day to release pollen. In A-type trees, the timing of that second opening is delayed compared to B-types. Researchers found a matching delay in when the A-type version of SDMYB switches on, roughly a two-hour lag relative to the B-type version. That shift in gene activity appears to account for the shifted timing of the flower’s second opening.

Two Gene Versions, Locked in Balance for 42 Million Years

Whether a tree behaves as A-type or B-type is largely determined by which version of the gene a tree carries. A-type trees carry the dominant version, called A1, while B-type trees carry two copies of the recessive version, A2. Trees with one copy of each version behave as A-types.

What keeps both versions circulating in plant populations? It comes down to reproductive strategy. When A-types are common, B-types have an advantage because they cross-pollinate more successfully with the abundant A-types, and vice versa. This back-and-forth creates a self-correcting system that prevents either version from disappearing. Researchers estimated that about 85% of the pollen fertilizing A-type mothers and 88% of the pollen fertilizing B-type mothers in their experiment came from trees of the opposite flowering type, confirming the strong pull toward cross-type mating.

This kind of long-term genetic balancing act is rare in plants. To determine how long it had been going on, the team built a detailed family tree of avocados and their relatives using nearly 8,000 shared genes across 15 genome sequences, anchored by fossil evidence. Their analysis estimated that the A1 and A2 versions of SDMYB split from a common ancestor approximately 42 million years ago, during the Eocene epoch, and have been maintained by natural selection ever since.

One Avocado Gene, Found in Dozens of Species

Researchers didn’t stop at avocado. Using genetic data from 72 individual plants representing 56 species in avocado’s broader plant family, they found the same two gene variants present across 26 non-avocado species, a genetic finding rather than direct proof of daily sex-switching, since the mating systems of many of these wild relatives have not been directly observed. In one case, the A1 variant turned up in a plant genus where this mating system had not previously been reported, raising the possibility that daily sex alternation may be more widespread among wild relatives of avocado than scientists have recognized.

Those same SDMYB variants were absent from true cinnamon and other members of the broader laurel family that have been reported to have similar mating systems. That absence points to a system that may have evolved more than once using different genetic tools, meaning evolution found multiple routes to the same floral behavior.

Faster Avocado Breeding for a Growing Market

Beyond the evolutionary story, this research has direct practical uses. Avocado trees take anywhere from five to twelve years to mature before growers can observe their flowering behavior in the field. Because orchards typically need both A-type and B-type trees to maximize fruit production, breeders must wait years to confirm what type of tree they have grown before making planting decisions.

With genetic markers now identified at the SDMYB gene, breeders could eventually screen seedlings for flowering type long before the trees ever flower, substantially reducing time, labor, and costs. That could shave years off the development cycle for new avocado varieties at a time when global demand for the crop keeps rising.

For a fruit that has become a staple of modern kitchens, the revelation that its daily reproductive rhythm traces back more than 40 million years to a single gene variant is a reminder that even the most familiar foods carry deep biological histories that science is still working to uncover.