UC Davis researchers traced the avocado’s daily male-female rhythm to a gene called SDMYB, whose two versions have persisted for about 42 million years.

An avocado tree has an unusual daily schedule. Each of its flowers carries both male and female parts, but it never uses them at the same time. Some trees open their flowers as females in the morning and as males in the afternoon. Others do the reverse. The arrangement keeps trees from pollinating themselves, and growers have known about it for more than a century.

What controls the rhythm has been a mystery. Now a team led by researchers at the University of California, Davis, has traced it to variation in a single gene, one that appears to have been doing the same job for about 42 million years. The study is published in Proceedings of the National Academy of Sciences.

“Our study provides a genetic explanation for a century-old mystery of avocado pollination,” said first author Jeffrey Groh, who did the work as a Ph.D. student at UC Davis and is now a postdoctoral fellow at UC Berkeley.

A-types and B-types

Avocado growers sort trees into two groups. About half are “A-type,” whose flowers function as female in the morning and male in the afternoon. The other half are “B-type,” following the opposite pattern. The popular Hass avocado is an A-type; Fuerte and Bacon are B-types.

The avocado, Persea americana, is native to Mexico and Central America, where people have eaten and cultivated it for thousands of years. It belongs to the laurel family, which also includes bay laurel and the trees that produce cinnamon. In orchards, its flowers are visited mainly by bees and other insects, which carry pollen between trees on opposite schedules.

Botanists call this system synchronous dichogamy. In practice, each flower opens twice, typically on consecutive days, first as a female ready to receive pollen and later as a male releasing it. Because all flowers on a tree keep the same timetable, a tree is effectively one sex at a time.

“If every plant was male at the same time, and then female at the same time, there’d be no exchange of pollen,” said senior author Graham Coop, a professor of evolution and ecology at UC Davis. “Having these two types that reciprocally and synchronously flower male and female allows avocados to exchange and use pollen throughout the whole day.”

The separation reduces self-pollination and inbreeding, which can weaken offspring. Commercial growers often plant a mix of A- and B-type trees to improve cross-pollination and fruit set.

Hunting for the switch

Collaborators at UC Riverside, home to one of the world’s major avocado breeding programs, grew hundreds of avocado trees. The team sequenced their genomes and compared them with each tree’s flowering behavior, an approach that links genetic differences to observable traits.

The search led to a single gene, called SDMYB. It belongs to a family of transcription factors, proteins that switch other genes on and off, and it regulates when flowers open. Its activity changes over the course of the day.

Avocados carry two versions, or alleles, of SDMYB. A-type trees have one copy of each. B-type trees have two copies of the recessive version.

“One allele of this gene is dominant, and it shows a unique pattern of temporal regulation that is closely linked to the difference in the timing of flowers opening and closing between A- and B- types,” Groh said.

Forty million years of rhythm

The most surprising finding came when the team looked beyond avocados. The same two versions of SDMYB appear in at least 26 related tree species. That suggests the two alleles arose about 42 million years ago and have been maintained ever since, a pattern known as balancing selection, in which natural selection keeps two versions of a gene in a population rather than letting one win out.

Here the reason is easy to see. If one type became too common, its trees would struggle to find partners on the opposite schedule, and the rarer type would gain an advantage. That keeps the two types in rough balance, which is consistent with the roughly even split between A- and B-type trees.

“We were blown away by the idea that these plants have been alternating male and female activity over the course of a day for over 40 million years, over potentially hundreds of species,” Coop said.

“Flowers are not just static decorations; they are highly evolved and dynamic structures,” Groh said. “We can see this in the pulsating rhythm of avocado flowers opening and closing throughout the day. Through the power of genetics, we can detect traces of this rhythm far into the ancient past.”

Faster breeding

The discovery has an immediate practical payoff. Until now, breeders could tell whether a seedling was an A- or B-type only by waiting for it to flower, which can take a very long time.

“These genetic markers will greatly speed up avocado breeding and research,” Coop said. “This has been a major impediment for avocado breeders. Previously, you could not tell whether a plant was an A- or a B- type until it flowered, and avocado plants may take up to 10 or 15 years to flower in the first place.”

That matters because demand for avocados has soared over recent decades, and the fruit is an important crop in Mexico, the world’s largest producer, as well as in California, South America and elsewhere. Faster breeding could help develop varieties suited to changing climates, new pests and different growing regions.

The release does not report exact sample sizes, statistical results or the specific genetic variants involved. The study was funded by the National Science Foundation, the National Institutes of Health, the UC Natural Reserve System, Eurosemillas and the National Institute of Food and Agriculture.

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Notes

1. Groh, J., et al. “Balanced polymorphism in a floral transcription factor underlies the ancient rhythm of daily sex alternation in avocado.” Proceedings of the National Academy of Sciences, 28 July 2026. DOI: 10.1073/pnas.2606876123.
2. University of California – Davis. “Study reveals how avocados reverse sexes throughout the day.” EurekAlert!, 29 July 2026. https://www.eurekalert.org/news-releases/1138094.

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