Study Reveals How Avocados Reverse Sexes Throughout the Day

To avoid self-pollination, avocado trees alternate between opening male and female flowers at different times of day. Some avocados are female in the morning and male in the afternoon, while others are on the opposite schedule. How avocados do this has been unclear — until now.

In a new study, a team led by researchers from the University of California, Davis, revealed that this rhythm is dictated by variation in a single gene that evolved about 42 million years ago. Published in Proceedings of the National Academy of Sciences, this finding could help avocado breeders and researchers quickly sort seedlings to maximize yield and develop new varieties.

“Our study provides a genetic explanation for a century-old mystery of avocado pollination,” said first author Jeffrey Groh, who led the project as a Ph.D. student at UC Davis and is now a postdoctoral fellow at UC Berkeley. “Flowers are not just static decorations; they are highly evolved and dynamic structures. 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.”

All avocado trees are hermaphrodites, but at any given time their flowers function either as male by releasing pollen, or as female by receiving pollen. Approximately half of all avocado trees (“A-type” plants) open female flowers in the morning and male flowers in the afternoon, whereas the other half (“B-type” plants) follow the opposite pattern. This partitioning minimizes the risk of self-pollination, which can result in inbreeding. Avocado breeders have known about this for over a hundred years, and because these two types easily pollinate one another, avocado farmers usually try to plant a mixture of A- and B-type trees.

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

To identify the genetic mechanism behind this daily rhythm, the team analyzed the genomes of hundreds of avocado trees grown by collaborating researchers at UC Riverside. By comparing the trees’ genomes in the context of their flowering behavior, they were able to pinpoint a single gene called SDMYB that was associated with the tree’s A- or B- type schedule.

SDMYB regulates the timing of flower opening, and its expression level oscillates throughout the day. In avocados, there are two alternative versions or “alleles” of the gene, similar to how many species have two alternative sex chromosomes. Whereas A-type plants carry one copy of each version, B-type plants carry 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,” said Groh.

By comparing avocado genomes to those of related tree species, the team showed that the same two versions of SDMYB are shared by at least 26 related tree species. This suggests that this genetic mechanism is regulating flowering behavior and preventing inbreeding far beyond avocados. 

“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,” said Coop.

The study provides a genetic framework for easily categorizing A- and B-type plants at the seedling stage — something previously only possible in mature, flowering plants.

“These genetic markers will greatly speed up avocado breeding and research,” said Coop. “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.”

Additional co-authors include Gracie Ackerman from UC Davis; Marllon F. Soares dos Santos, Emmanuel Avila de Dios, Rodrigo A. Iturrieta, Eric Focht, Danelle Seymour and Mary Lu Arpaia from UC Riverside; Edwin Solares from UC San Diego; and Brandon S. Gaut from UC Irvine.

The research was funded by the National Science Foundation, National Institutes of Health, the UC Natural Reserve Maurer-Timm Endowment, Eurosemillas and the National Institute of Food and Agriculture.

This project used the UC Davis High Performance Computing Core Facility, the UC Lindcove and South Coast Research and Extension Centers in Exeter and Irvine, and UC Quail Ridge Reserve.