For nearly a century, botanists and agricultural growers have known that avocado trees possess a complex biological routine, alternating their floral functions between male and female roles over the course of a day. Published in the Proceedings of the National Academy of Sciences, a new study has finally identified the genetic mechanism behind this daily sex-switching behavior.
Cracking a Century-Old Avocado Mystery
All avocado trees are hermaphrodites, but their flowers open twice over two consecutive days. During the first opening, the flower acts exclusively as a female to receive pollen. It then closes before reopening the following afternoon to release pollen. To minimize the risk of self-pollination and inbreeding, growers have historically categorized trees into two distinct schedules: A-type plants, which act as female in the morning and male in the afternoon, and B-type plants, which follow the reverse pattern. Because these two types easily pollinate one another, farmers traditionally plant a mixture of both varieties to maximize fruit production.
According to University of California, Davis researchers, this intricate daily rhythm is dictated by variation in a single gene called SDMYB.
How the SDMYB Gene Controls Floral Timing
The research team scanned the entire genome using 374 avocado seedlings, which were the offspring of a cross between an A-type variety called Gem and a B-type variety called Luna UCR. The analysis revealed that one location on chromosome 10 stood out as strongly linked to the tree’s flowering type.
The SDMYB gene regulates the timing of flower opening, and its expression levels oscillate throughout the day. The gene features two alternative versions, or alleles, operating much like sex chromosomes. A dominant version of the gene produces A-type flowering, while B-type plants carry two copies of the recessive version. In A-type trees, the timing of the second flower opening is delayed compared to B-type trees, matching a roughly two-hour lag in when the A-type version of SDMYB switches on.
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 Jeffrey Groh, a researcher involved in the study.
Graham Coop, a UC Davis professor of evolution and ecology and a senior author on the paper, noted the evolutionary significance of the discovery. Having these two types that reciprocally and synchronously flower male and female allows avocados to exchange and use pollen throughout the whole day,
Coop said.
An Evolutionary Standoff Spanning Millions of Years
These gene variants have been maintained by natural selection for more than 42 million years, representing one of the oldest known genetic balancing acts in flowering plants.
Furthermore, researchers found that these same two gene variants are distributed across at least 26 non-avocado species related to the avocado. 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.
Speeding Up Breeding Programs
The discovery holds practical benefits for agricultural producers and horticulturists.
By linking flowering behavior directly to the SDMYB gene, breeders can now utilize a simple DNA test at the seedling stage. This capability allows researchers and farmers to easily categorize plants long before maturity, significantly accelerating breeding programs and resource allocation.
These genetic markers will greatly speed up avocado breeding and research,
Coop said. This has been a major impediment for avocado breeders.
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