When it comes to sex, plants are pretty freaky. Most plants are firmly rooted in the ground, meaning they can’t simply head out to the nearest social event to meet someone new. Over evolutionary history plants have developed pretty creative solutions to navigate this challenge. As new research reminds us, avocado trees are no exception.
Flowers are one such creative solution. Not merely aesthetic adornments, flowers help plants sow their seed. Anatomically, flowers can separate the pistil (the “female” part that contains ovules, the equivalent of eggs) and the stamen (the “male” part containing pollen, the equivalent of sperm). Once pollinated the ovules will develop into seeds which, once released and germinated, can give rise to a new generation.
The arrangement of these parts is unique to each plant species. Some have both the female pistil and male stamen on the same individual plant, known as monoecious (Greek for “one household”) plants. Others further separate the stamen and pistil on different individuals, known as dioecious (“two households”).
Monoecious plants have a particular challenge because plants generally want to avoid pollinating themselves (self-pollination). To understand why, just consider what it would be like to have mini clones of yourself running around. Your clones would have not just your best qualities but also your absolute worst qualities. Over generations, the bad qualities pile up, limiting survival.
Since monoecious plants carry pistils and stamens on the same individual, the odds of self-pollination are high. One solution for monoecious plants is to temporally separate the maturation of pistil-containing flowers and stamen-containing flowers (known as heterodichogamy). Walnuts, maples, and avocados all use this strategy - separating the male and female sex organs across time.
For example, avocado trees exist as two types: A-types and B-types. A-type trees open pistil-containing “female” flowers early in the day before closing in the afternoon when stamen-containing “male” flowers open. B-types are the opposite: male flowers open early before closing when the female flowers open. Thus, over the course of a single day an avocado tree will change its functional reproductive sex twice.
“Female” and “male” avocado flowers | Groh et al. 2026. PNAS. doi: 10.1073/pnas.2606876123 | CC BY 4.0 | This figure was cropped.
However, it was unclear how these plants control flowering time. New research from researchers at the University of California (UC), Davis and UC Riverside identified a 42 million year old genetic system that avocados use to control these twice daily sex changes. This peer-reviewed research was published in the Proceedings of the National Academy of Sciences in July.
The UC researchers used a genetic technique known as a genome-wide association study (GWAS) to identify the region(s) in the avocado DNA sequence that control whether a tree is A-type or B-type. Their experiment narrowed down the possibilities to a single gene which they named SDMYB.
SDMYB has two different versions, known as alleles, in avocados. One allele is dominant, and a tree with just one (out of two) copies of the dominant allele will be an A-type tree. If instead the tree has two copies of second (recessive) allele, then it will be a B-type tree. The small genetic differences between the two SDMYB alleles are enough to produce the different flowering times for A- and B-type trees.
A-type trees appear to make less SDMYB than B-types. This difference in SDMYB levels between each tree type appears to account for the flowering time for the avocado tree. The UC researchers propose that SDMYB functions to turn genes on and off, and these target genes directly work to promote flowering opening/closing. Future experiments need to confirm this and identify which specific genes SDMYB turns on and off to influence flowering time.
By comparing the DNA sequences of plant species that are closely related to the avocado, the UC researchers were able to estimate that these two alleles are ancient: 42 million years old. The appearance of these two versions of the SDMYB gene in avocados and related species has conferred survival advantages for these monoecious plants by endowing their reproductive sex with natural plasticity to avoid self-pollination.
Ultimately, this study of avocado flowering sheds light on the natural variations in sex and reproduction across life - both on evolutionary time scales and across the life of a single individual. Across the tree of life, sex is complicated. True binaries are rare. Quirky plants like the avocado show us humans that our over-reliance on reductive notions of sex is an aberration, a crime against nature itself. We can and should take inspiration from the avocado tree and its twice daily sex changes. Or, at least embrace human sex’s shifting dynamics in our own lives.
