Handiness in flowers
Twist to the left, twist to the right. Mirror image flowers place their male and female organs on opposite side of the flower midline, preventing self-fertilization. But to ensure pollination, it helps if half of the flowers of a species are oriented the opposite way. Now a group of international researchers report that how South African butterfly lily (Wachendorfia paniculata) ensures this is remarkable similar to how sex in humans is decided.
In plants flowers can be right- or left-handed. In right-handed flowers the female style is located on the right of the flower midline, while the majority of its stamen are located on the left side of the midline. Left-handed flowers are organised in the opposite way. This prevents self-fertilization. But as pollinators are dusted with pollen on only one side of their body, pollination would only occur if some flowers of the population have an opposite handiness. In some species flowers on a single plant alternate, in other species an individual is either left- or right-handed. In both cases it is not clear how handiness is regulated.
Twisting styles
The authors of this latest paper wanted to find out and singled in on the South African butterfly lily, in which individuals are either left- or right-handed. After confirming that is indeed the case the researchers observed that the handiness was already noticeable in the closed buds. A time-lapse video showed twisting styles, indicating that the handiness is not simple bending to one side. Zooming in individual cells of the style, the researchers noticed that those at the base of the style where twisted.
After accidentally growing some of the buds upside down the researchers noticed that they showed a handiness opposite of what was expected. Suggesting gravity was involved. Looking at this more closely, showed that the cells that were at the bottom stretched themselves out more than those at the top, which enabled the twisting of the style or stamen in one direction. But growing the buds upside down means that the cells that where at the bottom are now at the top and the other way around, leading to a twist in the other direction.
Two to tango
To confirm that twisting cells and gravity are the only things that are needed to introduce left or right-handiness, the researchers put the whole thing through a simulation. Confirming that indeed twisting and gravity was all what was needed. What’s more, they are needed together, without one, the other has little effect, and the style and stamen don’t twist and are located in the middle of the flower. This was subsequently confirmed experimental.
After knowing how, mechanically, flower handiness works, the researchers wanted to know the genetics behind it. So, they sequenced left- and right-handed South African butterfly lilies. First, they found that in right-handed plants had a YUCCA-R gene while this gene was absent in the left-handed plants.
Underlying genetics
Diving in deeper to find out where on the genome this YUCCA-R gene was located let to the discovery that left-handed plants were missing a small stretch of a chromosome. In right-handed plants this stretch of DNA contained the YUCCA-R gene and two other genes.
Following up on those three genes, the researchers found that YUCCA-R is responsible for the twisting the style to the right, while the other two genes where responsible for the stamen twisting the opposite direction. If YUCCA-R or those two other genes are absent, then the stamen and style are located on the same side of the flower.
So, the presence or absence of that small stretch of chromosome determines if the flowers of the South African butterfly lily are right- or left-handed. This kind of resembles how in humans the presence or absence of the Y-chromosome determines if the individual is male or female. But also like in humans, it is not absolute, gravity and additional mutations also have their say.
Literature
Haoran Xue et al., Supergene control of chiral development in mirror-image flowers. Science 393, eaeb1157 (2026). https://www.science.org/doi/10.1126/science.aeb1157
Or for the BioRxiv version https://www.biorxiv.org/content/10.1101/2025.08.29.672924v1
Plant en Zo 