A fascinating tree on Earth has flowers that change gender every season, a remarkable trait it has maintained for 40 million years.

Nature holds an incredible number of secrets, and one of them is how plants manage to avoid self-pollination. For a long time, scientists had no answer to this question until they discovered that walnut trees evolved to prevent such confusion: they time their male and female flowers to bloom separately, with one gender flowering first and the other second, reports IFLScience.

The first to notice this was Charles Darwin. In 1877, he found that walnut trees could differ in whether they bloom with female or male flowers. Researchers also found that this behavior was observed among both domesticated walnut trees and their wild relatives. Interestingly, the ratio of female to male flowers typically stands at about 1:1.

It took another century for scientists to determine that the key to this phenomenon lies in a single genetic locus — this was discovered by Scott Gleeson, a graduate student at the University of California, Davis. However, it was still unclear to scientists how this process occurs.

In a new study led by Jeff Groh, a graduate student in population biology at the University of California, Davis, researchers uncovered insights into sex determination in the Juglandaceae family of trees, which includes walnuts, hickories, and pecans.

During the study, scientists gathered data from a walnut breeding program as well as from local black walnut trees. The team identified which of these flowers were female and which were male, and then sequenced their genomes to look for patterns related to this phenomenon. The results showed that there are two variants of a gene that the scientists identified for walnuts. It also became clear that this gene influences which sex will bloom first. This system was discovered in at least nine species of walnuts and has remained stable for an impressively long time — at least around 40 million years.

The study also examined pecan trees. The results suggest that they too have a flowering strategy controlled by a distinct genetic region, but in a different part of the genome than that of walnut trees. Scientists believe that the pecan system evolved 10 million years earlier than that of the walnut.

The research indicates that this system seems to work in favor of maintaining a 1:1 balance between female and male trees. According to Groh, it is quite unusual for trees to sustain this variation over such a long period.