Ginkgo: A Sexual Curiosity
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From above, the trees appear as a mass of vibrant yellow covering the western corner of Blandy Experimental Farm in Boyce, Virginia. At eye level, rows of tree trunks shade grassy hillocks that abut cattle and horse pasture on one side and a patch of forest on another, all framed by a winding gravel road. Amidst the grass itself, seeds sit enclosed in a pungent, fleshy covering that lends an inescapable aroma to the air of the grove. In China, these trees are called “silver apricots.” In the United States, we know them better by the Chinese-derived scientific name, Ginkgo biloba, or just “ginkgo.”
Ginkgo are not native to North America, or even the Western Hemisphere in general – their modern center of origin lies in Southeast Asia, having been spread around the world by scientists, explorers, gardeners and traders. The ones at Blandy, part of the arboretum managed by the University of Virginia, arrived here as a result of scientific inquiry.
A female ginkgo in spring, identifiable by the ovules which will develop into stinky seeds.
While some plants possess both male and female reproductive parts simultaneously, ginkgo do not – individuals are either male or female. The stinky seeds for which the tree is famous develop only on females, appearing in the fall and dousing the ground with offspring. Male trees, meanwhile, produce no seeds, but release pollen in the spring. This pollen fertilizes the females and allows for seed production in the fall. Without pollen, the seeds will not be fertilized, just as without females, pollen will simply go to waste. There is, however, a catch: although seeds are born male or female, it often takes about 20 years before the tree reaches maturity and displays distinct sexual characteristics. Identifying the sex of young ginkgo individuals is not yet possible unless a male or female branch has been grafted onto rootstock.
In 1929, biologist Dr. Orland E. White, the first Director of Blandy, had a question: of the seeds that a female ginkgo tree produces, what proportion are male and what proportion are female? White hypothesized that the sex ratio would be 1:1, meaning that the tree would produce approximately the same number of female seeds as male seeds. Taking seeds from a female ginkgo on the campus of the University of Virginia, White and his students eventually planted more than 600 saplings in what is now the Ginkgo Grove at Blandy. The experiment was simple: take the seeds from a female ginkgo, plant them, and determine the total sex ratio upon maturity. Twenty years later, researchers went back and recorded the sexes of the trees that had survived to maturity, tallying a final ratio right around 1:1. Although White did not live to see the experiment to its end, his hypothesis proved correct. Beyond answering the question of sex ratios in ginkgo offspring, the Ginkgo Grove is an enduring part of Blandy’s collections, brilliant leaves coloring the sky in autumn.
The evolutionary history of ginkgo goes back hundreds of millions of years. At one point it grew in forests around the world, coexisting with dinosaurs and early mammals. There was more than one species of Ginkgo plant – Ginkgo biloba is the sole remaining member of a long evolutionary lineage. As climates shifted and the world changed, though, Ginkgo species disappeared and lost habitat, eventually only surviving in what is now China. You can learn more about ginkgo’s evolution, and how it reemerged from China to populate streets and gardens around the world, in one of our earlier blog posts.
Although ginkgo trees have distinct sexes, this is not always set in stone. As Peter Crane recorded in a paper he wrote with Toshiyuki Nagata of Hosei University in Japan and several other colleagues, there are examples of individual branches on ginkgo trees changing sex from male to female. The phenomenon is rare, making it difficult to study. This difficulty is only compounded by the 19th century practice of grafting female branches onto male ginkgo trees around Europe, making some of the trees across the pond not ideal for this research. Studying sex-switching in predominately female ginkgo trees is much more difficult than examining the phenomenon in male ginkgo. In some years females do not produce seeds; and even when they do, for all but a brief period in the spring, a male branch appears as an innocuous, seedless patch on the otherwise fruiting tree. However, on a tree that is definitely male, a branch with seeds sticks out like a sore thumb. In Japan, Crane, Nagata and their colleagues recorded an example of a male tree with one female branch that produces ovules – but this particular tree happens to be a national monument, which means that sampling and study is strictly controlled and not especially easy.
Peter Crane examines a ginkgo branch up close from a tracked lift.
The Blandy ginkgo grove holds promise for elucidating how a Ginkgo tree changes sex in a way that the other trees do not. The trees have not undergone any grafting. Earlier this year, Crane and Nagata visited the ginkgo grove and identified at least one individual with sex-switching branches – a male tree with a small branch covered in seeds. This find is exciting, but the work of collecting samples and teasing out the molecular and genetic components of the sex-switching must now wait until the spring, when new growth appears. Then we will return to the ginkgo grove and seek more answers into how the trees are changing. With the aid of Blandy staff, we can take samples from sex-switching individuals and discover the processes going on at the molecular level.
Why change sexes to begin with? The evolutionary factors pushing the ginkgo towards having the occasional sex-changing branches are not yet entirely clear, but we have some ideas as to why the trees–especially males–exhibit a somewhat fluid sexual expression. For an isolated male, growing a few seeds on a single branch can act as a sort of failsafe. Of the millions of pollen grains the male produces, only a few are needed to fertilize the seeds, allowing for self-fertilization at the same time that the individual can still send out enough pollen to pollinate a female, should one happen to be nearby. If a female produces male pollen cones, however, it would only take a few pollen cones to fertilize an entire tree’s-worth of seeds. Instead of acting as an evolutionary backup plan, a female that produces male pollen could in fact prevent pollination by other male trees, reducing genetic diversity and resulting in less healthy offspring. For this reason, so-called “leaky” males (males which grow a few female branches) are more likely than leaky females.
This sex-switching may be one of the ways that ginkgo has cheated extinction for so long. Through millions of years, this tree has survived almost unchanged, thriving, declining, and rejuvenating through processes that we are still working to comprehend. In the midst of scientific inquiry and horticultural interest, careful cultivation and careless dispersal, processing for edible seeds and protection for spiritual and cultural heritage, ginkgo has continued to thrive, spreading with the help of people out of China and across the world. As we continue to discover the complicated life story of Ginkgo biloba, the trees at Blandy stand ready for the winter cold, yellow leaves waiting to drop in synchrony to the ground.