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The swimming of the Ginkgo sperm

by William (Ned) Friedman, Director of the Arnold Arboretum
September 11, 2015

GinkoSperm_DirectorsBlog6

The swimming of the Ginkgo sperm

GinkoSperm_DirectorsBlog6It is happening right now at the Arnold Arboretum – this very week. One of the most bizarre mating rituals imaginable is playing out on our female (seed-bearing) ginkgo trees. Inside the beautiful inch-in-diameter seeds, pairs of huge sperm are being produced by the pollen tubes that grew out of the pollen grains released by the male (pollen-producing) trees last spring. Now, four months later, sperm have been formed in these tubes and are swimming to the eggs contained inside the future seed. (Yes, plants, like animals, produce sperm and egg to make the next generation.)  An amazing thing to contemplate this unseen world inside a ginkgo seed.

So, why would a ginkgo tree, whose seeds may be found more than a hundred feet in the air, resort to swimming sperm to find a mate? The answer is rooted in history. The ancestors of all land plants (green algae) had swimming sperm that sought out (swam to) eggs to create the next generation. A pretty good idea if you live full time in the water.  Amazingly, after all of these years (475 million, to be precise, since the first green algae colonized land), sperm of ginkgo trees, cycads, ferns, horsetails, mosses, and other groups of plants still find their egg-mates by swimming, tracking the chemical signals that the conspecific (same species) egg is emitting to attract a suitor. Truly, this is a vestige of evolutionary history.

Pictured here are seeds of Ginkgo biloba ‘hayanari’ (824-83*A) from the grounds next to the Hunnewell Building. The third image is of a ginkgo sperm taken by a Japanese botanist, Tamaki Shimamura, in 1937. While a human sperm has a single “tail” (called a flagellum) that permits it to swim to an egg, a ginkgo sperm has approximately one thousand flagella! The surprising life cycle of the ginkgo tree was discovered in Japan in 1896 by a botanical research illustrator, Sakugoro Hirase, and this is a story in and of itself.  You haven’t really lived until you have seen a ginkgo sperm swimming under the microscope.  Trust me on this.

 

-Ned Friedman, Director of the Arnold Arboretum

3 thoughts on “The swimming of the Ginkgo sperm

  1. What do they swim through? Ginkgo trees do not live under water. Do they swim through the air? I’ve been through a book and two articles none of which explain how the sperm from the male tree reach the seeds. They just say they swim! I can’t swim from one tree to another even though I could swim from one clump of seaweed to another in the sea.

  2. Reply from Professor Friedman: In the spring, pollen flying through the air hits a small drop of viscous fluid on the small future seed. The fluid (pollination droplet) is retracted into the future seed, where the pollen grain will germinate and grow into a microscopic set of tubes that feed off of the tissues of the future seed. In the fall, the future seed is now fully enlarged and the pollen grain that germinated and produced a pollen tube forms two sperm with about 1000 flagella each (human sperm has one flagellum). These two sperm are inside of the future seed and when the signal arrives for them to burst out of the pollen tube, they do so. At that time, the future seed also forms a small swimming pool of fluid just above the eggs. The sperm swim into this pool and then track a chemical signal to find the eggs and fuse.

    So, when you look up at a female ginkgo tree in the fall, way up there, are lots of swimming sperm inside of the future seeds. All of this is a vestige of the ancestors of all land plants, that were algae, which naturally would rely in swimming for sperm to meet eggs.

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