Can trees really change sex?

November 5, 2015 by Caroline Wright, The Conversation
Credit: Willie Angus, CC BY-SA

The revelation that the UK's oldest tree is showing signs of switching sex has sparked much excitement in the world of horticultural science. The Fortingall yew (main image) in Perthshire, Scotland, having apparently spent 5,000 years as a male tree, has suddenly produced female berries. So what is going on?

Plant genders actually come in more varieties than the likes of humans. Many flowering bear flowers that are hermaphrodite, for example, with both male and female reproductive organs in every flower. There are quite a few in the rose family, for instance. Many hermaphrodite flowers have evolved complex mechanisms to ensure that they rarely pollinate themselves. This helps a species to endure by ensuring that different plants mix their genes.

Another plant gender variety is known as "monoecious", which refers to species that produce separate male and female flowers on the same plant. Anyone who has grown courgettes or cucumbers will recognise that only some of the flowers bear the swollen ovary at the base (which will become the courgette). The ones without are the males. In other words, when you eat a courgette you are eating the plant's ovaries.

There are also many species with more human-like genders, with individual plants having either male or female flowers. Known as "dioecious", they include many coniferous plants but also several flowering shrubs and non-coniferous trees.

Intersex and proud. Credit: T.Kiya, CC BY-SA

The within these species bear the flowers containing the ovary, style and stigma (collectively known as the pistil – see opposite). This will later produce the fruit. Knowing which is which is vital both for growing crops and flowers. If you want to produce kiwi fruit, for example, you need a female plant to bear the fruits and a male plant close by for pollination – the term "the birds and the bees" should not need explanation. Many decorative plants are also selected for their gender. If you have ever wondered why your holly doesn't bear any berries, it is because it is either a male or a very lonely female.

For those seeking to buy a plant of a particular gender, to produce fruit, say, there is a complication. You can't tell the gender of a plant grown from seed until it reaches sexual maturity and its flowers can be assessed, which can take many years. Nurseries usually get around this by taking a cutting or making a graft from a plant they already know the gender of. The new plant that this produces will have the same gender (with grafting, this sometimes means changing the sex of the stem of the other plant that was used).

In some cases, a buyer will want to know the gender of a plant for a different reason. One good example is the gingko, a large shade tree that is native to China. Female gingkos are most unwelcome as street , since their fruits give off a similar scent to a skunk. Town planners will make sure they select males to avoid this repulsive pong.

Yew are my everything

Yews are one of the species that clearly divide into male and female plants. You do occasionally see male flowers on female yews and vice versa, but you wouldn't expect it on as ancient a tree as the Fortingall. Yet having been male for all of living memory, it has definitely produced female flowers and red berries on some shoots.

It is possible that the tree has produced what is known as a "sport", which is a new growth that is morphologically different to the rest of the plant. This is relatively common. If so, you would expect to see a different type of foliage or a different colour or type of flower.

5,000 years old and full of surprises. Credit: Liz West, CC BY-SA

Either way, none of this means that the tree has technically changed sex. Some coniferous have been known occasionally to change sex, though scientists don't understand why. Indeed they are studying the Fortingall berries in the hope that it will further their understanding.

It is possible that the Fortingall's female flowers will spread, but it is unlikely that the whole tree will become female. Female plants require more water and nutrients than males in order to produce their fruits and seeds. In an ageing tree, a complete change would be a great source of stress. For an ancient gent like this one, that would probably be much more trouble than it was worth.

Explore further: Ancient British tree undergoing 'sex-change'

Related Stories

Ancient British tree undergoing 'sex-change'

November 2, 2015

A British tree thought to be up to 5,000 years old has started to change sex, a "rare and unusual" phenomenon not fully understood by scientists, a botanist said Monday.

Extra DNA creates cucumber with all female flowers

June 4, 2015

Ask a plant researcher how the sex of a cucumber plant is determined and the person will tell you, "It's complicated." Depending on a complex mix of genetic and environmental factors, cucumbers can be seven different sexes. ...

Threat posed by 'pollen thief' bees uncovered

October 9, 2015

A new University of Stirling study has uncovered the secrets of 'pollen thief' bees - which take pollen from flowers but fail to act as effective pollinators - and the threat they pose to certain plant species.

Science casts light on sex in the orchard

October 30, 2014

Persimmons are among the small club of plants with separate sexes—individual trees are either male or female. Now scientists at the University of California, Davis, and Kyoto University in Japan have discovered how sex ...

Recommended for you

Scientists shed light on biological roots of individuality

February 16, 2018

Put 50 newborn worms in 50 separate containers, and they'll all start looking for food at roughly the same time. Like members of other species, microscopic C. elegans roundworms tend to act like other individuals their own ...

Plants are given a new family tree

February 16, 2018

A new genealogy of plant evolution, led by researchers at the University of Bristol, shows that the first plants to conquer land were a complex species, challenging long-held assumptions about plant evolution.

1 comment

Adjust slider to filter visible comments by rank

Display comments: newest first

Moebius
not rated yet Nov 05, 2015
Marijuana changes sex and probably other plants too, why the surprise?

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.