Figs: Possibly Nature’s Strangest Fruit

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A common fig (Ficus carica): the fruits are there, but where are the flowers? Source: blogs.ubc.ca

The genus Ficus (fig) includes more than 850 species in the Moraceae family and occurs naturally on all continents except Antarctica. Figs are generally tropical, but some rare species tolerate warm temperate climates, including the common fig (Ficus carica), a well-known Mediterranean fruit.

Extremely varied, the genus includes tall trees up to 40 m tall, shrubs, epiphytes, climbing plants and also the famous “strangler figs,” hemi-epiphytes that germinate on the branch of a host tree, then end up strangling it and taking its place as a forest giant. Their abundant fruits of figs are a vital element of the diet of many rainforest animals. In fact, some birds and bats feed almost exclusively on figs … and deposit fig seeds in their excrement, thus helping spread the species.

Fruits Unlike Any Other

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The tiny fruits of the weeping fig (Ficus benjamina) may be tiny and hard, but they’re still figs. Source: Dinesh Valke, Wikimedia Commons

The fruits of all Ficus species are called figs, whether they are the relatively large, edible fruits of the common fig tree (F. carica) that you find fresh or dried at the market or the tiny hard fruits of your living room’s weeping fig (F. benjamina). And they’re all really very strange!

What is so odd is that fig trees never seem to bloom. With other fruits, a flower almost appears first, often covering the tree or shrub in spectacular bloom. The flower then turns into the fruit you harvest. But that’s not the case with figs. You only see small a fig that starts to form, then grows in size and eventually changes color to show the world it is ready to harvest. But there were no flowers in sight. Fruits without flowers? How is that even possible?

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Fig flowers are found inside the fruit. Source: www.tropicalfloridagardens.com

In fact, though, the fig tree does have blooms, but the flowers are inside the small “fruit” in formation. Technically, a fig is not a simple fruit, but an inflorescence: a fleshy, hollow receptacle whose inside surface is lined with numerous tiny flowers. This type of multiple fruit is called a syconium, a word usually only seen in crossword puzzles. (For convenience’ sake, I’ll be using the term fruit instead of syconium in this text.)

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Banyan (Ficus benghalensis) fruits. Note the small hole (ostiole) at the tip of each one. Source: Joe Mabel, fr.wikipedia.org

You’ll notice that there is a tiny hole at the tip of each fruit. It is through this hole (called an ostiole) that the pollinator reaches the flowers. When the fruit is ready to be pollinated, the ostiole opens marginally and the fruit emits an enticing scent containing specific pheromones to attract a female wasp of the right species.

And yes, it has to be just the right kind of wasp. Most fig species have their corresponding fig wasp. This is an example of extreme mutualism, because the fig cannot reproduce without its exclusive wasp and the wasp can only feed on its specific fig species. This is a big evolutionary risk to take, because if the fig species were to disappear, the wasp would quickly follow it into extinction and if the wasp were to be wiped out, the fig tree would have no pollinator!

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Fig wasp. Source: JMK, Wikimedia Commons

The tiny wasp penetrates through the ostiole*, bringing pollen from another fig tree of the correct species. While making its way through the tight hole, the female damages her antennae and her wings are shredded, but that’s all right, as she’ll never need them again. Once the female is inside, the ostiole begins to close again in order to prevent predators from taking the same route. However, often more than one female manages to work her way inside before the opening is completely plugged. When inside, the female visits the tiny flowers—there can be dozens or even hundreds of them!—inside the fruit, laying eggs on some and pollinating others. Then she dies, her role accomplished.

*In some species, the female wasp does not penetrate the fruit, but inserts its eggs from outside by means of an ovipositor, always passing through the ostiole.
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Immature fig cut open, showing the fig wasps inside. Source: www.naturamediterraneo.com

The larvae hatch and consume some of the small seeds that are forming … but that’s not a problem: the fig has planned for this. It’s more than willing to sacrifice a few seeds so that the others can mature.

In most species, wingless male wasps fertilize female wasps inside the fruit and then die, never having seen the light of day. The female, now pregnant, leaves the fruit, having to cross through a cluster of fertile male flowers as it leaves and thus picks up pollen. She then flies to a young fruit of the right species at the appropriate stage of maturity. She enters the young fruit … and the cycle repeats itself.

There are a lot of variants on this cycle. In some species, for example, the male wasp does leave the fruit before fecundating the female, although, wingless as he is, he doesn’t go far. To get out, he “drills” a hole out through the side of the fruit and waits just outside. The females are then able to work their way out the hole he prepared and hopefully one will mate with him. He then dies and the female abandons him, flying off in search of a new fig to pollinate.

Are There Dead Wasps Inside Figs?

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Mature fig: each seed is surrounded by a juicy fruit… and all the dead wasps have been absorbed. Note the ostiole, the opening that gave entrance to the female fig wasp, is still visible. Source: yle-the-hacker, Wikimedia Commons

And for those who are picky about what you eat, you can rest assured. When you eat a fig, you are not consuming the cadavers of the wasps that died inside. As it matures, the fig produces an enzyme that digests dead wasps and uses the resulting minerals for fruit development. So the fruit is perfectly bug-free by the time you eat it!

Fruits Without Pollination?

Wild figs as well as many cultivated varieties of common fig (F. carica) are pollinated by wasps as described above … but some common figs are exceptions to the rule. Many cultivars are parthenocarpic, that is, they are able to produce fruit without pollination. Since they aren’t pollinated, they don’t produce viable seeds either.

Parthenocarpic plants occur occasionally in nature, usually by mutation. As these plants either produce no seeds or infertile ones, they can’t reproduce in the wild and are quickly eliminated by natural selection.

In cultivation, though, parthenocarpic fruits are often considered desirable and are often carefully maintained for generations by humans through asexual means of propagation (cuttings, grafts, etc.). After all, who wants to have spit out banana and orange seeds? (Yes, originally bananas did produce huge and indeed, very hard seeds you had to dispose of somehow.) Or to have to grow space-hungry male persimmon trees as pollinators when a parthenocarpic female can do the job on all her own? Or to ensure the presence of pollinating wasps of just the right species in just the right place at just the right time when you could instead use parthenocarpic fig trees to produce fruit and never need worry about needing fig wasps.

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‘Brown Turkey’ fig is just one of many figs that is parthenocarpic. No wasp is needed for fruit production. Source: 3fatpigs.co.uk

Indeed, the wasp that pollinates the common fig (F. carica), Blastophaga psenes, is not found everywhere fig trees are grown today. The fig tree and its wasp are native to the Mediterranean, but while it has been relatively easy to acclimatize figs trees to other areas, the wasp has been less accommodating. In cooler climates, in particular, you can often grow figs (although you may have to bury them in trenches over the winter to help them survive), but the common fig wasp simply won’t take cold winters. Fortunately, that’s not a problem, since there are many parthenocarpic varieties that don’t need pollination!

And the history of parthenocarpic figs goes back a long, long way. In the Middle East, archaeologists discovered preserved fruit from a parthenocarpic fig tree estimated to date back at least 11,200 years! That’s well before the domestication of the second-oldest crop in the sector: wheat! So humans have been growing parthenocarpic figs since the very beginning of agriculture!


Figs: their flowers may be insignificant, but their fruits are definitely full of surprises!20180422A blogs.ubc.ca

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