Sex is Weird: Figs and Fig Wasps

In this episode of Sex is Weird: a coevolution double feature. Coevolution is when an evolutionary change in one organism is triggered by a change in a different organism. This will probably sound familiar in the context of the Red Queen Hypothesis, but the relationship does not have to be antagonistic. Many flowering plants rely on specialized pollinators, who in turn rely on the flowers in some way.

Bees and flowers are a classic example of two species that evolved to depend on each other. The bees get a tasty nectar snack, while the flower spreads its pollen to another plant. Photo credit: John Severns; wikimedia commons.

Bees and flowers are a classic example of two species that evolved to depend on each other. The bees get a tasty nectar snack, while the flower spreads its pollen to another plant. Photo credit: John Severns; wikimedia commons.

A delicious fig. You can see where the ostiole would be on the bottom of the fruit. Did you know that there are over 800 species in the Ficus genus? That's a lot of figs. Photo Credit: user byrev; pixabay.com

A delicious fig. You can see where the ostiole would be on the bottom of the fruit. Did you know that there are over 800 species in the Ficus genus? That's a lot of figs. Photo Credit: user byrev; pixabay.com

That’s how it all started for figs (Ficus spp.) and fig wasps (of the Agaonidae family). Everyone knows about those delicious fruits that are a staple of humans, monkeys, birds, and other animals all over the world. You may be surprised to learn that they don’t grow wrapped in a pastry crust, and that they’re not even fruits. When you eat a fig (would you call that a non-Newtonian fig?), you’re eating a cluster of flowers called a syconium. It's a thick bulb filled with tiny flowers that has a small opening at the end (an ostiole) just big enough for a fig wasp to enter. The wasp emerges from one fig covered in pollen, flies to the next one, and then crawls inside and spreads the pollen to the new fig. Sounds pretty normal right?

Now let’s take a closer look at the pollinator. It turns out that, for many Ficus species, the only animal that can fit through the ostiole is a fig wasp. Normally when a pollinator visits a flower it is looking for food (usually a bit of nectar). When a female fig wasp enters a fig she’s there to lay her eggs. As the wasp deposits her eggs she also spreads the pollen that she was carrying to the flowers. After laying her eggs, the female dies, and her corpse is broken down by the acids in the fig.

When the female wasp arrives, only the female flowers in the fig have developed. After the flowers are fertilized they begin to produce seeds (which is, after all, the whole point of flowers), and male flowers begin to develop.

A female fig wasp (Ceratosolen sp.) collected in south Taiwan. Not every species of fig wasp is a pollinator like this one. Some have modified stingers that they use to deposit eggs through a fig's skin. Photo credit: Anthony Bain; wikimedia commons.

A female fig wasp (Ceratosolen sp.) collected in south Taiwan. Not every species of fig wasp is a pollinator like this one. Some have modified stingers that they use to deposit eggs through a fig's skin. Photo credit: Anthony Bain; wikimedia commons.

While this is going on the wasp eggs are developing into mature wasps. The males develop faster than the females do, so they reach adulthood first. They have no wings, and will die shortly after completing two important tasks: mate with the still developing females (those are siblings, by the way) and dig a hole to the outside. Interestingly, if there are multiple males in the same fig, they will work together to dig the exit tunnel. No other insects are known to show post mating cooperation like this (Suleman 2012). The females use the exit made by the male and fly off to find another fig.

By the time the female wasps emerge, the male flowers have fully developed and are producing pollen. The wasps crawl towards the outside world picking up pollen as they go. Since they are already pregnant, the wasps will immediately search for an unfertilized fig where they can lay their eggs and start the cycle again. With the wasps gone the fig is fully ripe and ready to be eaten by a monkey, a human, or whatever else comes along. The seeds go with the animal to be deposited in their own little pile of fertilizer.

So, a bunch of flowers that look like a fruit can only be pollinated by an inbred wasp that is looking for a place to lay eggs that will develop and mate inside the flower/fruit. Sex is weird.

References

"Aggression prevents the better part of valour… in fig wasps" University of Leeds. Date: Decemper 1, 2011. Web. http://www.leeds.ac.uk/news/article/2716/aggression_prevents_the_better_part_of_valour_in_fig_wasps

Kline, Katie. "The story of the fig and it’s wasp." Ecological Society of America. Date: May 20, 2011. Web. http://www.esa.org/esablog/research/the-story-of-the-fig-and-its-wasp/

Suleman, N., S. Raja, and S. G. Compton. 2012. Only pollinator fig wasps have males that collaborate to release their females from figs of an Asian fig tree. Biology letters 8:344-346.