Not just another Newton

Flowers release chemical cues to attract pollinators, some smell pleasant to humans while others smell like feces or rotting flesh, the chemical signature is designed for each plants’ ideal pollinator. But what happens when the flower is inside of the fruiting body? The answer is complex but we are starting to understand.

chemical-compound-flower

Our story starts with the delicious and nutritious fruiting bodies of the ficus genus, figs. Unlike some other plants which attract bats, birds, bees, flies, butterflies or moths to pollinate their flowers, figs depend on wasps only a few millimeters long. You see, the flowers of figs are inside the unripe fruit and only these tiny wasps can crawl inside the ostiole (small opening) of the fruit. Without fig wasps, figs would not survive. Figs and fig wasps have evolved together for 70-90 million years. There are around 850 species of figs and each has at least one dominate pollinator (wasp species). Frequently there will be two species or an entire list of wasp species that pollinate a single species of fig. These wasps have mostly been identified morphologically, leading scientists to believe that we only described a small portion of the existing species.

pollinators

Diversity of pollinators: hummingbirds, sweat bees (copyright Clay Bolt/claybolt.com), bumblebees (copyright Clay Bolt), and bats.

The female wasp who enters the fig is known as the foundress. The foundress will enter the fig and crawl to a cluster of flowers inside known as the inflorescence. There she will pollinate some of the female flowers and lay her eggs, both male and female, before she dies. Galls form where the eggs are deposited. Male wasps emerge from the galls first. The only job of the males is to fertilize their sisters who are trapped inside galls and chew a hole to let the females escape. Then the males will die, never leaving the fig, but the females emerge fertilized and gather pollen before finding a new unripe fig to deposit their eggs. Their life outside the fig sometimes only lasts a day. Sounds bizarre, right? Well fig wasps aren’t the only insects with such behavior. Many species of Heliconius butterflies exhibit pupal mating strategies.

dsc_2878

Male Heliconius butterfly mating with a female pupa. Notice an adult female has deposited two eggs on the female pupa’s head.

All figs on a particular tree will ripen at the same time so that when the female fig wasps emerge they are forced to go to another tree. How does a tiny wasp find another fig tree of the same species with unripe fruit over a long distance? Well scientists believe that they use olfactory signals and chemostimulation (Gibernau & Hossaert-McKey 1998). Finding another tree doesn’t ensure oviposition, in fact some females must fight to the death to crawl into a fig to pollinate it, deposit eggs, and die. Aggressive species will even decapitate competitors (Dunn et al. 2015).

chemical-attractants-of-figs

Gas chromatography showed that receptive fig extract is composed of ten compounds. The principal chemical compounds, making up over 75%, are linalool, linalool oxide, and benzylalcohol (Gibernau & Hossaert-McKey 1998).

The relationship described here is a mutualism between figs and fig wasps but nature is known to be filled with cheats and fig wasps are no exception. What happens if a foundress decides not to pollinate the fig in which she deposits her eggs? Frequently fig trees will abort any fruit that has not been pollinated and this will kill the wasp’s eggs in it, punishing the cheaters.

fig-wasp-mutualism

Mutual relationship between figs and fig wasps on the left. Cheating situation on the right where the female fig wasp never pollinates the fig, only deposits her eggs.

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Adult female fig wasps emerging from a fig.

So, the question becomes, are you eating helpless baby wasps when you grab a fig newton? The answer is no. Most commercial fig production comes from dioecious trees meaning there are two sexes. The male fig tree will produce fruit which will have the wasp galls and when the wasps hatch they will pollinate the fruits of both the female and male trees but will only deposit their eggs in the fruit of the male tree. Thus, leaving those figs of the female tree with only seeds inside. Some people call the figs of the male tree goat figs because people will purchase a bag of the figs from a male ficus and hang it in their female tree to make use of the wasps but then they collect only the fruit from the female tree and feed the left-over goat figs to…well their goats. Goats have such an extensive history of eating figs that some fig wasp species have changed their oviposition site due to goat predation (Zamora & Gómez 1993). Some commercial varieties of figs are sterile, like seedless watermelons or bananas, and do not require pollination. Other ficus trees are monoecious meaning the tree will have both male and female flowers on the same tree. These figs will have the seeds and wasp galls. If you ate the figs of these trees, you might end up with a dissolved foundress, the dead males or some wasp larva but figs produce an enzyme (ficain) which digests any dead wasps and use those nutrients to grow so it is unlikely you’ll find any presents. Knowing all this, will it affect your consumption of figs? Hopefully it just makes you appreciate this supple fruit even more and its little wasp friends. If it still grosses you out, then fig get about it.

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References

Dunn DW, Jandér KC, Lamas AG, Pereira RAS. 2015. Mortal combat and competition for oviposition sites in female pollinating fig wasps. Behavioral Ecology 26(1): 262-268.

Gibernau M, Hossaert-McKey M, Frey J, Kjellberg F. 1998. Are olfactory signals sufficient to attract fig pollinators? Ecoscience 5(3): 306-311.

Zamora R, Gómez JM. 1993. Vertebrate herbivores as predators of insect herbivores: an asymmetrical interaction mediated by size difference. Oikos 66: 223-228.

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