Mutualism Between Fig Wasps & Fig Trees
Most of us think a fig is a fruit, but for this fascinating story of mutualism we need to look a bit closer to what a fig actually is.
A fruit is generally the fleshy part of a plant that contains seeds. So far, this definition holds true for the fig at a certain phase of its development. But a fig is also an enclosed cluster of numerous flowers.
Flowers contain the reproductive structures essential for plants to carry on their quest to populate. For many flowering plants, a cooperative effort of a pollinator such as a bee, wasp or butterfly is necessary to transfer pollen in order to initiate fertilization.
These pollinators are attracted to flowers based on shape, scent and color. They can not see all the colors we can, but their visual acuity extends into the ultraviolet range which is invisible to us. Because of this, flowers often go to extremes to get noticed by pollinators. That is one reason there are so many amazingly showy flowers.
However, for the Fig Tree the flowers are hidden within the “fruit.” This demands a totally different strategy when it comes to pollination.
And nature has selected a remarkable solution. There are around 750 worldwide species of figs that require pollination. And some say that each type of fig tree has a single species of Fig Wasp from the family Agaonidae, that is responsible for pollinating it. This view is not shared by all but even if there are exceptions, and only 75% of the cases hold true, this is an amazing statistic. Through the process of co-evolution the symbionts are paired up to provide for the needs of the other. The fig is pollinated by the wasp and in return furnishes a place for the wasp to live and breed. This relationship is so interdependent that neither organism would survive without the other.
Although there is some variation between species and the exact interaction of fig and wasp, it generally starts with female wasp. She enters a tiny opening in the fig called the ostiole. The female is laden with pollen, which she uses to pollinate the fig by brushing up against the flowers as she travels through the cavity laying her eggs. After the eggs are laid, the queen dies and her body provides nourishment for the fig.
The eggs that she laid develop and the larvae hatch, feeding on the flesh inside the fig. The offspring mature and mate. In some species there are several females that lay eggs within a single fig, allowing more diversity in the DNA. After mating, the females gather pollen. In the mean time the males dig a hole though the “fruit”, fall to the ground and die or in some cases remain within the fig. This may seem like a pointless act, however, in doing so, the males have provided a means of escape for the winged females.
The females are now free to fly off to another fig and start the whole cycle over again as a queen wasp.
Animals that eat the figs disperse its seeds to a new location when they defecate.
There are also several thousand varieties of parasitic wasps that lay their eggs in the fig by employing long ovipositors to drill though the outer cover. These wasps do not pollinate the flowers since the female does not actually enter the fig. The photo below shows several parasitic wasps. Notice the female just below the middle fig with a ovipositor twice the length of her body.
There is an interesting side note to this story that is worth mentioning. Scientists have discovered that in species where it is necessary for the females to actively gather pollen, if one does not, there are serious consequences. If a female does not bother to collect pollen and uphold her part of the mutualistic arrangement, once she becomes queen of her own fig, if no pollen is delivered, the tree will drop the fig she is in, and the queen’s offspring will be killed.
This story may not sell many Fig Newtons, but it is one of many that shows the amazing creativity, resilience and even partnerships that exist in the world around us.
Note: To ease the minds of those who like figs, a vast majority of them processed for human consumption, are said to be self-polinating.
Header Illustration: Courtesy of www.plantillustrations.org
Curtis’s Botanical Magazine, vol. 61 [ser. 2, vol. 8]: t. 3305 (1834) [Miss Young]