Fig trees and their pollinating wasps, which are principally groups of tropical organisms, are obligatory mutualists, a form of symbiosis wherein the survival of one partner requires the survival of both. The fig-wasp feedback loop works as follows:
Fig wasps began to pollinate and co-evolve with figs 90 million years ago, even before continental drift separated what we today think of as the Old World and New World groups of figs, of which there are over 750 recognized species. As for the fig wasps, they form a complex of cryptic species that evolved separately for more than 1.5 million years. A cryptic-species complex is a group of species that satisfies the biological definition of species—that is, they are reproductively isolated from each other but are virtually indistinguishable on a morphological basis. The only way to tell them apart is through DNA sequencing.
Some cryptic species of fig wasps are actually sibling species, which means they not only shared the same ancestor but also probably evolved within a single species of host fig or very closely related species. However, genetically identical wasps may also be found in fig hosts of two different species, a finding that suggests new associations are formed now and then.
Each species of fig has one or more species of small wasp, the female of which pushes her way into the fig while it's still green and hard. As she squeezes herself inside, her wings are torn off. Once within the fig, she is confronted with three kinds of flowers: male, short female, and long female.
To fulfill her life's purpose, she must make her way past the immature male flowers, which, as yet, have no pollen. She then moves further down into the hollow, where she dusts the female flowers with the pollen she brought from the fig in which she grew up. A female wasp can only reach the ovaries of the short female flowers with her ovipositors, in which she lays her eggs and dies soon after. Meanwhile, the long flowers become the fig's seeds.
The eggs laid, the fig tree chemically detects their presence and surrounds them with plant tissue. The eggs hatch and the fig provides the rapidly developing larvae with enough food to grow and restart the cycle. While the larvae grow inside the fig, the male flowers mature.
The male wasps are born first, but look nothing like a wasp because they have neither eyes nor wings; yet they soon detect the baby females and mate with them. Before their brief lives are forfeited, the males perform one other duty: they enlarge the original entrance, which allows the females to exit with their wings intact.
For their part, the female wasps are dusted with pollen as they squeeze past the male flowers to exit the fig. Once outside, they carry the pollen to another green fig, which they, in turn, will pollinate on their way to laying their eggs. After the wasps have left or died, the figs become bright in color, soft, and succulent as they ripen.
In return for providing a home and nourishment for the wasps, fig trees get their flowers pollinated. Without their tiny symbiotic wasps, the figs would not ripen, and the tree would eventually become extinct. Nevertheless, the population of wasps can be maintained only if figs are produced year-round, and because individual fig trees flower synchronously, the wasps that pollinate them have to locate a new individual host tree at each generation.
ENDNOTES
The preceding story of figs and fig wasps is based on: (1) Marie-Charlotte Anstett, Martine Hossaert-McKey, and Doyle McKey. Modeling the Persistence of Small Populations of Strongly Interdependent Species: Figs and Fig Wasps, Conservation Biology 11 (1997): 204-213; (2)Drude Molbo, Carlos Machado, Jan Sevenster, and others. Cryptic Species of Fig Pollinating Wasps: Implications for the Evolution of the Fig-Wasp Mutualism, Sex Allocation and Precision of Adaptation, Proceedings of the National Academy of Sciences 100 (2004): 5867-5872; and (3) Ian Giddy. Cloudbridge Nature Reserve, Nature Notes, no. 19 (2004), http://cloudbridge.org/fig-wasp.htm (accessed on January 25, 2009).
©chris maser 2009. All rights reserved.