FIG TREE & FIG WASP MUTUALISM

Introduction:
"For the pollination of their flowers, figs (Ficus) are dependent upon Hymenoptera Chalcidoidae of the family Agaonidae (fig wasps). For the propagation of their kind, the fig wasps are dependent upon the ovaries of the figs, in which their larvae develop. Thus the figs and fig wasps are interdependent" (Wiebes).

The life cycle of both organisms is entirely related and is shown in the diagram below.

Life Cycle of the Fig and Fig Wasp Mutualism
Life Cycle of the Fig and Fig Wasp Mutualism



Basics of the Symbiosis:
Pollination is a critical element of most plants life cycles and therefore the mechanism of pollination is literally life or death. Many pollination relationships are easy to predict based on the type of flower and its characteristics which in turn predict what possible pollinators the plants have. Not all plants however have flowers that demonstrate such easy to identify characteristics of flower; Ficus sycamorous, or fig trees, are a perfect example of this. In its most basic form the symbiosis supplies a place for reproduction to the wasp and a source of pollination to the fig tree. Based on the figs dependence on the wasp, and the wasps reciprocal dependence on the fig trees this relationship is a classic example of an obligate mutualism. “Fig trees are completely dependent on tiny wasps, a couple of millimeters long, for their propagation and survival. These fig wasps are the sole pollinators of fig trees and in turn, fig wasps can breed nowhere else but inside figs” (Noort). The relationship demonstrates further specificity however because usually one species of wasp is associated with one species of fig tree. This relationship has been found to be untrue in some cases however so it is still unclear how many more wasp species are yet to be discovered as well as how many pollinating wasp species there actually are (Noort). The symbiotic relationship is one of the most widely studied associations between animal and plants because it is one of the best studied species-specific examples of both mutualism and coevolution. Although there are over 1ooo species of figs that grow, they are all pollinated by a certain species of wasp. (Paracer 187-188).













Fig Trees:
The fruit of the fig trees is the part associated with the symbiotic relationship, specifically the fig fruit. “The fig fruit is actually an inflorescence called a syconium, with its margins folded to a pear-shaped structure that has a small opening, the ostiolum, which is lined with hairlike scales.” (Paracer 194). The female wasp is attracted to a fig that is flowering and receptive and they enter through the ostiolum.

fig_tree.jpg
Fig Fruits on Fig Tree
















The inside of the fig is lines with a variety of flowers, some of which are female uniovulate flowers and some of which are male flowers. Fig trees are unique in that the flowers are completely concealed within the fig, an enclosed inflorescence, with the hundreds of tiny florets lining the inside of a central cavity” (Noort). Usually those that are deeper towards the center of the flower are female (Paracer 194). In addition to sex, they differ in size, some are short while others are tall. Based on this unique flower structure it is expected that the process of pollination would be highly specific for the fig trees. This has been found to be true based on the plants association with the small wasps, Ceratosolen arabicus, that breed within the figs and serve as pollinator.


flower.gif
Flowers on Interior of Fig Fruit



Fig Wasp:
The species associated with Ficus (fig trees) is the species Agaonidae. They are found worldwide and most known for their symbiotic relationship with Ficus (FigWeb).

Kingdom: Animalae

Phylum: Anthropoda

Class: Hexapoda

Order: Hymenoptera

Superfamily: Chalcidoidea

Genus: Agaonidae





Fig Wasp: Agaonidae
Fig Wasp: Agaonidae








Fertilization within the Fig:
The female wasp is the one that first enters into the flowering fig to deposit its eggs within the female flowers of the fig. She enters into the fig through the one opening that the surface of the fig has called the ostiolum.

ostiolum.jpg
Ostiolum of the Fig Fruit


This task is difficult because of how narrow the ostiole is, but the female wasp "is remarkably adpated to do so. Her body in particular her head and thorax, is extremely flattened and elongate. She also has rows upon rows of backward pointing teeth on her madibular appendage, situated on the underside of her head, as well as a few strong teeth on her legs" (Noort). The teeth are extremely important becuase they not only help her to move fowards through the ostiole but also prevent her from slipping backwards through the area in which she came (Noort).


Courtella_wardi_mandible_200.jpg
Underside of head of Courtella wardi showing mandibles and mandibular appendages, bearing many rows of backward pointing teeth.

During the process of entering the fig, she often loses her wings as well as antennae but still is able to deposit eggs for reproduction. “The female gall wasp deposits an egg into the ovary of a female fig flower with its ovipositor and induces the ovary to form a tumor-like gall, which encloses the insect egg” (Paracer 194). There are two female flower types that the wasps can be attracted to once they enter through the ostiolum, but typically the female deposits the egg on the short styles of flowers which are borne on stalks (Paracer 194). While the height of the long style are not a problem to reach, the female is unable to reach her ovipositor (the structure inserted into the flower to reach the flower ovary in order to deposit the egg) to the ovary of the flower, the wasp will not release the egg until she has found a short style flower (Paracer 194). The life span of the fig wasp is short, and the female dies almost immediately after depositing the egg.


VIDEO of Fig Wasp entering into the Ostiolum!
http://www.naturefootage.com/video_clips/NZ58a_169

Once the eggs are laid on the flower of the fig fruit they emerge after only a few short days. Once they come out of the egg they feed on the surrounding gall tissue. “Larvae then transform into pupae from which adult wasps with strong mandibles develop and eat their way through the gall” (Paracer 195). The male and female wasps have very different roles right away and the male wasps surface and develop first. The male’s job is then to search for galls that have female wasps within them and fertilize them. They do so by puncturing the wall of the gall and inserting the tip of their abdomen into the gall. Only after fertilization from a male does the female wasp emerge from the galls (Paracer 195). After the females have emerged both the male and female wasps begin to tunnel through the syconium near the ostiolum (where the female fig initially entered). The females are the only wasps to exit the fig however because due to “their sensitivity to the light, the male wasps withdraw into the interior of the fig fruit, where they eventually die” (Paracer 195).

Ceratosolen capensis emerging from her gall inside a fig of Ficus sur.
Ceratosolen capensis emerging from her gall inside a fig of Ficus sur.


fertilization.jpg
Larvae emerging from Gall within the Fig Fruit


Process of Pollination:
Pollination actually occurs as a result of the male flowers. At the same time that the tunnel is completed for the female wasp to exit the fig through, the male flowers mature and produce the pollen which then is picked up by the female. This part of the symbiosis is spectacular and demonstrates the high specificity of the association. “Before the female wasps leave the syconium they fill specialized pouches on their thoraxes with pollen from anthers of male flowers” (Paracer 195). This however is an aspect of research on fig wasps that is up for some discussion as a result of some finding suggesting not only the presence of both passive and active pollinating wasps, but also the possibilty for a wasp to chear and receive the benefits from the wasp without returning the favor and pollinating the fig. Active pollinators participate in a specific mode of collecting pollen from within the fig fruit and then transport it out. Passive pollination however occurs when the wasp does not change behavior in any way to collect pollen, instead the fig fruit produces enough pollen to cover the wasp in pollen prior to exiting the fig. The fig wasp then emerges from the fruit covered in pollen and as a result serves as pollinator despite the fact that they do not perform a specific behavior to do so. There are significant differences in how the fig produces pollen based on what type of pollinator it has. "Passively pollinated fig species produce numerous, large male flowers that release abundant pollen onto the waps as they leave the fig to disperse. Therefore, in these fig species, trees invest considerable resources in producing abundant pollen, and no aspect of pollen transfer relies on specialized wasp behavior" (Jander). On the otherside, in active pollination, the fig does not have to invest as much energy and therefore they produce a smaller number of male flowers that are also smaller in size (Jander). This is all very importnat research because it questions the mutualism and how highly specified it is expected to be.

The idea of cheaters is very interesting to scientists today because it interupts the concept of the realtionship being obligate. A paper by Charlotte Jander and Edward Allen Herre speaks more to this! Click here to read more on Cheaters!

In addition to the research being done on the presence of cheaters there is also more research being done in regards to the modes of pollination. A paper by Finn Kjellberg et al speaks more to this! Click here to read more on Active vs. Pollination Modes!

Once the fig wasp has gathered pollen, either actively or passively, she begins the next step of the pollination process by leaving the fig fruit. Once the female exists the host fig, she searches for a new one and the process is repeated. It is impressive however that the female wasp is able to travel to a new host species given both the rigor of the travel and also the short life span of the wasp.
















One to One Specificity:
The life cycle of the mutualism is very important but is also more understood than some of the more detailed aspects of it such as cheaters and the method of pollination like we saw above. Another interesting new discovery is that sometimes the symbiosis does not demonstrate a one to one specificity as expected. Historically, it has been believed that "their relationship is strictly specific--i.e. every species of fig has its own species of pollinator wasp" (Wiebes). This belief comes from observations that suggest that every fig leaving the fig are always of the same species (Wiebes). However, there have been situations found where this is not the case. There have been Ficus found to have more than one species of Agaonid associated with it. In article called "Coevolution of Figs and Their Insect Pollinators" by J.T. Wiebes, he found there to be two species for Ficus sycomorus dispersed across Africa. He found evidence of both Ceratosolen arabicus and C.galili (Wiebes). He also found evidence of cheaters here because only the C.arabicus proved to be a pollinator, while C.galili was "ovipositing in the flowers without pollinating the stigmas" (Wiebes). There were also situations found where more than one tree was pollinated by the same wasp. Again, this presented data to question the specificity of one wasp for one fig tree. When looking at what might cause the break in the normal trend, Wiebes found that "quite a number of double occurrences concern geographical variation of the wasp species while the host fig remains uniform over the whole range" (Wiebes). Not only does this evidence interrupt one to one specificity, but it also presents more support for the idea of cheater wasps within this highly specified mutualism.

Another article by Eleanor R. Haine, Joanne Martin and James M. Cook offers more insight into the questions regarding one to one specificity of the fig-fig wasp mutualism. Click here to read more on One-to-One Specificity!



Fun Facts:

Who eats Figs? and What about FIG NEWTONS?
Humans are not the only species that eat figs; bats, birds, as well as chimpanzees also eat figs


Green Pigeon Duncan Butchart.
Green Pigeon Duncan Butchart.
Chimpanzee feeding on Ficus ottonifolia lucanda Diogo Verissimo.
Chimpanzee feeding on Ficus ottonifolia lucanda Diogo Verissimo.

But what about those fig newtons your mother bought you as a child? When we eat fig fruit products do we consume fig wasps as well? The answer is two-fold; when eating dried figs, usually yes, but when eating fresh fig fruit, they are generally without fig wasps. The name of fig species that is eaten by humans is the domesticated fig Ficus carica. It has been cultivated for thousands of years and is found to grow naturally in the Mediterranean region. Interestingly enough, many of the figs that we might eat are from commercialized production and therefore there are no fig wasps even involved in the pollination process. Many of these figs come from areas like California and South Africa. There are situations though where wasps are consumed. For instance, many indigenous people eat wild figs and as a result of natural pollination and cultivation it is likely that they also consume the fig wasps when eating the fig fruit. (Noort).




Works Cited:


Haine, Eleanor R., Joanne Martin and James M. Cook. Deep mtDNA divergences indicate cryptic species in a fig-pollinating wasp. BMC Evolutionary Biology, 2006 Vol. 6:83. Published October 13, 2006. © 2006 Bio Med Central.

Jander, Charlotte and Edward Allen Herre. Host sanctions and pollinator cheating in the fig tree-wig wasp mutualism. The Royal Society 10. 1098, January 12, 2010.

Kjellberg, Finn. et al. Pollination Mode in Fig Wasps: The Predictive Power of Correlated Traits. The Royal Society 10.1098, February 14,2001

Noort, Simon van. Fig Web” Iziko Museums of Capt Town.< http://www.figwe b.org> Accessed April 2011.

Paracer, Surindar and Vernon Ahmadjian. Symbiosis: An Introduction to Biological Associations; Second Edition. © 2000 by Oxford University Press, Inc.

Ronstead, Nina et al. 60 Million Years of Co-Divergence in the Fig-Wasp Symbiosis. The Royal Soceity 10.098, September 37, 2005.

Wiebes. Co-Evolution of Figs and Their Insect Pollinators. Annual Review of Ecological Systems, Vol 10:1-12. © 1979 by Annual Reviews Inc.
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