coevolution

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Year of Pollination: Figs and Fig Wasps

This post originally appeared on Awkward Botany in November 2013. I’m reposting an updated version for the Year of Pollination series because it describes a very unique and incredibly interesting interaction between plant and pollinator. 

Ficus is a genus of plants in the family Moraceae that consists of trees, shrubs, and vines. Plants in this genus are commonly referred to as figs, and there are nearly 850 described species of them. The majority of fig species are found in tropical regions, however several occur in temperate regions as well. The domesticated fig (Ficus carica), also known as common fig, is widely cultivated throughout the world for its fruit.

common fig

Common Fig (Ficus carica) – photo credit: wikimedia commons

The fruit of figs, also called a fig, is considered a multiple fruit because it is formed from a cluster of flowers. A small fruit develops from each flower in the cluster, but they all grow together to form what appears to be a single fruit. The story becomes bizarre when you consider the location of the fig flowers. They are contained inside a structure called a syconium, which is essentially a modified fleshy stem. The syconium looks like an immature fig. Because they are completely enclosed inside syconia, the flowers are not visible from the outside, yet they must be pollinated in order to produce seeds and mature fruits.

This is where the fig wasps come in. “Fig wasp” is a term that refers to all species of chalcid wasps that breed exclusively inside of figs. Fig wasps are in the order Hymenoptera (superfamily Chalcidoidea) and represent at least five families of insects. Figs and fig wasps have coevolved over tens of millions of years, meaning that each species of fig could potentially have a specific species of fig wasp with which it has developed a mutualistic relationship. However, pollinator host sharing and host switching occurs frequently.

Fig wasps are tiny, mere millimeters in length, so they are not the same sort of wasps that you’ll find buzzing around you during your summer picnic. Fig wasps have to be small though, because in order to pollinate fig flowers they must find their way into a fig. Fortunately, there is a small opening at the base of the fig called an ostiole that has been adapted just for them.

What follows is a very basic description of the interaction between fig and fig wasp; due to the incredible diversity of figs and fig wasps, the specifics of the interactions are equally diverse.

First, a female wasp carrying the pollen of a fig from which she has recently emerged discovers a syconium that is ready to be pollinated. She finds the ostiole and begins to enter. She is tiny, but so is the opening, and so her wings, antennae, and/or legs can be ripped off in the process. No worries though, since she won’t be needing them anymore. Inside the syconium, she begins to lay her eggs inside the flowers. In doing so, the pollen she is carrying is rubbed off onto the stigmas of the flowers. After all her eggs are laid, the female wasp dies. The fig wasp larvae develop inside galls in the ovaries of the fig flowers, and they emerge from the galls once they have matured into adults. The adult males mate with the females and then begin the arduous task of chewing through the wall of the fig in order to let the females out. After completing this task, they die. The females then leave the figs, bringing pollen with them, and search for a fig of their own to enter and lay eggs. And the cycle continues.

But there is so much more to the story. For example, there are non-pollinating fig wasps that breed inside of figs but do not assist in pollination – freeloaders essentially. The story also differs if the species is monoecious (male and female flowers on the same plant) compared to dioecious (male and female flowers on different plants). It’s too much to cover here, but figweb.org is a great resource for fig and fig wasp information. Also check out the PBS documentary, The Queen of Trees.

 

 

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Year of Pollination: Most Effective Pollinator Principle and Beyond, part two

“The most effective pollinator principle implies that floral characteristics often reflect adaptation to the pollinator that transfers the most pollen, through a combination of high rate of visitation to flowers and effective deposition of pollen during each visit.” – Mayfield, et al., Annals of Botany (2001) 88 (4): 591-596

In part one, I reviewed a chapter by Jose M. Gomez and Regino Zamora in the book Plant-Pollinator Interactions: From Specialization to Generalization that argues that the most effective pollinator principle (MEPP) “represents just one of multiple evolutionary solutions.” In part two, I summarize a chapter by Paul A. Aigner in the same book that further explains how floral characteristics can evolve without strictly adhering to the MEPP.

maximilian sunflower
Aigner is interested in how specialization develops in different environments and whether or not flowering plants, having adapted to interact with a limited number of pollinators, experience trade-offs. A trade-off occurs when a species or population adapts to a specific environmental state and, in the process, loses adaptation to another state. Or in other words, a beneficial change in one trait results in the deterioration of another. Trade-offs and specialization are often seen as going hand in hand, but Aigner argues that trade-offs are not always necessary for an organism to evolve towards specialization. Plant-pollinator interactions provide an excellent opportunity to test this.

“Flowers demand study of specialization and diversification,” Aigner writes, not only due to their ubiquity, “but because much of the remarkable diversity seen in these organisms is thought to have evolved in response to a single and conspicuous element of the environment – pollination by animals.” If pollinators have such a strong influence on shaping the appearance of flowers, pollination studies should be rife with evidence for trade-offs, but they are not. Apart from not being well-studied, Aigner has other ideas about why trade-offs are not often observed in this scenario.

Aigner is particularly interested in specialization occuring in fine-grained environments. A course-grained environment is “one in which an organism experiences a single environmental state for all of its life.” Specialization is well understood in this type of environment. A fine-grained environment is “one in which an organism experiences all environmental states within its lifetime,” such as “a flowering plant [being] visited by a succession of animal pollinators.” For specialization to develop in a fine-grained environment, a flowering plant must “evolve adaptations to a particular type of pollinator while other types of pollinators are also present.”

It’s important to note that the specialization that Aigner mainly refers to is phenotypic specialization. That is, a flower’s phenotype [observable features derived from genes + environment] appears to be adapted for pollination by a specific type of pollinator, but in fact may be pollinated by various types of pollinators. In other words, it is phenotypically specialized but ecologically generalized. Aigner uses a theoretical model to show that specialization can develop in a fine-grained environment with and without trade-offs. He also uses his model to demonstrates that a flower’s phenotype does not necessarily result from its most effective pollinator acting as the most important selection agent. Instead, specialization can evolve in response to a less-effective pollinator “when performance gains from adapting to the less-effective pollinator can be had with little loss in the performance contribution of the more effective pollinator.”

Essentially, Aigner’s argument is that the agents that are the most influential in shaping a particular organism are not necessarily the same agents that offer the greatest contribution to that organism’s overall fitness. This statement flies in the face of the MEPP, and Aigner backs up his argument with (among other examples) his studies involving the genus Dudleya.

Dudleya saxosa (panamint liveforever) - photo credit: wikimedia commons

Dudleya saxosa (panamint liveforever) – photo credit: wikimedia commons

Dudleya is ecologically generalized. Pollinators include hummingbirds, bumblebees, solitary bees, bee flies, hover flies, and butterflies. “Some Dudleya species and populations are visited by all of these taxa, whereas others seem to be visited by only a subset.” Aigner was curious to see if certain species or populations were experiencing trade-offs by adapting to a particular category of pollinators. Aigner found variations in flower characteristics among species and populations as well as differences in pollinator assemblages that visited the various groups of flowers over time but could not conclude that there were trade-offs “in pollination performance.”

In one study, he looked at pollination services provided by hummingbirds vs. bumblebees as corolla flare changed in size. In male flowers, bumblebees were efficient at removing pollen regardless of corolla flare size, while hummingbirds removed pollen more effectively as corolla flare decreased. Both groups deposited pollen more effectively as corolla flare decreased, but hummingbirds more strongly so. Ultimately, Aigner concluded that “the interactions did not take the form of trade-offs,” or, as stated in the abstract of the study, ” phenotypic specialization [for pollination by hummingbirds] might evolve without trading-off the effectiveness of bumblebees.”

Aigner goes on to explain why floral adaptations may occur without obvious trade-offs. One reason is that different groups of pollinators are acting as selective agents for different floral traits, “so that few functional trade-offs exist with respect to individual traits.” Pollinators have different reasons for visiting flowers and flowers use the pollination services of visitors differently. Another reason involves the “genetic architecture” of the traits being selected for. Results can differ depending on whether or not the genes being influenced are linked to other genes, and genetically based fitness trade-offs may not be observable phenotypically. Further studies involving the genetic architecure of specialized phenotypes are necessary.

And finally, as indicated in part one, pollinators are not the only floral visitors. In the words of Aigner, “if floral larcenists and herbivores select for floral traits in different directions than do pollinators, plants may face direct trade-offs in improving pollination service versus defending against enemies.” These “floral enemies” can have an effect on the visitation rates and per-visit effectiveness of pollinators, which can drastically alter their influence as selective agents.

Like pollination syndromes, the most effective pollinator principle seems to have encouraged and directed a huge amount of research in the field of pollination biology, despite not holding entirely true in the real world. As research continues, a more complete picture will develop. It doesn’t appear that it will conform to an easily digestible principle, but there is no question that, even in its complexity, it will be fascinating.

I will end as I began, with an excerpt from Thor Hanson’s book, The Triumph of Seeds: “The notion of coevolution implies that change in one organism can lead to change in another – if antelope run faster, then cheetahs must run faster still to catch them. Traditional definitions describe the process as a tango between familiar partners, where each step is met by an equal and elegant counter-step. In reality, the dance floor of evolution is usually a lot more crowded. Relationships like those between rodents and seeds [or pollinators and flowers] develop in the midst of something more like a square dance, with couples constantly switching partners in a whir of spins, promenades, and do-si-dos. The end result may appear like quid pro quo, but chances are a lot of other dancers influenced the outcome – leading, following, and stepping on toes along the way.”

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Figs and Fig Wasps

Recently I was listening to a past episode of Caustic Soda Podcast in which the hosts briefly discussed fig wasps. I was intrigued by this discussion, having previously never heard of fig wasps, and so I did a little research. As it turns out, what I am about to share with you here is just the tip of the iceberg. The relationship between figs and fig wasps is a complex topic, to the extent where you could easily spend a lifetime studying this relationship and there would still be more to discover.

Ficus is a genus of plants in the  family Moraceae that consists of trees, shrubs, and vines. They are commonly referred to as figs, and there are between 755 and 850 described species of them (depending on the source). The majority of fig species are found in tropical regions, however many of them are found in temperate regions as well. The domesticated fig (Ficus carica), also known as common fig, is widely cultivated throughout the world for its fruit.

common fig

Ficus carica – common fig

photo credit: wikimedia commons

The fruit of figs, also called a fig, is a multiple fruit because it is formed from a cluster of flowers. A fruit is formed by each flower in the cluster, but they all grow together to form what appears to be a single fruit. Now here is where it starts to get bizarre. The flowers of figs are contained inside a structure called a syconium, which is essentially a modified fleshy stem. The syconium looks like an immature fig. Because they are contained inside syconia, the flowers are not visible from the outside, yet they must be pollinated in order to produce seeds and mature fruits.

This is where the fig wasps come in. “Fig wasp” is a term that refers to all species of chalcid wasps that breed exclusively inside of figs. Fig wasps are in the order Hymenoptera (superfamily Chalcidoidea) and represent at least five families of insects. Figs and fig wasps have coevolved over tens of millions of years, meaning that each species of fig could potentially have a specific species of fig wasp with which it has developed a mutualistic relationship. However, pollinator host sharing and host switching occurs frequently.

Fig wasps are tiny, mere millimeters in length, so they are not the same sort of wasps that you’ll find buzzing around you, disrupting your summer picnic. Fig wasps have to be small though, because in order to pollinate fig flowers they must find their way into a fig. Fortunately, there is a small opening at the base of the fig called an ostiole that has been adapted just for them. What follows is a very basic description of the interaction between fig and fig wasp – remember with the incredible diversity of figs and fig wasps, the specifics are sure to be equally diverse.

First a female wasp carrying the pollen of a fig from which she has recently emerged discovers a fig that is ready to be pollinated. She finds the ostiole and begins to enter the fig. She is tiny, but so is the opening, and so her wings and antennae are ripped off in the process. No worries though, she won’t be needing them anymore. Inside the fig there are two types of flowers – ones with long styles and others with short styles. The female wasp begins to lay her eggs inside the flowers, however she is not able to lay eggs inside the flowers with the long styles. Instead, these flowers get pollinated by the wasp. After all her eggs are laid, the female wasp dies. The fig wasp larvae develop inside galls in the ovaries of the fig flowers, and they emerge from the galls once they have matured into adults. The adult males mate with the females and then begin the arduous task of chewing through the wall of the fig in order to let the females out. After completing this task, they die. The females then leave the figs, bringing pollen with them, and search for a fig of their own to enter and lay eggs. And the cycle continues.

But there is so much more to the story. For example, there are non-pollinating fig wasps that breed inside of figs but do not assist in pollination – freeloaders essentially. And how is the cycle different if the species is monoecious (male and female flowers on the same plant) compared to dioecious (male and female flowers on different plants)? It’s too much to cover here, but visit figweb.org for more information. FigWeb is an excellent resource for learning all about the bizarre and fascinating world of the fig and fig wasp relationship. Also check out the PBS documentary, The Queen of Trees.

This is the first of hopefully many posts on plant and insect interactions. Leave a comment and let me know what plant and insect interactions interest you.