How figs keep their symbiont wasps from cheating

On May 18 Dr. Charlotte Jander, a Swedish physician who decided that she’d rather spend her life doing scientific research rather than practicing medicine, spoke at the Smithsonian’s Tuesday lecture series. The Uppsala University professor had a larger-than-usual crowd, including a number of audience members from Michigan State University, for the presentation of her findings about the relationship between fig species and the symbiotic fig wasps that pollinate them.

“Species don’t live in isolation,” Jander pointed out, adding that interactions between species are an important aspect of their evolution.

Symbiosis is an interaction between different species for mutual benefit. But what keeps this mutualism stable? As Jander put it, “What ensures that one partner doesn’t ‘cheat’?”

A very common stabilizing mechanism is the vertical transmission of symbionts, for example the passing of the bacteria that allow an insect to digest its food through its eggs to its offspring.

But there are exceptions to this pattern, one of which is the relationship between fig trees and fig wasps.

The wasps --- of which there are a number of species, which frequently specialize in just one of the many species of figs. What we think of as the fruit of a fig is actually a collection of flowers, with the females inside. The female wasp must enter the fruit through a pore called its ostiole to lay her eggs. In so doing, the general rule is that the wasp will have visited male flowers --- generally conveniently located in or adjacent to the ostiole --- and coated herself with pollen, which is then spread on the female flowers inside the fig. Some wasp species are passive pollinators --- they don’t have any specialized organs for pollination, tend to visit a lot of male flowers, and are rather haphazard about how they spread pollen about the inside of the figs. Others are active pollinators, which have specialized pollen receptacles, into which they collect pollen from the relatively few male flowers they visit, and from which they precisely sprinkle pollen into the female flowers within the fig.

Active pollination is more common than passive, and according to Jander is “probably beneficial for the tree because the tree does not have to invest so much energy in the production of pollen.”

But what about wasps --- whether active or passive pollinators --- who “cheat” by just boring into figs and laying their eggs, without bothering to do the job of pollination?

There are lazy wasps out there, but trying to identify and follow them, then determine their effects on another species would be a daunting task. What Jander did instead was to look at six fig trees and six wasp species in an experiment in which one group of fruits had their male flowers removed and the control group was left unaltered. The fruit of both groups were bagged before the female flowers matured so as to prevent pollination before the study began. Then, after pollination, the ostioles of the fruit were glued shut, and finally both the pollen-free and control fruit were again bagged to exclude predators.

The results varied among the different fig species, but in general unpollinated fruit were much more likely to be aborted --- in nature shed from the tree before ripening --- which also effectively aborts the eggs laid within them.

So if you are a fig wasp who cheats, the tree is likely to take its revenge on your eggs.

“These data are consistent with co-evolution,” Jander opined. Both figs that don’t penalize cheaters and wasps that don’t bother to pollinate tend to have fewer offspring, and that in turn encourages cooperative behaviors in both symbionts, the plants and the insects.

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When the Americans brought their medicine to Panama
How figs keep wasps from cheating
Termite evolution
Biological climate control
Interpersonal violence as a public health issue

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How do figs keep wasps from cheating?