Parasite Helps Balance a Mutualistic Relationship

a wasp deposits eggs in a green fig against a green background

Biological Strategy

Fig wasp

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Pollinate

One of the ways that plants can produce offspring is by making seeds that contain the genetic information to produce a new plant. Seeds can only be produced when pollen grains reach the hidden ovaries within a flower. Flowers have developed wildly diverse forms and activities, such as the production of sweet nectar to attract organisms such as bees and butterflies, that move from flower to flower, transferring pollen. Flowers will also adapt to make use of environmental influences such as wind and water as pollinators.

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Cooperate/Compete Between Different Species

From parasitism to mutualisms, there are endless interactions between organisms in nature. Interactions between species that lead to a negative outcome for one species are known as competition. Cooperation occurs when organisms work together for the benefit of each organism or species. While competition often occurs over resources or mates, the more biologists have begun to look for examples of cooperation in nature, the more they have found. For example, around 90% of plants have a beneficial partnership with fungi. Fungi provide the plant with nutrients such as nitrogen and phosphorus, in exchange for sugars from the plant.

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Protect From Animals

Animals–organisms that range from microscopic to larger than a bus–embody a wide variety of harms to living systems, including other animals. They threaten through predation, herbivory, defense, and parasitism, and they compete for resources such as water, nutrients, and space. Any given living organism commonly faces threats from a variety of animals, requiring strategies that effectively defend from each. Trout and other bony fish, for example, escape predators by having scales made of very thin, flake-like pieces of bone covered with slippery mucus. They also have behavioral strategies such as camouflage, fast swimming, and twisting and turning to achieve release from a predator’s grip.

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Insects

Class Insecta (“an insect”): Flies, ants, beetles, cockroaches, fleas, dragonflies

Insects are the most abundant arthropods—they make up 90% of the animals in the phylum. They’re found everywhere on earth except the deep ocean, and scientists estimate there are millions of insects not yet described. Most live on land, but many live in freshwater or saltwater marshes for part of their life cycles. Insects have three distinct body sections: a head, which has specialized mouthparts, a thorax, which has jointed legs, and an abdomen. They have well-developed nervous and sensory systems, and are the only invertebrate that can fly, thanks to their lightweight exoskeletons and small size.

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Parasitic wasps increase fig tree production by placing limits on the mutualism between figs and fig wasps.

Mutualism is cooperation between species that helps each of them survive. One species provides something for the other and in exchange, receives something good in return. One example is the relationship between fig trees and fig wasps that has evolved for longer than 60 million years.

In any mutual relationship, there needs to be some way to maintain balance. Otherwise, one partner might take advantage of the other. In Australia, there is a species of fig tree that is pollinated by a specific wasp, known as a fig wasp.

The figs of the tree are sweet fruit full of small seeds. The flowers are tiny and found inside the fig, attached to the inside wall (see photo and illustration). As with any flowering plant, a fig can’t produce seeds without being pollinated. The tiny fig wasp pollinates the plant as it enters the fig. It spreads pollen around as it lays its eggs inside the flowers’ ovules. The eggs eventually grow into adult wasps and leave to pollinate other trees. However, the fig wasp doesn’t lay its eggs in all of the ovules, leaving some to grow into the seeds that crunch when you eat the fig.

What keeps the fig wasp from laying eggs in all of the ovules? Another partner in this three-way mutualism is a parasitic wasp. It uses its long egg-laying tube to pierce the outside of the fig and lay its own eggs into the ovules with the fig wasps’ eggs (the red ovals in illustration). The parasitic wasps’ young then eat the young of the fig wasp. However, the egg-laying tube isn’t long enough to reach all of the ovules, just the ones closest to the surface. As a result, to escape the parasite, the fig wasp tends to lay most of its eggs in the “enemy-free zone” deepest inside the fig (the blue ovals in the illustration). This leaves the outer ones (yellow ovals) free to develop into seeds and create the next generation of trees.

What can we learn from the mutualistic relationship between the fig tree, wasp and parasite? Sometimes, we need to rethink something we might automatically think of as “bad”, like parasitism. The parasitic wasp is an important member of this relationship. By limiting where the fig wasp lays its eggs, the parasite allows the fig tree to produce seeds to make the next generation of fig trees. This in turn benefits the fig wasp, as it has another generation of trees to pollinate and lay eggs for its young.

Image: Wang et al. 2019 / CC BY - Creative Commons Attribution alone

Figure 1. (a,b) Schematic spatial stratification pattern of fig contents. (a) After entering figs, pollinator wasps prefer to lay their eggs in ovules close to the center of a fig, but pollinate flowers irrespective of their style lengths. Conversely, parasitoids lay eggs from the outside of figs and are more likely to attack hosts located in ovules nearer the fig wall. (b) After pollination and oviposition, galls of pollinator offspring are aggregated towards the center of the figs (blue ovules) and developing seeds and galls containing parasitoid larvae are mainly located nearer the fig wall (red ones parasitized, yellow ones turn into seeds).

Cross-section of a fig showing flowers (red)

Image: Gubin Olexander / CC BY SA - Creative Commons Attribution + ShareAlike

Cross-section of a fig showing the flowers (red)

Last Updated June 18, 2020

References

“Much biodiversity ultimately relies on cooperation between different species, interactions called mutualisms. Benefits to one partner are gained by obtaining resources from the other, presenting a problem: what prevents one partner from exploiting the other at an unsustainable level? Fig trees are pollinated by tiny wasps that only develop successfully themselves by each destroying a single female fig flower that would otherwise become a seed. Wasps tend to occur in long flowers near the fruit’s centre, with seeds developing near the outer wall. Female wasps therefore favour long flowers for their offspring, and leave short flowers to develop into seeds. To understand why wasps exploit fig trees sustainably, we need to explain why this preference has evolved. The fig-pollinator mutualism is exploited by small parasitic wasps that attack pollinators from outside the fruit. In three Australasian fig species, we found that pollinator offspring in the outer layer of flowers were more likely to be parasitized than those in the inner layer. Our data thus indicate that long flowers provide enemy-free space for pollinator offspring at the fruit’s centre. We suggest that the provision of variable length flowers by fig trees may contribute to mutualism stability by indirectly involving a third party: parasitic wasps, previously regarded as detrimental to both mutualists.” (Dunn et al. 2008:0491)

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