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Sanctions and cheating in pollination and protection mutualisms

The most-cited paper from my lab is one by Toby Kiers, showing that soybean plants impose fitness-reducing sanctions on "cheating" rhizobia, which multiply inside root nodules but then fail to provide their hosts with nitrogen. This week I will briefly discuss two recent papers on the role of sanctions in two different kinds of mutually beneficial interactions between species.

Toby is also one of the coauthors on the first paper by Ryutaro Goto and others. The author/year citation (Goto 2010 ) reminds me of programming computers in Fortran, but the full title is "Selective flower abortion maintains moth cooperation in a newly discovered pollination mutualism." The second paper, by David Edwards and others, discusses ants that protect trees from browsing animals. This paper asks, "Can the failure to punish promote cheating in mutualism?"

Clochidion trees in Japan are pollinated by moths. Like the moths that pollinate yuccas and the wasps that pollinate figs, these moths lay eggs as they pollinate, and their larvae then consume some seeds. What keeps the moths from laying too many eggs in a given flower?

There are two general mechanisms that might work. The plants could somehow coerce or manipulate individual moths to limit how many eggs they lay in one flower. The moth species might then evolve resistance to that manipulation, however.

Or, the plants could "domesticate" the moths, as humans domesticated wolves, by somehow increasing the relative reproductive success of more-beneficial moth genotypes. This would tend to reduce the frequency of "cheating" moths over generations. This is what the trees apparently do.

Flowers containing more moth eggs were more likely to be aborted, killing any surviving seeds but also the moth eggs. As a result, a single egg per flower was more than twice as likely to survive, relative to when there were two or more eggs per flower. This "host sanction" imposes selection on the moths to lay only one egg per flower, minimizing damage. Domestication appears to have worked; moths tend to lay only one egg per flower. So, by imposing sanctions, the trees may allocate more of their resources to their own seeds, rather than to developing moth larvae. But they also indirectly benefit future generations of trees, by guiding the evolution of the moths in a beneficial direction.

Pollination is not the only benefit insects can provide plants, however. Various tree species are protected, to varying extent, by ants that attack animals browsing on their leaves. In return, the trees provide housing and/or food to the ants. If browsing kills the tree, the ants need to look for a new home, which might provide some incentive for the ants to protect the tree. But I wouldn't call that "sanctions." Some ant-trees, however, reduce the growth of new housing ("domatia") for ants when their leaves are damaged, a response that has reasonably been called a sanction. Basically, they make the housing and a new leaf at the same time, so damage to the leaf destroys the housing.

The rattan palms studies by Edwards et al., on the other hand, seem to be more trusting. They make the domatia first, then the leaf. The authors suspected that this would prevent the palms from reducing domatia when ants failed to protect their leaves. Sure enough, cutting leaves off had no effect on the abundance of domatia.

Does failure to impose sanctions allow cheaters to invade? Apparently so. Different trees were occupied by different ant species, which differed in how vigorously they patrolled and protected their plant hosts. About half the trees have an ant species that protects their host very effectively, but the other half have a much less-effective species.

But, without sanctions, why hasn't the good ant species evolved into a cheater? They mostly protect the tree from other insects, and they eat those insects. Protection may therefore be a by-product of their search for food. This contrasts with at least one tree species with sanctions, where the patrolling ants attack, but cannot eat, invaders.

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