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April 16, 2010

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.

Collaboration via Skype and the joys of 3 monitors

Carl Zimmer will be interviewing me soon for his "Microbe World -- Meet the Scientist" podcast, via Skype. I hadn't paid much attention to Skype, because my long-distance collaborations have almost all used email rather than phone. But, when I set it up, I discovered a feature that seems really useful, that some people may not know about yet.

Right-clicking a contact name brings up various options, including Send File and Share Your Screen. When I tested the latter option, it asked which of my two screens I wanted to share. The selected screen shows up on the contacts computer, live. In other words, I can use the mouse pointer to highlight something we want to discuss, then talk about using the audio link. This seems like it would be really useful for some kinds of long-distance collaborations.

In my home office, I've recently upgraded to 3 monitors. Two are mounted with one above the other, mainly for use in Microsoft Word. Using Word's Window/Split command gives two views of the same document, one in each monitor. This is useful in working on manuscripts, because figures and tables are usually at the end, but it's helpful to be able to see them while you're writing about them. In working on my book, though, I usually had one screen in outline mode and the other in normal text. That made it easy to go back and forth between rearranging paragraphs and editing within paragraphs.

April 8, 2010

Sex bias in lizards

When humans can control the sex of their offspring, they usually choose to have more sons than daughters. This collective behavior results in a shortage of females, so females are more likely to marry than males are. Parents who chose to have only sons are therefore less likely to have grandchildren. This week's paper shows that lizards are more sophisticated.

"Cryptic sex-ration bias provides indirect genetic benefits despite sexual conflict" was published in Science by Robert Cox and Ryan Calsbeek.

In many species, females prefer to mate with larger males. In species where this is true, males are often larger than would be optimal (in terms of food needs, ability to squeeze through tight spaces, etc.) in the absence of this preference.

To the extent that size is inherited, the sons of females who mate with larger males will be more likely to find mates, because... females often prefer to mate with larger males. But what about their daughters? They may also inherit genes for large size, so will they have the disadvantages of large size, without the male-mating-success advantage? If so, that cost could counter-balance the offspring-fitness benefit of mating with larger males.

One possible solution for this problem would be for females lucky enough to mate with large males to produce mostly male offspring, while those who mate with smaller males produce mostly female offspring. But is this possible?

It is for anole lizards. They have about 20% more sons than daughters, after mating with a large male, but the reverse if their mate was small.

This would make evolutionary sense if sons of large males were larger and therefore more fit, while daughters of large males were larger and therefore less fit. In field tests, however, only the first of these was true. So, by producing relatively more sons when their mate was large, they increase the fitness of their offspring. But why bias sex ratio the other way, when their mate is small?

The paper doesn't seem to answer this question. Maybe their genes reason (to use what Richard Dawkins has called a "harmless anthropomorphism") as follows: "Some lizards mated with larger males. They will have mostly sons. So if I make this female have mostly daughters, they are sure to find mates, whereas sons might not. So I'm more likely to make it into the next generation by biasing reproduction in favor of daughters than if I contribute to the male surplus."

If only humans were so smart.

For those who may be interested, I have finally sent a draft of my book, "Darwinian agriculture: where does nature's wisdom lie?" off to Princeton University Press. They will be sending it to two scientists for review, after which I assume I will have some revision to do. Also, my student Ryoko Oono has a paper accepted for publication in New Phytologist about the evolutionary history of an interesting interaction between legume plants and the symbiotic rhizobial bacteria that live in their root nodules and, to varying extents, provide their plant hosts with nitrogen. I'll write more about that when it comes out.