Evolution 2008: sexy plants, battling bacteria, durable cooperation
About 1500 scientists attended Evolution 2008 here last week. The four-day meeting was filled with 15-minute talks (usually ten at once, in different rooms), plus two evening poster sessions (like a science fair, for grownups, with discussions rather than judging), scenically located on a pedestrian bridge over the Mississippi. Reports that “scientists are abandoning evolution�? appear to be exaggerated.
Here are summaries of some of the talks I enjoyed.
It has been suggested that harsh environments may select against elaborate sexual displays. For example, peacocks with fancier tails may mate more often (but see last week’s guest post), increasing the frequency of fancy-tail genes in the population. But if predators are very common, the increased risk of being eaten may outweigh sexual selection. OK, but why worry about animals, when plants are so much more interesting?
Chris Herlihy talked at Evolution 2008 about a plant that lures pollinators with elaborate clusters of male flowers. Male plants with more flowers attract more pollinators, but the flower clusters also have lots of small, thin leaves, which lose more water, increasing the risk of death during a drought.
Herlihy and Lynda Delph imposed selection, under greenhouse conditions, for more elaborate male flower clusters, analogous to what would be imposed by choosy pollinators) and found that they evolved readily. Given plenty of water, lines selected for more elaborate flower clusters made more pollen and would presumably attract more pollinators, achieving more male reproductive success.
Herlihy also mentioned some related field experiments. Lines selected for more elaborate flower clusters were more likely to die in the field, apparently due to their greater water use. They saw a similar pattern in wild plants, not subject to artificial selection, in the field. Males with more flowers were more likely to die at dry sites, whereas at wet sites they survived well and presumably had more offspring than plants with fewer flowers. These observations were consistent with the hypothesis that, for plants as well as animals, the cost of elaborate male displays is more of a constraint in harsh environments.
Our understanding of infectious disease has advanced from the naïve expectation that all pathogens evolve to cause less harm, to the realization that, although "it may be better to keep alive the goose that lays the golden eggs than to kill it… this argument depends on the assumption that, if you do not kill the golden goose, no one else will either�? (Maynard Smith, 1989). This more realistic view of self-restraint in pathogens led to the conclusion that “competition between species or clones of one species within a host results in the evolution of greater virulence than would be favored with only one strain of pathogen�? (Williams and Nesse, 1991). More recently, it has been recognized that cooperation among genetically related pathogens within a host may take the form of a collective attack on the host rather than restraint, in which case mixed infections may actually be less virulent (Brown, et al., 2002).
But what if unrelated pathogens actually attack each other? At Evolution 2008, Farrah Bashey-Visser (and coauthors Hadas Hawlena, Fabienne Vigneux, and Curtis Lively) presented data showing that insects infected by multiple strains of bacteria died less quickly, apparently because unrelated bacteria were killing each other using chemical warfare. Field sampling showed that many strains of these bacteria do indeed produce chemicals that kill other strains. They are, of course, resistant to their own toxic chemicals, because those that weren’t left no descendants. So mixed infections do matter, but whether they are more or less harmful depends on the details of how pathogens interact within their hosts.
Mixed infectious also have important implications for beneficial interactions with hosts, a topic I emphasized in my own talk. For example, rhizobium bacteria inside root nodules on legume plants benefit collectively when some or all of them supply their plant with nitrogen, but this process is costly for the rhizobia. Free-riders would therefore undermine this cooperation, were it not for sanctions imposed by the host plant on root nodules (West, et al., 2002)(Kiers, et al., 2003). But rhizobia, too, are known to engage in chemical warfare, at least in the soil. I wonder what happens when two strains share the same nodule.
We generally assume that those paying the cost of mutually beneficial activities, such as nitrogen fixation, will be among the beneficiaries. But this may not always be true. Sam Brown spoke at Evolution 2008 on his work with coauthor Francois Taddei, on how “durable public goods�? (those whose benefits may outlast those that paid for them) can affect cooperation. The collapsed bridge being replaced a short walk from the meeting site was paid for by gasoline taxes decades ago. (Today, we take the opposite approach, saddling future generations with debt to pay for current benefits, such as the war in Iraq. If you don’t think this is a “current benefit�?, you must not work for Halliburton, Blackwater, or Al-Queda, rebuilding at a safe distance near the Afghanistan-Pakistan border. But I digress.) Similarly, bacteria produce and release expensive molecules that float around picking up essential nutrients, like iron. After the bacteria take the iron, they release the molecule back into the medium. Thus, the concentration of this “public good�? depends on past, not just current production. Brown showed that, when public benefits come partly from past public benefactors, that can change the evolutionary stability of cooperation. For example, the frequency of cooperators and cheaters can oscillate, reaching a stable equilibrium much more slowly than if there were greater short-term dependence of public goods on the current frequency of cooperators. Brown and Taddei provide details in the open-access journal, PLoS One (Brown and Taddei, 2007).
More talk summaries next week...
Brown, S.P., Hochberg, M.E., Grenfell, B.T., 2002. Does multiple infection select for increased virulence? Trends in microbiology 10, 401-405.
Brown, S.P., Taddei, F., 2007. The durability of public goods changes the dynamics and nature of social dilemmas. PLoS One 2, 593.
Kiers, E.T., Rousseau, R.A., West, S.A., Denison, R.F., 2003. Host sanctions and the legume-rhizobium mutualism. Nature 425, 78-81.
Maynard Smith, J., 1989. Generating novelty by symbiosis. Nature 341, 284-285.
West, S.A., Kiers, E.T., Simms, E.L., Denison, R.F., 2002. Sanctions and mutualism stability: why do rhizobia fix nitrogen? Proceedings of the Royal Society of London B, Biological Sciences 269, 685-694.
Williams, G.W., Nesse, R.M., 1991. The dawn of Darwinian medicine. Quarterly Review of Biology 66, 1-22.