July 24, 2009

Microbes evolve; flies evolve and learn

"Can plants predict the future?" asked one of my Crop Ecology lectures at UC Davis. Yes, they can. Plants use decreasing daylength to predict oncoming winter, and flower early enough to finish seed development before it gets too cold. Some plants detect early signs of drying soil and reduce their own water use, saving water in the soil for later.(Davies & Zhang. 1991, Bano, et al. 1993) Others detect "distress signals" from neighbors under insect attack, turning on chemical defenses in anticipation (Karban, et al. 2004).

But these anticipatory responses do not require learning: a beneficial change in individual behavior in response to individual experience. An alfalfa plant will never learn that the farmer always irrigates before it actually runs out of water. At least, I assume it won't. An evolving alfalfa population is a different story. Over a few generations under irrigation, genotypes that reduce their water use as the soil starts to dry (thereby reducing their growth) will be out-competed by genotypes that keep using water and growing.

Like plants, microbes can predict the future. As in plants, this trait can evolve. As they pass through the gut, bacteria typically see lactose before they see maltose. So they have evolved to "anticipate" maltose availability, turning genes for maltose use on as soon as they are exposed to lactose. After 500 generations of evolution on lactose without maltose, however, the bacteria have lost lose this anticipatory response, so that they turn maltose genes on only when they actually see maltose.(Mitchell, et al. 2009) The title of the news story in Nature about this work asked whether microbes can "learn from history", but this is clearly not an example of individual cells modifying their responses to lactose based on their individual experience.

Individual insects can learn. But is learning always a good thing? Aimee Dunlap, a grad student in my department working with David Stephens, just published a paper in Proceedings of the Royal Society exploring the conditions under which natural selection will favor learning (Dunlap & Stephens. 2009).
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