Owl Ears, Evolutionary Escape, and Evolutionary Not-Escape

Today I Learned:
1) Many owls have asymmetric ears! One ear is higher than the other, so that they can use use binocular (binaural?) hearing to locate prey in the up-down axis as well as the left-right axis.

2) I’ve been wondering for a while how much of a deal evolutionary escape of genetic circuits is. The idea is that when you put a genetic circuit to, say, detect the presence of a small molecule or metabolize a drug, into a bacteria (or other creature), it’s often not particularly healthy for the organism. It might not be *terrible*, but the bacteria that’s spending energy making insulin isn’t going to grow as effectively as the wild-type bacteria — in other words, there’s a fitness penalty associated with the genetic circuit. Which means there’s evolutionary pressure against having that circuit. Evolutionary escape is what happens when one bacteria happens to evolve a trait that somehow shuts down your circuit, a trait which then rapidly spreads and takes over your population.

So… how often does this happen? How big a problem is it? It turns out the answer is, “a lot”, and “potentially big”. For instance, it turns out that it’s really hard to clone and miniprep (i.e., isolate DNA from) some plasmids with constitutively active genes on them because the activity is *so deleterious* that you end up cloning almost exclusively broken plasmids that don’t do anything anymore.

It’s also a problem in industrial contexts — apparently most industrial bioreactors run in “batches”, where a reactor gets filled with bacteria, runs for a few days or a couple of weeks, then gets cleaned out and replaced with a new batch. One of the reasons is that the bacteria start escaping their circuits quite rapidly, and after a few days there’s no point in keeping the reactor going.

3) More on evolutionary escape — turns out circuits in yeast cells escape much more slowly, and can be run continuously on the order of months or years. Why would this be? (For those who wonder, lots of places still use bacteria because it’s much easier to develop genetic circuits that work in bacteria.)