Friday, January 18, 2013

Burrowing into the unending nature vs nurture debate

Behavior modules
Two interesting stories about behavior this week.  You've probably read about the mouse that builds its nest in a stereotypical way.  The Nature paper (Weber, Peterson, and Hoekstra) describing this was reported in The New York Times, as a story about how genes control complex behavior. 

The research was motivated by the question of how genetics influences the evolution of complex behaviors, but the researchers also wonder whether the environment influences variation in heritable behavior, and whether many or few genetic changes affect behavioral evolution.

The authors describe the burrow built by oldfield mice as having a long entrance and escape tunnels, and they say that the length is consistent wherever these mice build them, although tunnel depth depends on the soil in which they build.

Deer mice make a smaller, simpler burrow, and when the two species are interbred in the lab, the offspring reproduce the more complex burrow of the oldfield mouse, though with varying length.  First generation backcross mice (offspring of the interbreeding then bred with the parental strain) build burrows of varying length, and not all built escape tunnels, which suggests to the researchers that these behaviors are separately determined. 

The researchers then looked for genetic loci that might be influencing these behaviors and found three different loci that seem to contribute to tunnel length, and another that influences the building of escape tunnels.  As explained in the NYT:
All complicated behaviors are affected by many things, Dr. Hoekstra said, so these regions of DNA do not determine tunnel architecture and length by themselves. But tunnel length is about 30 percent inherited, she said, and the three locations account for about half of that variation. The rest is determined by many tiny genetic effects. As for the one location that affected whether or not mice dug an escape tunnel, if a short-burrow mouse had the long-burrow DNA region, it was 40 percent more likely to dig a complete escape tunnel. 
Well, if tunnel length is about 30 percent inherited (and there is variation in tunnel length; average length is 181 cm, with a standard deviation of ±53), and the three locations account for about half of that variation, or perhaps 15% of the variation in tunnel-building, then the rest of the causation is not due to a few major genes.  Though, if it's 70% non-genetic, then one wonders how the environment specifies that a tunnel will have an escape hatch.

The authors conclude that "discrete genetic modules" control complex behaviors and that, "Together, these results suggest that complex behaviours—in this case, a classic ‘extended phenotype'—can evolve through multiple genetic changes each affecting distinct behaviour modules."  One could challenge this rather strong genetic-determinism view.  But how genes (much less environments) determine this type of thing is interesting and challenging to think about.

Overcoming behavior modules
Manduca sexta, or Hawkmoth;
Wikimedia Commons
The second story this week is from a paper in Science reporting that while the hawkmoth seems to innately prefer the nectar of night-blooming flowers, it can learn to collect nectar from other sources.  These moths seem to innately prefer flowers that give off a specific class of aromatic compounds, or at least the aromatics of their preferred flowers are processed similarly by the moth's olfactory response.

But the researchers wondered whether olfactory response necessarily reflects the moth's preference.  To test this, they exposed naive moths to paper flowers scented with different classes of compounds from those they generally prefer.  The moths were then observed to visit flowers emitting these scents at similar frequencies to those they innately prefer.  The authors conclude that "olfactory conditioning provided moths with flexibility in their foraging behavior but did not extinguish their innate preferences for scents from the moth-pollinated flowers." They suggest that "olfactory conditioning may operate in an olfactory "channel" separate from, but parallel to, that involved in the innate responses."  Modularity again. 

So much for exquisitely fine-tuned co-evolution of flower and pollinator. 

Glass half full/half empty
There's something for everyone here. The mouse researchers conclude that behavior is modular and evolves by the mixing and matching of genetic modules. Maybe, but there's plenty of non-heritable influence on mouse burrowing behavior as well, if tunnel length is only "about 30 percent inherited."

Modularity is a fundamental principle of life, and the idea that behavior, as well as morphological traits, can be modular is certainly a possibility -- every 4th grade class has its clown and its bully and its teacher's pet, after all.  But this doesn't mean that it must be genetically determined.  It could be, but the evidence isn't yet there. Maybe the non-mapped variation has to do with general cognitive function and how an animal scopes out its environment and decides what would be a good strategy for living in it.

We are all ultimately reducible to genes, in the sense that they are the basis of who and what we are.  But, as the hawkmoth study shows, the fact that even a lowly moth can easily overcome "innate" behavior with a whiff of something it wasn't born to crave is yet further evidence that organisms are inherently adaptable, a fundamental principle of life.  Given that environments can change quickly and unpredictably, that's a good thing. 

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