Can an NPR gene explain gregarious and solitary behavior?
This week's issue of Science has a special section on the genetics of behavior. In humans, at least, conflicting results seem to be common. According to the overview paper, one study found that people who were abused as children have a two-thirds chance of depression as adults, if and only if they also have a particular version of a serotonin transporter gene; an analysis of several studies on this gene rejected this conclusion, however. Other combinations of genes affecting testosterone and the rate at which certain brain chemicals break down may increase criminal activity, but maybe not. One company is already offering genetic testing of potential mates for a gene possibly linked to divorce. How soon will we see "in-vitro tourism", to countries that offer genetic modification of behavior-linked genes in human germ-line cells? Clinics that currently offer untested or dangerous "cancer cures" might not wait for answers to the many scientific, technical, and ethical questions that such services would raise. But genetic effects on behavior are complex and hard to predict, even in the simplest cases, as seen in this week's paper.
"npr-1 regulates foraging and dispersal strategies in Caenorhabditis elegans" was just published in Current Biology by Andrea Gloria-Soria and Ricardo Azevedo.
C. elegans is a tiny worm, barely visible without magnification. Two alleles of the npr gene produce two different versions of the NPR signal-receptor protein, differing in a single amino acid. (Despite the connection to signals, the NPR gene actually has as much to do with National Public Radio as the famous "sonic hedgehog" gene has to do with hedgehogs.)
Worms with different versions of npr have consistent behaviors referred to as "gregarious" and "solitary," but these traits may be rather different from their human equivalents. "Gregarious" worms appeared to hang out together more than "solitary" worms, but it's not clear that they necessarily enjoy each other's company more. The worms eat bacteria, which (in the lab), spread out from the center of a Petri dish. They used fluorescent bacteria as worm-food, so feeding locations could be measured from changes in fluorescence, controlling for bacterial reproduction.
When there were lots of worms on each Petri plate, 79% of "gregarious" worms apparently preferred the growing edge of the bacterial swarm, whereas "solitary" ones were indifferent to where they fed, with only 35% feeding on the edge. So "gregarious" worms might just end up relatively close to each other because they all like life on the edge. Similarly, if beer were only available in bars, we might see beer-lovers as gregarious, whether or not they actually enjoy socializing. But when they tested single worms, they found no genetic difference in the percent of time spent on the edge. "Solitary" worms appeared to dislike other worms: they spend most of their time on the edge, but not if there are other worms there. So maybe "tolerant of crowds" would a better description of the "gregarious" worms.
If "gregarious" worms are more tolerant of crowding, you might expect them to disperse less. But they found that the opposite was true. When worms started at the center of a Petri plate with rings of bacteria, most of the "solitary" worms stayed in the center, whereas the "gregarious" ones distributed themselves across the plate.
Groups of worms consume more oxygen, lowering its concentration. If "gregarious" worms are more tolerant of low oxygen, that could explain their apparent tolerance of crowding. But their greater dispersal tendency seems inconsistent with this, as dispersal takes them into regions of higher oxygen.
Bacteria may be younger and tastier at the growing edge. This may be less important to "solitary" worms, relative to their aversion to crowding or low oxygen. Could greater dispersal by "gregarious" worms also be a search for younger bacteria?
I have focused on behavioral issues in this paper, but the authors also asked an evolutionary question which comes up repeatedly whenever two or more genotypes persist over generations: why doesn't one genotype out-compete the other? They showed that spatial structure may be important, given differences in dispersal, etc. But the "gregarious" strain lost in competition, whether or not the environment was fragmented. Still, this paper is a nice illustration of the complexity of behavioral differences that can result from the simplest possible genetic difference.
Incidentally, they cite a paper by fellow blogger John Dennehy, whose summaries of keystone papers are themselves classic.