Wednesday, March 14, 2012

The buzz about novelty-seeking

One topic I have always been interested in is how complex behaviors are driven by genetics and molecular biology.  For instance, all the courtship rituals of a male fruit fly are basically encoded by a single gene, and more specifically one small exon (a chunk of DNA within a gene).  A recent paper by Liang et al. which was published in Science, looked at novelty-seeking behavior in honeybees.  The nice thing about this paper is that they started out by finding bees that like to scout for food and nests and then worked backwards to figure out what is different about these bees compared to their boring, steady-as-she-goes sisters.

Novelty-seeking in bees
 There are two types of novelty-seeking behaviors which the authors investigate. 

1) Food scouting: Even when there are plentiful known food sources, some bees (5-25% of foragers) still go out to look for new sources.  The other foraging bees rely on these food scouts, who tell them where the new food source is.  The food scouts are obviously important for a colony, so the bees always know where to get food.
Photo taken by Rebecca Wenk 2010.

2) Nest scouting: When a swarm has left its colony in search of a new location, a few bees (<5%) will search out for the best place to build a new hive.  They will tell the rest where to go and lead them there.

Are nest scouts the same bees as food scouts?

The researchers identified nest scouts in artificial and natural swarms and marked them.  Then they waited to see if these nest scouts would become food scouts as well.  They moved the hives every night to a new location.  The bees that seek new food sources will not be thrown by the new environment and will be able to find new food sources quickly.  Under these circumstances, the authors found there was a trend for the former nest scouts to also be food scouts, though it wasn’t a hard-and-fast rule.

Molecular underpinnings
All worker bees are female and have the same genetic make up— they’re all identical sisters.  What would make some bees more likely to explore novel environments?  To examine this question, the authors collected food scouts.  While the bees slept in their hives, the authors introduced a new feeder in different locations each day.  They collected bees that visited the new feeder twice, when it was in two different locations.  Once they had the food scouts, they performed a microarray to determine differences in gene expression between food scouts and non-scout controls.

If you want to know more about how microarrays work, visit my methods section.  All you really need to know is that a microarray looks at every single gene and determines how much it is expressed.  Since all bees have the exact same genes, the only differences are going to arise by how these genes are expressed (remember: not all genes are expressed at a given time).

The authors found that scouts have differential expression of various neurotransmitter systems.  Neurotransmitters are the signals that are sent from one neuron to another.  They found that scouts had higher expression of genes involved in GABA and glutamate neurotransmission.  In general, GABA is used to inhibit neurons, and glutamate is the standard excitatory neurotransmitter.  GABA and glutamate may control different neuron circuits in the brain.  They also saw a decrease in a receptor for dopamine.  What this implies is that when the brain has increased GABA and glutamate signaling, but decreased dopamine, it makes the bees want to seek out novel environments (or maybe it makes them less afraid to try something new). 

To prove that this hypothesis is correct, the authors took some non-scouts and fed them glutamate.  These non-scouts with extra glutamate were more likely to search for new food.  In other words, they switched the behavior of non-scouts to novelty-seeking by changing the balance of neurotransmitters in their brain.  Unfortunately, this didn’t work when they tried to do the same thing with dopamine.  They inhibited the dopamine receptor, which should also increase food scouting, but it actually made the non-scouts even less likely to scout for food.

It’s cool that the authors did behavioral studies in bees, but I feel like something was missing in this paper.  Okay, neurotransmitter systems are expressed differently, but why?  A paper from last year showed that queen bees have different epigenetic marks than worker bees, so maybe this is what’s going on with the novelty seekers.  Is there some difference in their environment as they develop, which causes epigenetic changes and differential gene expression?  There is a lot more to explore in this topic.

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