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Comparative Genome Sequence of Bees Elucidates Control of Social Behavior

The complete genome sequence of an organism, whether that of man or a honey bee is an encyclopedia of information written with only four letters (A, T, G, and C) and no punctuation.  To complicate matters, the number of ATG&C differs a great deal among organisms.  The smallest known insect genomes have about 100 million letters, while the honey bee genome has 275 million letters. You and I have a lot more letters in our genome (1.5 billion), and yet the humble grasshopper may have many, many more, (up to 18.5 billion) letters.  We are only starting to learn what all this means. 

Scientists have many questions they would like to ask of the genome sequence.  Among these is, “How is behavior determined?”  The honey bee, with its many complex behaviors seemed an obvious place to start in an effort to answer this question.  The honey bee genome sequence is relatively small, complete and accurate.  The genes have largely been identified and the information gained to date is readily available at hymenopteragenome.org/beebase.  Further, comparative genetics seemed the way to address the behavior question.  The idea was, lets sequence other bees with easily compared behaviors and see if we can find genome sequences that change in concert with the behavior.  Dr. J Spencer Johnston, Department of Entomology at Texas A&M Unniversity, was engaged early in this process to measure the number of nucleotides in bees that seemed good candidates for this comparative study.

Dr. Johnston’s findings had a profound influence on the direction of a study he recently co-authored in the journal Science "Genomic signatures of evolutionary transitions from solitary to group living".  He found that the small honey bee genome is the exception.  Other bees can have up to 4 billion letters in their genomes.  That meant a great deal more effort and that extra effort limited the number of species that could be compared.  That hurtle addressed, the sequences were generated for four bees with differing levels of social behavior, and comparable sequences from four other bees were added to the study as they became available,. The comparison of the genome sequence and the level of social behavior provided answers that were fascinating; it isn’t the sequence itself that changes coincident with behavior.

First, it was found that the more social insects share with man the ability to control the expression of genes by adding a simple CH3 (Methyl) group to one of the letters  (usually a methyl group will be added to a C that is followed by a G).  The more social the bee, the more genes contain a methylated C.  The honey bee appears to takes this methylation one step further than in other organisms.  As honey bees age, they take on increasingly risky jobs.  Only the oldest bees take the high risk jobs leaving the hive to forage and scout.  Younger bees tend the queen and maintain the hive.  What does that behavioral change have to do with these methylated Cs?  Coincident with this change in behavior is a change in methylation.  Older bees have less and less of their genes controlled (usually reduced in activity) by methylation of a CG couplet.  Methylation is one signature social behavior.

The scientists went on to ask, what else changes with sociality.  The answer opens what will likely be a lively area of research.  The answer was that the number of interactacting genes (the gene network) grows increasingly complex as bees become more social.  In the most extreme form (eusociality) where workers give up their own fertility to tend a sister (the queen) and her brood, the level of networking is extensive.  The increasing level of networking we observe as insects become more social raises many interesting questions.  Is this same true in man?  Are altruists characterized by an increasingly complex gene network.  How about other vertebrates? Is the complexity of the gene network one of the ways we differ from other primates?  What about our favorite pet?  Has domestication changed the level of networking?  Some would even ask, do cats even have such a network?

In the long run, the study of eusociality is a study of man.  We know a great deal about the sequence of letters in the human genome, but we have much more to learn. The sequence is inherited, but the changes we identified here are not in the sequence itself.  The environment plays a role.  Our future may depend upon knowledge of that role.

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