Thursday, April 19, 2012

The birds and the bees, for bees

Sex-determination is simple: omitting errors, everyone gets an X chromosome from their mom.  Dad supplies either an X or Y, which makes you either a male or a female, respectively.  End of story… unless you’re not a mammal or a fruit fly or a ginkgo tree (or a few other things).  

Indeed, most of the living world does not share our determination of sex.  In fact, the largest order of insects, which themselves make up a vast proportion of all living things, operates on a sex-determination system wholly foreign to us.  Hymenoptera (ants, bees and wasps) are haplodiploid, meaning some members are diploid, like us, and have two full sets of chromosomes (one from the mother and one from the father), while the rest are haploid and have only one set of chromosomes.   

Here’s how it works:  a male drone fertilizes a female, which becomes a queen.  The queen establishes a colony in one of many possible ways.  Winter becomes spring, spring becomes summer, summer changes back into winter, and winter gives spring and summer a miss and goes straight into autumn.   

Eventually a mature colony is formed.  For utility’s sake, here’s where I’ll begin in earnest.  In a mature colony, the queen mates rarely and stores sperm for long periods of time.  Meanwhile, she constantly produces eggs, sometimes fertilizing them, sometimes not.  Fertilized eggs receive half of her DNA and half of the father’s.  These invariably become females.  Unfertilized eggs still develop into fully functional ants, bees or wasps,  but only contain one set of maternal DNA.  These become the males, known as drones. 

It is the job of drones to fertilize potential queens.  Typically, the meeting of the two occurs away from the hive or colony, and afterward the queen leaves to start anew.  Thus, one queen usually inhabits a hive, and all the workers (female), future queens (female) and drones (male) are her offspring.  Drones and queens get to mate, but workers do not.  Rather, they slave their lives away in service to the queen, living and dying with no hope of passing on their genes. This raises questions in the face of Darwinian evolutionary understanding:  how could such a system evolve?  Why would workers be willing to sacrifice their lives to the queen, in perfect altruism, against the prospect of their own procreation? 

The answer lies in the subtlety of hive-interrelatedness.  Consider the following: 

Males have only one set of chromosomes to contribute when they mate, so they pass on 100% of their DNA to female offspring.  Females, on the other hand, have two sets of chromosomes, so 50% of their DNA passes on to daughters and 50% to sons.  Whether the queen produces daughters or sons, there will be a shared gene proportion of ½ between mother and offspring. 

Now, consider the shared gene proportion between sisters.  Any female Hymenopteran will have 100% of her father’s genes and 50% of her mother’s DNA.  So will her sisters (because most of the time all hive members are of the same parents).  The total relationship between sisters is then the average between 100% and 50%, or 75%.  This means sisters are more closely related to each other than they are to their mother or to any potential offspring.  It is therefore in the best interest of a worker to aid her mother in the production of sisters.  Although she cannot directly transmit her genes to offspring, they can be passed on even more effectively through the success of the queen and of the colony.

In parting, I offer you this image of a wasp wearing a parka, courtesy of

My sister got it for me!

1 comment:

  1. This just came up last week in genetics class--including questions about the 50% vs. 75% relatedness of brothers and sisters. Perhaps I'll direct the class towards this post...