TWO B, or not two B? That is the question. Especially if you are a young
queen fire ant just reaching reproductive maturity. You had better hope that you
are carrying the right gene, because if you have BB, rather than Bb, you are in
trouble. Any of your servants who are not also BB will turn on you, ripping you
limb from limb.
On the face of it, this looks like a clear case of nepotism. You don鈥檛 share
my genes, you are not family, so it鈥檚 off with your head! While such behaviour
seems a little barbaric, its evolutionary credentials are flawless鈥攐ne
selfish gene is simply ensuring its own genetic legacy at the expense of
another. A closer look shows that things are not quite so simple, because all
queens鈥攄oomed or not鈥攁re related to all worker ants in their nest.
Even so, the discovery that fire ants discriminate on the basis of a queen鈥檚
genes makes a crucial link between behavioural evolution and genetics. This is
the first example of what theorists have called a 鈥済reen beard鈥� in nature.
In the fight for survival, an organism that looks after its relatives will
promote the survival of identical copies of its own genes. So it is hardly
surprising that many animals favour their kin above other individuals.
Distinguishing relatives from outsiders is not always easy, though. But what if
all family members carried a distinctive signal, such as a green beard? A gene
that linked such a marker with the urge to be nice to others with the same
marker should ensure that nepotism was not misplaced.
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For three decades it remained just a theory, then last year Laurent Keller
from the University of Lausanne in Switzerland and Kenneth Ross from the
University of Georgia, discovered a 鈥済reen beard鈥� in North American fire ants
(Solenopsis invicta). Rather than colourful facial hair, the b gene
gives all its carriers a distinctive scent, which is what saves Bb queens.
Without this, BBs are betrayed to mutinous workers. What鈥檚 more, the only
workers to play no part in this assassination are the BBs. Keller and Ross see
this as a sure sign that the b gene鈥攐r a gene very closely linked to
it鈥攃auses the aggressive behaviour. Here is the green beard. A gene that
both signals kinship, through scent, and prompts kin selection, through
aggression to queens that lack b.
鈥淯sually, a green-beard gene would quickly spread until the entire population
carried it,鈥� says Keller. This is why green beards have been so hard to find in
nature. So why hasn鈥檛 the fire ant population not simply lost the
disadvantageous B altogether? One reason is that a double dose of b is lethal to
queens. 鈥渂b queens always die young,鈥� says Keller. So the assassination of BBs
is cancelled out. Since only Bbs survive to reproduce they will pass on B to
half of their offspring and b to the other half. 鈥淭he system is stable,鈥� he
says.