| |
Pest
Control
 Along
with bees, wasps and ants, termites are one of the
main groups of social insects. These fascinating
insects are characterized by a reproductive division
of labour: the vast majority of colony members are
usually sterile and perform work, while just a lucky
few undertake all the reproduction. This phenomenon
has traditionally been considered a kind of evolutionary
conundrum. How could genes that cause sterility
be passed on to offspring?
The answer appears to be that such 'altruism' genes
are switched off in some individuals (reproductives),
and switched on in others (workers).
Termites can thus be considered a type of derived
cockroach. Cryptocercus is a sub-social cockroach:
an adult male and female raise a single clutch of
offspring over a period of several years. The ancestor
of termites and cockroaches presumably had this
characteristic. A key step in the evolution of termites
would have been the production of second clutch
of offspring, which would have been raised by the
first group of offspring. This would have allowed
the queen to focus on egg-laying, and so on.
Cellulose, the main component of wood, is the most
abundant organic compound on earth, with global
annual production estimated at 100 billion tonnes.
Breakdown of cellulose by living organisms helps
the earth's carbon cycle to continue. It is widely
believed that such 'recycling' is done only by microbes
such as bacteria, fungi
and protozoa, and that animals can't digest cellulose
by themselves. My research has shown that this belief
is unfounded.
Termite protozoan. Photo: Visuals Unlimited
Termites are a text-book example of 'symbiosis':
they provide a home (their lower intestines) to
protozoa and/or bacteria, who, in return, convert
the wood into products usable by the termite. Together
with some Japanese colleagues, I identified the
first animal cellulase gene, in a Japanese termite.
The cellulase protein is expressed in the saliva,
where microbes don't live.
Thus termites have two sources of enzymes for cellulose
breakdown, which helps to explain
why they are among the few organisms on earth that
can digest living wood - well known for its strength
and durability. This discovery of termite genes
has led to the determination of the three-dimensional
structure of the enzyme, which will hopefully lead
to the development of environmentally-friendly inhibitors
of these enzymes that can be impregnated into wood
to inhibit termite activity.
Structure of a termite cellulase. Source: Protein
Data Bank Where did these genes come from? If
one looks through the genomes of humans, fly,
nematode, rat, fish and various other animals,
no cellulase genes are present. On the other hand,
these genes are common in bacteria, fungi and
protozoa. Did an ancestor of termites 'pick up'
the gene from a microbe by horizontal transfer?
By looking for cellulase genes in other insects,
and scanning the DNA databases, we have found
evidence for the presence of these genes in very
primitive animals. Thus, these genes have been
lost over evolutionary time by many lineages,
including that leading to vertebrates like ourselves,
but retained in some lineages, like that leading
to insects.
. |
|
|
|
