Science of Discworld III

on Earth, for a special purpose – which is what most people believed at the time – then God had deliberately designed parasitic wasps, whose purpose was to eat other species of insect, also designed by God. To be so eaten, presumably.
    Darwin was fascinated by such wasps, ever since he first encountered them in Botafogo Bay, Brazil. He eventually satisfied himself – though not his successors – that God had found it necessary to permit the existence and evolution of parasitic wasps in order to get to humans . This is what the quote at the end of Chapter 10 alludes to. That particular explanation has fallen out of favour among biologists, along with all theist interpretations. Parasitic wasps exist because there is something for them to parasitise – so why not? Indeed, parasitic wasps play a major role in controlling many other insect populations: nearly one-third of all of the insect populations that humans like to label ‘pests’ are kept at bay in this manner. Maybe they were created in order for humans to be possible … At any rate, the wasps that so puzzled Darwin still have much to tell us, and the latest discovery about them threatens to overturn several cherished beliefs.
    Strictly, the discovery is not so much about the wasps, as about some viruses that infect them … or are symbiotic with them. They are called polydnaviruses.
    When mother wasp injects her eggs into some unsuspecting larva, such as a caterpillar, she also injects a solid dose of viruses, amongthem said polydnaviruses. The caterpillar not only gets a parasite, it gets an infection. The virus’s genes produce proteins that interfere with the caterpillar’s own immune system, stopping it reacting to the parasite and, perhaps, rejecting it. So the wasp larvae munch merrily away on the caterpillar, and in the fullness of time they develop into adult wasps.
    Now, any self-respecting adult parasitic wasp obviously needs its own complement of polydnaviruses. Where does it get them? From the caterpillar that it fed on. And it gets them (just as mother did) not as a separate infective ‘organism’, but as what is called a provirus: a DNA sequence that has been integrated into the wasp’s own genome.
    Many genomes, probably most if not all, include various bits of viruses in this way. Our own certainly does. Transport of DNA by viruses seems to have been an important feature of evolution.
    In 2004 a team headed by Eric Espagne worked out the DNA sequence of a polydnavirus – as one does – and what they found was dramatically different from what anyone had expected. Typical virus genomes are very different from those of ‘eukaryotes’ – organisms whose cells have a nucleus, which includes most multi-cellular creatures and many single-celled ones, but not bacteria. The DNA sequences of most eukaryote genes consist of ‘exons’, short sequences that collectively code for proteins, separated by other sequences called introns, which get snipped out when the code is turned into the appropriate protein. Viral genes are relatively simple, and typically they do not contain introns. They consist of connected code sequences that specify proteins. This particular polydnavirus genome, in contrast, does contain introns, quite a lot of them. The genome is complex, and looks much more like a eukaryote genome than a virus genome. The authors conclude that polydnavirus genomes constitute ‘biological weapons directed by the wasps against their hosts’. So they look more like the enemy’s genome than that of an ordinary virus.
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    Numerous examples, old and new, disprove every aspect of the folk version of evolution and DNA. We end with one that looks especially important, discovered very recently, and whose significance is just becoming seriously apparent to the biological community. It is probably the most severe shock that cell biology has received since the discovery of DNA and the wonderful ‘central dogma’: DNA specifies messenger-RNA which specifies proteins. The discovery was not made through some big, highly publicised research programme like the human genome project. It was made by someone who wondered why his petunias had gone stripy. When all the world is chasing ‘the’ human genome, it’s not easy to get research grants to work on stripy petunias. But what the petunias revealed is probably going to be far more important for medicine than the entire human genome project.
    Because proteins are the structure of living