[.ca] The Art of Being a Parasite (ISBN 0226114384)

The little \oand some not so little\c buggers are everywhere!:
The term "parasite" usually evokes the image of some little critter feasting off some "host" unable to reject it or cast it away. Claude Combes wishes us to revise that simplistic description in favour of a more realistic view. "Parasitism" needs better definition. He prefers a more descriptive term, "mutualism" which covers more biological territory. In this wonderfully conceived and beautifully written account of what science has learned about parasites, he explains how species interact, sometimes to mutual benefit. The "art" of being a parasite resides in their evolutionary history. Some creatures, once free-living, have managed to occupy others at various surface contact areas or internally. The mitochondria in our cells, the "energy engines" were clearly once free-living bacteria. Invading cells, they paid a "rent" of genes donated to the main genome in the nucleus. The arrangement is apparently incomplete, as mitochondria still make bids for independence. In some cases, the intruder merely occupies the host, generally on its way to another species to enter its reproductive phase. Other invaders proved to interact so well with their hosts that they have become entirely dependent on each other for survival. Combes lays all these situations out for us, describing the process as part of the "evolutionary arms race". That arms race has other applications such as predator-prey interaction, but the result in that scenario has no mutual benefit - the predator wins, eating the prey, or loses and goes hungry. The key to parasite-host relations lies in two filters. The invader must pass an encounter filter, which might reflect little more than availability. If a potential host is not close to the parasite, there's nothing to attach itself to. If the parasite is species specific - as is the case with the wasp-fig associations, the potential parasite will expire. The host may have mechanisms in place to resist the intrusion. If the parasite gains entry, a "compatibility" filter situation arises. The host may have immunity elements that cast off the intruder. Both these filter systems are the basis of the evolutionary arms race between parasite and host. That situation has been credited with being the foundation for all evolution. The erection of the filters by the host and changes to circumvent them by the parasite may have brought about selection changes. This is the basis for much of what's called "coevolution" - an ongoing process over time in which each species changes in response to changes in the other. While "parasitism" is generally considered to be one species utilising another's resources - even if the parasite is using the host as a way-station to another host - there are many cases in which the arrangement is more mutual. The wasp-fig liaison is fairly well known now with the wasps acting as pollinators between the fig plants. Except that the wasp lives in galls formed in the fig's branches, it resembles the action of bees with flowers. A less known mutual arrangement is the inhabiting of molluscs by bacteria. The mussel provides a sheltered environment, but feeds on the bacteria. Yet enough are permitted to survive to allow them to reproduce and infest other mussels. As Combes notes, "Who is exploiting whom?" It's a big question, since "parasites" make up more than half the planet's biomass and human beings are subject to more parasites than any other species. Not all parasites are microscopic nor even small. One of the more recognised "parasites" are the cuckoos of Europe and cowbirds of North America. Both lay eggs in the nests of other species. These, in turn, have sometimes learned to recognise the intruder's eggs and cast them from the nest, or the nest is abandoned with the mating pair relocating to a new site. Less commonly known is a tapeworm inhabiting whale intestines. Combes declares it to be the longest creature living - at 40 metres! All these elements are presented in a beautifully written \othank you, David Simberloff for an excellent translation\c and effectively illustrated book covering a topic many would avoid. They shouldn't. Given that parasites are so complex and prevalent they are creatures and lifestyles we need to know more about. \ostephen a. haines - Ottawa, Canada\c