Parasites Take the Biological Spotlight
By NATALIE ANGIER
Published: July 17, 1990
TO the ancient Greeks, the word ''parasite'' meant one who eats at the table of another. But far from having the decency to sit down for dinner, most parasites in nature suck the blood, sip at the gastric juices of the intestines, pierce into the nourishing warmth of muscle tissue and otherwise leech rudely off the fluids and labors of their unwilling hosts.
Yet for all their repulsive traits, worms, mites, fungi, viruses and a rogues' gallery of other parasitic organisms that derive their nutrients from a larger species hold an increasing fascination for evolutionary biologists. After long dismissing the freeloaders as bit players in the grand theater of evolution, scientists now propose that parasites may explain many of the outstanding features seen in a broad spectrum of animals and plants.
They say that the relentless pressure of a diverse battery of parasitic species ready to colonize and exploit every square millimeter of larger creatures has helped shape many of the traits of the beleaguered hosts. And because parasites breed so quickly and so prolifically, scientists say, their effects on their hosts is particularly powerful.
At a meeting of evolutionary biologists last month in College Park, Md., researchers presented a wealth of new data suggesting that parasitism is what gave birth to sex. They proposed that animals and plants mix their genes together sexually, rather than simply generating clones of themselves, to create diverse offspring as a way of assuring some resistance to parasites. An article on the subject by one of the foremost theoreticians of evolutionary biology, Dr. William D. Hamilton of Oxford University appeared in the May issue of The Proceedings of the National Academy of Sciences.
Other biologists have gathered compelling evidence that the need to dodge parasites may have been the force prodding some species to become migratory or to spend part of every year in isolation from their potentially pest-ridden fellows.
''Historically, the study of parasites has been utterly, completely and egregiously neglected,'' said Dr. Douglas E. Gill, a parasite expert at the University of Maryland in College Park. ''Now that's all changing. There's a great deal of interest at the medical, molecular and ecological level of what parasites are doing out there. It's a new frontier.'' Some researchers are considering the story from the parasite's perspective. They are trying to understand why many species of parasites go through multi-stage life cycles, passing from one host to another. In probing the parasitic reproductive cycle, researchers have unearthed examples of host-parasite relationships that border on the macabre.
Dr. Manfred E. Rau, a parasitologist at McGill University in Montreal, recently found that two types of closely related parasitic worms can dramatically influence the behavior of mice to suit their own needs. One worm will prompt the mouse to become hyperactive, scampering through fields so frenetically that it attracts the attention of a predatory bird that will eat the mouse and the worm with it. When the bird eats the mouse, it provides the necessary next home for the parasitic larvae.
By contrast, the related worm species will cause a mouse to become sluggish, heightening the chance it will be easily stalked down by the carnivorous mammals this worm prefers for its second shelter.
Other parasitic larvae have been found to drive host snails mad, forcing the creatures to make a suicidal ascent to the top of a blade of grass, rather than hiding underneath the foliage. At the same time, a few of the invading larvae migrate to the snail's antennae, turn bright colors and pulsate, transforming the hapless gastropod's feelers into a reasonable facsimile of a caterpillar. That resemblance catches the attention of birds, which then consume the infested snails. Once in the guts of the birds, the larval worms can mature and reproduce.
''There are fantastically interesting questions to be answered,'' said Dr. Gerhard A. Schad, professor of parasitology at the University of Pennsylvania and the president of the American Society of Parasitology. ''How do these hosts and parasites come to live together? Who's ahead at any given moment in the evolutionary game plan?''
Offshoot of Immune System Studies
Some researchers say the new interest in parasites stems partly from the extraordinary advances in the study of the human immune system. As investigators have deciphered its staggering complexity, they have come to consider the varied spectrum of parasites and pathogens that the immune system evolved to attack.
''Parasites have had a tremendous impact on human evolution, which we're only now beginning to appreciate,'' said Dr. Dickson D. Despommier, a professor of public health and microbiology at Columbia University. ''About half of all people who have ever lived on earth have died from malaria or malaria-related problems, and malaria is caused by the Plasmodium parasite. That's an astonishing notion. The early American colony of Jamestown had to be re-established three times because of malaria.''