o 



o 

 > 



the relationships between H. aoronymph- 

 ium and their fruit fly hosts can be. Several 

 years ago, after determining that wild 

 strains of the nematodes can infect both D. 

 falleni and D. putrida, I began to maintain 

 one strain in the laboratory using D. falleni 

 as the host. (At that time, I found it easier 

 to keep the parasite strain going in this 

 host species.) A couple of years later, in 

 the course of doing some other experi- 

 ments, I discovered that these nematodes 

 could no longer infect D. putrida. 

 (Whether the parasite lost the ability to 

 recognize the fly as a suitable host or 

 whether it lost some specific ability to 

 penetrate the fly larvae's cuticle remains a 

 mystery.) 



Just to be sure of my observation, I 



tested two other strains of H. aoronymph- 

 ium that I had obtained from the wild more 

 recently. These nematodes infected about 

 two-thirds of both species of fruit fly. My 

 original laboratory strain could still infect 

 D. falleni with the same frequency as these 

 wild strains, but it had completely lost its 

 ability to infect D. putrida after being iso- 

 lated from it for fifty generations. Because 

 a fruit fly generation is only two weeks in 

 the laboratory, fifty generations is but a 

 blink of an eye on an evolutionary time 

 scale. Presumably, the abihty to infect a 

 fruit fly host can be reacquired in a com- 

 parably short time. 



How does the parasite's ability to shift 

 rapidly from species to species affect the 

 evolution of its hosts? Consider a case in 



which the outcome of competition be- 

 tween different host species is mediated by 

 a shared parasite species, ff, for instance, a 

 nematode infects one fruit fly species 

 more frequently than another, that can tip 

 the scales in favor of the less-affected spe- 

 cies. But if this less-affected host becomes 

 more abundant, the parasite may respond 

 by becoming speciahzed on it, thus reduc- 

 ing the host's competitive advantage. In 

 this manner, a rapidly evolving parasite 

 may enable competing host species to co- 

 exist — as in the case of D. falleni and D. 

 putrida. 



A parasite's ability to rapidly shift hosts 

 might have large effects on island popula- 

 tions. Suppose that on the mainland, a ne- 

 matode like H. aoronymphium parasitizes 

 two competing fruit fly species, such as D. 

 falleni and D. putrida. An island may be 

 colonized by one of tiiese host species and 

 later — or simultaneously — by the nema- 

 tode. Given my laboratory results, I would 

 expect the newly arrived nematodes to 

 quickly become specialized on the island 

 fruit fly. When the other host eventually 

 colonizes the island, it wiU be at a compet- 

 itive advantage because the resident nema- 

 tode can no longer parasitize it. 



Finally, the Howardula results may be 

 relevant to the evolutionary enigma of sex. 

 The abihty of a parasite to exploit a partic- 

 ular host, and the response of that host to 

 the parasite, can depend on the specific 

 genotypes of the players. For instance, 

 monocultures of crop plants, often bred to 

 be genetically identical, are particularly 

 susceptible to epidemics of parasites 

 adapted to specific crop genotypes. Such 

 observations have led to the hypothesis 

 that parasites favor the maintenance of 

 sexual reproduction in their hosts because 

 sex serves to reshuffle the genetic deck 

 every generation, preventing any one 

 genotype from becoming especially com- 

 mon, and thus vulnerable. 



Parasites of one kind or another have 

 managed to invade almost every organism, 

 from the mushroom-dweUing fly larva to 

 the human body. And although they wiU 

 never be apparent to most people who 

 sttoll through flie woods, their effect is 

 great. Interacting with otiier species, they 

 play an important role in balancing eco- 

 logical communities. 



A fly and insect pupae cling to the 

 underside of a broken mushroom cap. 



Joy Spurr 



48 Natural History 6/94 



