Behind-the- 

 Scenes Role 

 of Parasites 



^ The fates of mushrooms, flies, 

 Q and parasitic nematodes are 

 , r>H intricately intertwined 



by John Jaenike 



*^ As every mushroom picker knows, 

 p]^ many of the mushrooms that spring from 

 the earth after a summer rain are crawling 

 with insect larvae. Cut open the stalk of a 

 bolete or an amanita and you may find 

 hundreds of tiny, white larvae tunneling 



through the fungus. In addition to humans 

 and other mammals such as deer and 

 squirrels, numerous insects compete for 

 the mushroom's flesh. In turn, these in- 

 sects are eaten by beetles, ants, and toads 

 that visit the mushrooms. By drawing nu- 

 trients from the surrounding trees and soil 

 through a network of mycorrhizae, mush- 

 rooms form the base of a small, but com- 

 plex, food web. 



In the 1970s, I became interested in 

 mushroom-feeding fruit flies while con- 

 ducting fieldwdrk on an island off the 

 coast of Maine. All around me, the tiny 

 flies were busy laying eggs on hundreds of 

 mushrooms that would provide food and 

 shelter for the developing larvae. Because 

 they were in the same genus as Drosophila 

 melanogaster, the workhorse of laboratory 

 genetics, I realized that these flies would 

 be ideal for studying ecological relation- 

 ships. Later, when I learned that two of the 

 most common mushroom-feeding fruit 



flies, D. falleni and D. piitrida, can be in- 

 fected by a parasitic nematode worm 

 called Howardula aoronymphium, I 

 wanted to leam whether this microscopic 

 parasite affected the relative abundance of 

 the flies. 



Traditionally, ecologists studying why 

 one species tends to be more abundant 

 than another will focus solely on such fac- 

 tors as competition or predation. Increas- 

 ingly, however, we are realizing that the 

 parasites an organism harbors can be 

 equally important in determining a 

 species' place in the larger ecological 

 community and, ultimately, in its evolu- 

 tionary history. 



Some parasites may evolve with a par- 

 ticular host for millions of years, presum- 

 ably in a state of equilibrium. The chewing 

 lice that parasitize pocket gophers, for ex- 

 ample, are so genetically different from re- 

 lated hce that their association with the go- 

 phers must be a long one. Although some 

 host-parasite associations are long-stand- 

 ing, many other parasitic relationships are 

 quite ephemeral — at least on an evolution- 

 ary time scale. The Howardula nematodes 

 are internal parasites of various arthro- 

 pods, including flies, beetles, thrips, and 

 mites. Such a broad range of hosts sug- 

 gests that the worms can occasionally shift 

 from one host to another. 



The worms enter the fruit fly larvae by 

 piercing their outer cuticle when they are 

 feeding within the mushroom. The nema- 

 todes thrive inside the developing flies, 

 which as adults spread the parasites to 

 other mushrooms. The major deleterious 

 effect that H. aoronymphium has on some 

 fruit fly species is that it makes the females 

 sterile. Nematodes, therefore, could have 

 drastic effects on susceptible fruit fly pop- 

 ulations. The parasites don't eliminate 

 their hosts, however, because if the fly 

 populations fall too low, transmission 

 rates decline, so fewer flies become para- 

 sitized, thus allowing the fly populations 

 to recover. 



Recently, I observed just how fleeting 



A stinkhom mushroom, left, attracts 

 fruit flies by the dozen. These 

 mushrooms can often be smelled before 

 they are seen. A trio o/ Amanita 

 muscaria mushrooms, right, push up 

 through fall leaves. As they age, 

 they will attract a host of insects 

 and their parasites. 



J. L. Lepore; Photo Researchers, Inc. 



46 Natural History 6/94 



