RAPID EVOLUTION OF HOST SPECIFICITY IN A PARASITIC NEMATODE

In eastern North America, the nematode Howardula aoronymphium parasitizes four species of mushroom-breeding Drosophila: D. falleni and D. recens of the quinaria species group, and D. putrida and D. testacea of the testacea group. One strain of H. aoronymphium, designated Mendon-87, was initially capable of infecting all four of these host species. After less than 3 years in laboratory culture using D. falleni as the sole host, this strain had completely lost the ability to infect D. putrida. Two other nematode strains parasitized D. falleni and D. putrida at equal rates. These results demonstrate the existence of genetic variation for host specificity within this nematode species. More importantly, they show that host specificity can evolve rapidly when only one host is available for parasitization. Ecological conditions are such that natural populations of H. aoronymphium may comprise numerous host races, lineages incapable of parasitizing the full range of host species. However, I argue that such host races are probably ephemeral and thus unlikely to persist long enough to undergo speciation.