Selection can favor a diversity of host and parasite genotypes

As mentioned earlier, plant defense systems include a specific response that makes particular plant genotypes resistant to particular parasite genotypes. Such gene-for-gene interactions are well documented in a number of plant species, including wheat, flax, and Arabidopsis thaliana.

Changes in the frequencies of host and parasite genotypes also occur in natural systems. In the lakes of New Zealand, a trematode worm (Microphallus sp.) parasitizes the snail Potamopyrgus antipodarum. The worm has serious negative effects on its snail hosts: it castrates the males and sterilizes the females. The parasite has a much shorter generation time than its host, and hence we might expect that it would rapidly evolve the ability to cope with the snail's defensive mechanisms. Lively (1989) tested this idea in an experiment that pitted parasites from each of three lakes against snails from the same three lakes. He found that parasites infected snails from their home lake more effectively than they infected snails from the other two lakes (FIGURE 13.11). This observation suggests that the parasite genotypes in each lake had evolved rapidly enough to overcome the defenses of the snail genotypes found in that lake.

FIGURE 13.11 Adaptation by Parasites to Local Host Populations Thegraphshowsthe frequencies with which Microphallus parasites from three lakes in New Zealand (Lake Mapourika, Lake Wahapo, and Lake Paringa) were able to infect snails (Potamopyrgus antipodarum) from the same three lakes. Error bars show one standard error of the mean.

Do snails with poor defenses against parasites from their own lake also have poor defenses against parasites from other lakes? Explain.

(After C. M. Lively. 1989. Evolution 43: 1663-1671.) View larger image

The snails also evolved in response to the parasites, albeit more slowly. Dybdahl and Lively (1998) documented the abundances of different snail genotypes over a 5-year period in another New Zealand lake. The snail genotype that was most abundant changed from one year to the next. Moreover, roughly a year after a snail genotype was the most abundant one in the population, snails of that genotype had a higher-than-typical number of parasites. Together with Lively's earlier study (1989), these results suggest that parasite populations evolve to exploit the snail genotypes found in their local environment. Refining this idea further, Dybdahl and Lively hypothesized that as a result of evolution by natural selection, parasites would be able to infect snails with a common genotype at a higher rate than they could infect snails with a rare genotype. That is exactly what they found in a laboratory experiment (FIGURE 13.12). Hence, snail genotype frequencies may change from year to year because common genotypes are attacked by many parasites, placing them at a disadvantage and driving down their numbers in future years.

FIGURE 13.12 Parasites Infect Common Host Genotypes More Easily Than Rare Genotypes In a laboratory experiment, Dybdahl and Lively compared rates of Microphallus infection in four common snail genotypes (A-D, represented by blue dots) and in a group of 40 rare snail genotypes (E, represented by a red triangle). The parasites and snails in this experiment were all taken from the same lake. (After M. F. Dybdahl and C. M. Lively. 1998. Evolution 52: 1057-1066.) View larger image