Speci5c and Non-speci5c Defense against Parasitic Attack

Speci"c defense protects against some parasite genotypes but not others, whereas non-speci"c defense is e!ective against all genotypes of a parasite. Some empirical studies observe hosts with variability only in non-speci"c defense, other studies "nd only speci"c defense. I analyse a model with combined speci"c and non-speci"c defense to determine the conditions that favor detectable variation in each form of defense. High variation in non-speci"c defense is often maintained when resistance increases in an accelerating way with investment, whereas low variation tends to occur when resistance increases at a decelerating rate with investment. Variation in speci"c defense rises as the parasite pays a higher cost to attack a broad host range (high cost of virulence), as the number of alternative speci"cities declines, and as the average level of non-speci"c defense increases. The last condition occurs because greater non-speci"c protection tends to stabilize the gene frequency dynamics of speci"c defense. Selection favors a negative association between costly components of speci"c and non-speci"c defense*hosts defended by one component are favored if they have reduced allocation to other costly components. A negative association confounds the measurement of costs of resistance. Individuals with speci"c defense may have reduced investment in costly nonspeci"c defense. This leads to an apparent advantage of speci"cally defended hosts in the absence of parasites and a measured cost of resistance that is negative. ( 2000 Academic Press