Specific and non-specific defense against parasitic attack

Specific defense protects against some parasite genotypes but not others, w hereas non-specific defense is effective against all genotypes of a parasit e. Some empirical studies observe hosts with variability only in non-specif ic defense, other studies find only specific defense. I analyse a model wit h combined specific and non-specific defense to determine the conditions th at favor detectable variation in each form of defense. High variation in no n-specific defense is often maintained when resistance increases in an acce lerating way with investment, whereas low variation tends to occur when res istance increases at a decelerating rate with investment. Variation in spec ific defense rises as the parasite pays a higher cost to attack a broad hos t range (high cost of virulence), as the number of alternative specificitie s declines, and as the average level of non-specific defense increases. The last condition occurs because greater non-specific protection tends to sta bilize the gene frequency dynamics of specific defense. Selection favors a negative association between costly components of specific and non-specific defense-hosts defended by one component are favored if they have reduced a llocation to other costly components. A negative association confounds the measurement of costs of resistance. Individuals with specific defense may h ave reduced investment in costly nonspecific defense. This leads to an appa rent advantage of specifically defended hosts in the absence of parasites a nd a measured cost of resistance that is negative. (C) 2000 Academic Press.