A MODEL EXAMINING THE EFFECT OF ENVIRONMENTAL SEX DETERMINATION IN PARASITES ON THE BREAKDOWN OF MONOGENIC HOST-RESISTANCE

If sex determination of a parasite depends on feeding conditions on it s host, then good feeding conditions may favour development into one s ex, but poor feeding conditions may enhance development into the other sex. This implies that a resistant host may exert a selection pressur e towards virulence via one sex? but a selection pressure towards avir ulence via the other sex. Continuous cultivation of a resistant host m ay therefore lead to an equilibrium frequency of avirulent parasites, and consequently to a durable partial resistance to the population. In this way the parasite may maintain its genetic diversity, at the expe nse of its reproduction rate. In a previous paper(Schouten, 1994) this phenomenon has been elaborated mathematically for complete, monogenic resistance. Tn the present paper the derivation is widened towards pa rtial monogenic resistance to avirulent parasites, and is worked out f urther for potato cyst nematodes (Globodera rostochiensis and G. palli da). The equilibrium frequency of avirulent nematodes ranges from 0 to 100 %, but the ensuing durable resistance is lower than 50 %. Thin ro ots and high nematode densities lower the equilibrium frequency of avi rulent nematodes and the matching resistance level.