A Model Examining the Effect of Environmental Sex Determination in Parasites On the Breakdown of Monogenic Host Resistance

Abstract

If sex determination of a parasite depends on feeding conditions on its host, then good feeding conditions may favour development into one sex, but poor feeding conditions may enhance development into the other sex. This implies that a resistant host may exert a selection pressure towards virulence via one sex, but a selection pressure towards avirulence via the other sex. Continuous cultivation of a resistant host may 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 expense of its reproduction rate. In a previous paper (Schouten, 1994) this phenomenon has been elaborated mathematically for complete, monogenic resistance. In the present paper the derivation is widened towards partial monogenic resistance to avirulent parasites, and is worked out further for potato cyst ncmatodes (Globodera rostochiensis and G. pallida). The equilibrium frequency of avirulent nematodes ranges from 0 to 100 %, but the ensuing durable resistance is lower than 50 %. Thin roots and high nematode densities lower the equilibrium frequency of avirulent nematodes and the matching resistance level.