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Preservation of Avirulence Genes of Potato Cyst Nematodes Through Environmental Sex Determination: A Model Involving Complete, Monogenic Resistance. Henk J. Schouten, Centre for Plant Breeding and Reproduction Research (CPRO-DLO), P.O. Box 16, 6700 AA Wageningen, the Netherlands; Phytopathology 84:771-773. Accepted for publication 20 April 1994. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/Phyto-84-771.

Host-parasite compatibility strongly influences the sex determination of potato cyst nematodes (Globodera rostochiensis and G. pallida). The probability of a juvenile developing into a male may be higher in the case of poor compatibility than in the case of full compatibility. As a result, avirulence toward the resistant host may lead to the production of more males. In contrast, virulence favors the production of females. These conflicting sexual trends imply opposing selection pressures by a resistant host. This may create an equilibrium frequency between the avirulent and the virulent nematodes, thereby preserving host resistance. In this way, parasite populations with environmental sex determination could maintain their genetic diversity at the expense of their reproduction rate. The equilibrium frequency of avirulence and the matching level of durable resistance are derived mathematically for complete, monogenic resistance.