GENETIC-VARIATION IN NATURAL-POPULATIONS OF THE CEREAL CYST-NEMATODE (HETERODERA-AVENAE WOLL) SUBMITTED TO RESISTANT AND SUSCEPTIBLE CULTIVARS OF CEREALS

The purpose of the present work was to study the genetic characteristi cs of cereal cyst nematode (Heterodera avenae) populations re-establis hed after the long-term use of resistant oat cultivars in field condit ions, Population features were analyzed through fitness components and variation in enzymatic polymorphism (esterase and malate dehydrogenas e loci). The longest (6 year) use of the same resistance genes (oat cv Panema) at high frequency (Rotation IB) led to the selection of a res istance-breaking pathotype and to a decrease in viability which sugges ted either a founder effect or a lower reproductive potential for the new pathotype, Analysis of esterase allelic frequencies led to the con clusions that: (1) the genetic constitution of this pathotype was diff erent from the reference population maintained on the susceptible host (oat cv Peniarth), and (2) that the esterase locus may develop a dise quilibrium linkage with loci involved in virulence(1). Random mating w as recorded at the whole-field level but not always at the single-plan t level, suggesting that cultivation practices such as annual ploughin g could play a major role in homogenizing subpopulations developed in the vicinity of a plant. These phenomena showed that the long-term use of highly effective resistance could provok marked genetic modificati ons in populations, These risks should be taken into account when deci ding strategies for optimal use of resistance genes in nematode manage ment programs.