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 characteristics of cereal cyst nematode (Heterodera avenae) populations re-established after the long-term use of resistant oat cultivars in field conditions. Population features were analyzed through fitness components and variation in enzymatic polymorphism (esterase and malate dehydrogenase 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 resistance-breaking pathotype and to a decrease in viability which suggested either a founder effect or a lower reproductive potential for the new pathotype. Analysis of esterase allelic frequencies led to the conclusions that: (1) the genetic constitution of this pathotype was different from the reference population maintained on the susceptible host (oat cv Peniarth), and (2) that the esterase locus may develop a disequilibrium linkage with loci involved in virulence Nematodes overcoming the resistance of cv Panema did not differ from H. avenae species following RFLPs in ribosomal DNA . Random mating was recorded at the whole-field level but not always at the single-plant level, suggesting that cultivation practices such as annual ploughing 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 modifications in populations. These risks should be taken into account when deciding strategies for optimal use of resistance genes in nematode management programs.