Genetic diversity in relation to sexual and asexual reproduction in populations of Melampsora larici-epitea

Genetic diversity of Melampsora larici-epitea leaf rust from three cultivat ed stands of the willow Salix viminalis was studied using AFLP polymorphism s at 60 loci. One population was located in Northern Ireland and two in Swe den. Analysis of molecular variance (AMOVA) showed that most of the genetic variation was distributed on a fine scale within the field in all populati ons. Both Swedish populations displayed a very high genotypic diversity (no rmalized Shannon's indices of 0.95 and 1.00) and random association among l oci. These results suggested that sexual reproduction had an important infl uence on the Swedish populations. The occurence of the alternate host (larc h) adjacent to one of the Swedish rust populations did not affect the genet ic diversity. However, severe rust attacks started earlier in the season in this population. The M. larici-epitea population in Northern Ireland was c haracterized by a low genotypic diversity (normalized Shannon's index = 0.5 4) and non-random association among loci was indicated by test of multilocu s association and by pairwise tests among loci. These results suggested tha t asexual reproduction had a major effect on the genetic structure of this population, probably because of the overwintering of asexual spores and/or a population bottleneck associated with the annual sexual phase.