Presence of natural genetic resistance in Fraxinus excelsior (Oleraceae) to Chalara fraxinea (Ascomycota): an emerging infectious disease.

Fraxinus excelsior, common ash native to Europe, is threatened by a recently identified pathogenic fungus Chalara fraxinea, which causes extensive damage on ash trees across Europe. In Denmark, most stands are severely affected leaving many trees with dead crowns. However, single trees show notably fewer symptoms. In this study, the impact of the emerging infectious disease on native Danish ash trees is assessed by estimating presence of inherent resistance in natural populations. Disease symptoms were assessed from 2007 to 2009 at two different sites with grafted ramets of 39 selected clones representing native F. excelsior trees. A strong genetic variation in susceptibility to C. fraxinea infections was observed. No genetic or geographic structure can explain the differences, but strong genetic correlations to leaf senescence were observed. The results suggest that a small fraction of trees in the Danish population of ash possess substantial resistance against the damage. Though this fraction is probably too low to avoid population collapse in most natural or managed ash forests, the observed presence of putative resistance against the emerging infectious disease in natural stands is likely to be of evolutionary importance. This provides prospects of future maintenance of the species through natural or artificial selection in favour of remaining healthy individuals.

Natural hybridisation between Quercus petraea (Matt.) Liebl. and Quercus pubescens Willd. within an Italian stand as revealed by microsatellite fingerprinting.

Interspecific gene flow is frequently reported in the genus Quercus. However, interfertile oak species often seem to remain distinct, even within areas of sympatry. This study employed molecular markers to verify, at a fine scale, the presence of interspecific gene flow in a natural population of Quercus petraea and Quercus pubescens. Within a delimited area of 6 ha, all adult trees belonging to the studied oak complex and seeds from a subsample of such trees were collected and analysed using molecular microsatellite markers. A low interspecific genetic differentiation and a high level of interspecific genetic admixture suggested past hybridisation. Paternity inference of seeds allowed the estimation of pollination frequencies from the three groups of pollen donors (Q. petraea, Q. pubescens, intermediate). We also assayed pollen viability and germinability of each species group. We observed natural hybridisation between Q. petraea and Q. pubescens, with a predominant component in the direction Q. petraea --> Q. pubescens: Q. pubescens displayed a higher level of heterospecific pollination by Q. petraea (25.8%) and intermediate morphotypes (14.7%), compared to Q. petraea acting as pollen receptor (with less than 5% heterospecific pollinations). Intermediate 'mother trees' were pollinated in similar proportions by Q. petraea (23.1%), Q. pubescens (37.8%) and intermediate morphotypes (39.1%). The asymmetrical introgression observed for the studied generation may be caused, among other factors, by the relative abundance of trees from each species group in the studied area.