Common ash is a temperate forest tree with a colonizing behaviour, a discontinuous spatial distribution and a peculiar and poorly known mating system. Microsatellite markers were used to study the genetic structure in natural populations of common ash. Twelve populations located in northeastern France were analysed at five loci. Levels of genetic variability within and among stands were estimated for the seedling and adult stages. As expected for a forest tree, our results reveal high levels of intrapopulation diversity and a low genetic differentiation between stands. However, a general and significant heterozygote deficiency was found, with a mean F(IS) of 0.163 for the seedlings and of 0.292 for the adult trees. The different explanations for such an excess homozygosity are discussed: a nonMendelian inheritance of alleles, the presence of null alleles, a Wahlund effect and assortative mating.

A simplified method for the isolation of mitochondrial DNA (mtDNA) from several angiosperm tree species is described. The procedure does not require gradient ultracentrifugation or organic solvent extractions and can be performed in a single day. mtDNA was isolated from flower buds collected from adult trees just after the first pollen mitosis and purified by gravity flow through disposable columns containing an anion-exchange resin. The method led to restrictable mtDNA from the tree species analysed. Restriction patterns enabled us to distinguish between European and American ashes.