Allozyme variation in Ulmus species from France: analysis of differentiation.

In the present paper, the geographical structure of genetic variation in three French elm taxa is described using electrophoretic data. For three species, Ulmus laevis, U. glabra and U. minor, two kinds of analyses were performed. First, the genetic parameters (genetic diversity indices) of individuals sampled from naturally regenerating forest were compared to samples of cultivated ornamental trees. Secondly, when sample sizes were sufficient, the genetic parameters of trees sampled from different regions of France were compared to detect geographical differentiation. From these analyses, the ultimate aim was to offer recommendations concerning the conservation policies of the genetic resources of French elms. The heterozygosity of Ulmusxhollandica (the presumed hybrid between U. minor and U. glabra) was compared to its putative parent taxa to determine whether the hybrid harbours great levels of genetic variation. In spite of the ravaging effects of Dutch Elm disease in the past, all three species exhibit high levels of electrophoretic variation. The three species surveyed displayed similar levels of genetic diversity, proportions of polymorphic loci and levels of allelic diversity in trees harvested from naturally regenerated forests relative to cultivated ornamental trees. High levels of genetic diversity in U. minor within geographical regions of France were detected, with only moderate levels of genetic differentiation detected between regions. Ulmusxhollandica is not more heterozygous than either of its parent species, suggesting that extant representatives of this taxon are the result of past and ongoing backcrosses with the parental taxa. Ongoing efforts to preserve the genetic variation still present in French elms should take advantage of their high levels of electrophoretic variation and target genetically distinct, vegetatively reproducing genotypes in their natural environment.