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 anal yses were performed. First, the genetic parameters (genetic diversity indices) of individuals sampled from naturally regenerating forest wer e compared to samples of cultivated ornamental trees. Secondly, when s ample sizes were sufficient, the genetic parameters of trees sampled f rom different regions of France were compared to detect geographical d ifferentiation. From these analyses, the ultimate aim was to offer rec ommendations concerning the conservation policies of the genetic resou rces of French elms. The heterozygosity of Ulmus x hollandica (the pre sumed hybrid between U. minor and U. glabra) was compared to its putat ive parent taxa to determine whether the hybrid harbours great levels of genetic variation. In spite of the ravaging effects of Dutch Elm di sease in the past, all three species exhibit high levels of electropho retic variation. The three species surveyed displayed similar levels o f genetic diversity, proportions of polymorphic loci and levels of all elic diversity in trees harvested from naturally regenerated forests r elative to cultivated ornamental trees. High levels of genetic diversi ty in U. minor within geographical regions of France were detected, wi th only moderate levels of genetic differentiation detected between re gions. is not more heterozygous than either of its parent species, sug gesting that extant representatives of this taxon are the result of pa st and ongoing backcrosses with the parental taxa. Ongoing efforts to preserve the genetic variation still present in French elms should tak e advantage of their high levels of electrophoretic variation and targ et genetically distinct, vegetatively reproducing genotypes in their n atural environment.