RAPD of controlled crosses and clones from the field suggests that hybridsare rare in the Salix alba-Salix fragilis complex

The polyploid Salix alba-Salix fragilis hybrid complex is rather difficult to study when using only morphological characters. Most of the features hav e a low diagnostic value for unambiguously identifying the hybrids, introgr ession patterns and population structures, though morphological traits have proved to be useful in making a hybrid index. Morphology and molecular var iation from RAPDs were investigated in several case studies on willows from Belgium. A thorough screening of full-sib progenies of interspecific contr olled crosses was made to select homologous amplification products. The sel ected amplified products proved to be useful in a principal coordinate anal ysis for the estimation of variability of hybrid progenies. On the basis of genetic similarities and ordination analysis, a method for the identificat ion of clones in the field was established using presumed pure species and presumed introgressants. The chosen reference clones were checked against a dditional European samples of putative pure species to ensure the reliabili ty of the method beyond a regional scale. The RAPDs suggested that both spe cies have kept their gene pools well separated and that hybridization actua lly does not seem to be a dominating process. The observation that molecula r markers do not always follow the morphological traits or allozyme data is discussed.