Genetic variation and putative hybridization in Salix alba and S-fragilis (Salicaceae): evidence from allozyme data

Genetic variability was estimated by enzyme electrophoresis in 239 Belgian clones from the Salix alba-S. fragilis complex. This morphological complex suggested a high frequency of hybrids. To test this hypothesis, the clones were pooled as a single coadapted species complex and secondly as belonging to either species, i.e. being S. alba-like or S. fragilis-like. The standa rd genetic variability measures showed higher values for the complex than f or the separate species. However, the observed mean heterozygosity was lowe r in the putative hybrid complex than for each of the species separately. T he fixation indices were more variable at the species level and indicated t hat most S, fragilis locations appeared fixed for lap-1, whereas no fixatio n occured in S. alba locations. Averaged at the regional (i.e. catchment) l evel, this difference between the two species remained and values ranged fr om 0.457 to 0.617 in S. alba and were much higher, fixed homozygous or mono morph in S. fragilis. Hierarchical F-statistics revealed that most of the d ifferentiation occured at the lower levels of localities and tributaries an d that there was no further differentiation between catchments. Tributaries which are 10-25 km in length were proposed as the most likely entities for further examination of putative hybridization and events of allelic fixati ons. By considering the two abovementioned approaches of data input, it cou ld be suggested that most of the allozyme differentiation was between the s pecies and less between the regions.