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 co-adapted species complex and secondly as belonging to either species, i.e. being S. alba-like or S. fragilis-like. The standard genetic variability measures showed higher values for the complex than for the separate species. However, the observed mean heterozygosity was lower in the putative hybrid complex than for each of the species separately. The fixation indices were more variable at the species level and indicated that most S. fragilis locations appeared fixed for lap-1, whereas no fixation occured in S. alba locations. Averaged at the regional (i.e. catchment) level, this difference between the two species remained and values ranged from 0.457 to 0.617 in S. alba and were much higher, fixed homozygous or monomorph in S. fragilis. Hierarchical F-statistics revealed that most of the differentiation occured at the lower levels of localities and tributaries and that there was no further differentiation between catchments. Tributaries which are 10–25km in length were proposed as the most likely entities for further examination of putative hybridization and events of allelic fixations. By considering the two abovementioned approaches of data input, it could be suggested that most of the allozyme differentiation was between the species and less between the regions. [ABSTRACT FROM AUTHOR]