Variation and Inheritance of Isozyme Loci in Controlled Crosses of Salix alba and Salix fragilis

Summary The expression of polymorphism in different enzyme systems was investigated in clones of Salix alba and Salix fragilis. Dormant buds and sprouting leaves were used but only few consistent enzyme patterns could be revealed and were retain- ed for further testing. When considering all obtained enzyme profiles, a total of 43 genes could be scored, of which 15 may show polymorphism within S. alba or S. fragilis. Poly- morphism was observed in aconitase, alcohol dehydrogenase, phosphoglucomutase, leucine aminopeptidase, glutamate oxa- loacetate transaminase, β-esterase, isocitrate dehydrogenase, glutamate dehydrogenase, peroxidase, superoxide dismutase and xanthine dehydrogenase. The genetic basis of phosphoglu- comutase, leucine aminopeptidase glutamate oxaloacetate transaminase and shikimate dehydrogenase could be inferred directly from intraspecific and interspecific controlled crosses. Full-sib progeny showed mendelian segregation for lap-1 and most of the pgm-2 alleles, whereas in other genes such as β-est- 1 and adh-2, there was only fixed heterozygosity. Most of the isozymes can not be used directly as good markers at species level. Differences between S. alba and S. fragilis are mainly in the allele frequencies. The number of genes and alleles of the polyploid S. alba and S. fragilis were compared to those of earlier reports on diploid Salix species. The polyploids contain less genetic diversity in their soluble enzymes than the diploids. Duplicated genes did not led to a substantial increase in the heterozygosity levels in the S. alba-S. fragilis complex. Isozyme studies based on full-sib families are important since such data are needed for a correct genetic interpretation of the phenotypic pattern. In the case of polyploids, the zymo- grams often are more complicated due to duplications and fixed heterozygosities. Enzyme variation that is physiologically induced at different developmental stages or that is environ- mentally induced should be excluded from the standard genetic variability analysis in populations. Inheritance studies are also required for the detection of segregation distortion (STRAUSS and CONKLE, 1986) and of linkage, which is especially useful in the case of polyploids. The amount and pattern of genetic variation within and among populations and closely related species further can be influenced by a wide range of factors, including mating system, natural selection, geographic distribution, and historical events (LOVELESS and HAMRICK, 1988). This study focuses on the genetics of Salix alba and Salix fragilis which are closely related species with a widely sympa- tric distribution throughout Europe. The boundary between the two species is defined by a relatively poor amount of diagnostic features in their morphology. In many localities the two species coexist in mixed stands. The taxonomic identification as well as the determination of so-called functional population units is still uncertain. These willow species are dioecious and thus obligate outcrossers. Clonal growth however is very common as known for most polyploid species. The high chromosome number (2x=76) may indicate ancient auto-or allopolyploid origins. Isozyme studies on Salix exigua NUTT. showed that in spite of its relatively high chromosome number (2x=38), the willows seems to remain functionally diploidized. Some enzymes retain their duplicate control due to homologous genes, while others lose their diploid expression, possibly due to gene silencing or hybrid dysgenesis (ARAVANOPOULOS et al., 1993). The lack of clear-cut diagnostic characters, together with the fact that interspecific controlled crosses are successful and that intermediate morphological forms largely dominate on the field, support the hypothesis that S. alba and S. fragilis may hybridize frequently. Hybrids and introgressed hybrids seem to dominate when considering the morphology (DE BONDT, 1996). Studies on allozyme variation in other Salix species (e.g. S. silicicola RAUP., S. alaxensis (ANDERS.) COV. ) revealed that differentiation between populations is low (PURDY and BAYER, 1995). This is consistent with the reviewed data on trees with a dioecious breeding system and wind-dispersed seeds (HAMRICK and GODT, 1989). Factors that promote high levels of genetic diversity within populations in Salix species include dioecism, high fecundity, wind-dispersed seeds and long-lived clonal growth. Characteristic for Salix thus might be the allelic evenness of allozyme distribution (BRUNSFELD et al., 1991) as well as the low probability for genetic drift (PURDY and BAYER, 1995). In this study, we first investigated a large number of clones to elucidate the polymorphism in the different enzymes and thereafter selected the most reliable polymorphic enzymes such as GOT, LAP and PGM for studying their inheritance in controlled interspecific and intraspecific crosses between S. alba and S. fragilis.