SEX DETERMINATION AND SEX-RATIO IN THE DIOECIOUS SHRUB SALIX-VIMINALIS L

Various ecological factors (e.g. herbivory, difference between males a nd females in colonising ability) have been invoked to explain female- biased sex ratios in populations of willow species. It was implicitly assumed that genetic factors would lead to a balanced sex ratio in the absence of ecological disturbances. In an experiment carried out in a homogeneous environment and in the absence of herbivores the progeny sex ratio of 13 crosses of basket willow (Salix viminalis L.) was obse rved to range from extreme female bias to extreme male bias. The obser ved sex ratio cannot be explained by the presence of sex chromosomes w ithout assuming that additional loci are also involved in the sex dete rmination. Alternatively, the sex ratios in this study can be explaine d by a sex determination mechanism governed by multiple independent lo ci.