Sex expression, sex-specific traits, and inbreeding depression in freshwater and salt marsh populations of Amaranthus cannabinus (L.) Sauer (Amaranthaceae), a dioecious annual

Dioecy, the condition in plants in which male and female flowers are produced by separate individuals, is fairly uncommon among flowering plants. The avoidance of inbreeding and several ecological factors are frequently cited as selective pressures which may have led to dioecy. One purpose of this study was to investigate some of the ecological and genetic factors which may have been involved in the evolution of dioecy in Amaranthus cannabinus (L.) Sauer, an annual wetland plant. In order to investigate some of the ecological factors, the sex expression, sexual dimorphism, sex ratios, and spatial distribution of the sexes were examined in New Jersey freshwater and salt marsh populations for two seasons. Plants were also grown in the greenhouse at three salinity levels to examine further sex-specific differences in morphology and flowering phenology. To investigate the effects of inbreeding on A. cannabinus, crosses were performed in the greenhouse between closely related and between distantly related individuals over two generations.; Adult sex ratios were found to be 1:1 in the field and in the greenhouse. Temporal deviations from a 1:1 sex ratio were due to differences between the sexes in flowering phenology and/or differences in mortality. Males began flowering earlier than females at a salt marsh population, but the sexes did not differ in flowering phenology at freshwater populations. Greenhouse studies verified that these population-specific patterns of flowering phenology were genetically determined. Males tended to be taller than females and allocated fewer resources to the production of leaves and branches. Females were larger than males, produced more leaves and branches, and continued to grow after males had senesced. The increased accumulation of resources by females is probably necessary for seed maturation. Sex expression was found to be stable in the vast majority of plants. There was no evidence of spatial segregation of the sexes in the field. Inbreeding depression was observed in the percentage of germination, leaf size, and plant height. Results of this study suggest that avoidance of inbreeding and the optimization of male and female functions were both important selective drives for the evolution of dioecy in A. cannabinus.