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Sex expression, sex-specific traits, and the effects of salinity on growth and reproduction of Amaranthus cannabinus (Amaranthaceae), a dioecious annual¹

⁰ Department of Ecology, Evolution, and Natural Resources, Rutgers University, New Brunswick, New Jersey 08901-1582 USA

Amaranthus cannabinus was studied to investigate some of the ecological factors thought to be involved in the evolution of dioecy and to investigate the effects of salinity on sex expression and sex-specific selection. In the field portion of this study, sex ratios, stability of sex expression, spatial distribution, allocation strategies, and phenologies of the sexes were investigated in New Jersey freshwater and salt marsh populations of water hemp. To examine the effects of salinity on vegetative and reproductive development of males and females, plants were grown in the greenhouse at three salinity levels. Adult sex ratios were found to be 1:1. Temporal deviations from a 1:1 sex ratio varied by population and were due to differences in flowering phenology and mortality between the sexes. No plants were observed to change sex expression, and there was no evidence of spatial segregation of the sexes in the field. In both the field and the greenhouse, females allocated more resources to vegetative tissues and had a longer growing period than males. The results of this study suggest that increased reproductive efficiency through sex-specific growth patterns may have been an important selective factor involved in the evolution of dioecy in A. cannabinus.

Key Words: Amaranthaceae • Amaranthus cannabinus • dioecy • flowering phenology • freshwater marsh • salt marsh • sex expression • sex-specific selection

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