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American Journal of Botany 87(11): 1609–1618. 2000.

SEX
OF

EXPRESSION, SEX-SPECIFIC TRAITS, AND THE A

EFFECTS OF SALINITY ON GROWTH AND REPRODUCTION

AMARANTHUS CANNABINUS (AMARANTHACEAE),
DIOECIOUS ANNUAL
1

MARGOT R. BRAM2

AND JAMES

A. QUINN

Department of Ecology, Evolution, and Natural Resources, Rutgers University, New Brunswick, New Jersey 08901-1582 USA Amaranthuscannabinus 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 freshwaterand 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 plantswere 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 factorinvolved 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.

Due in part to its relative rarity in the plant kingdom, there has been a lot of interest in dioecy and the selective factors that may have led to the evolution of dioecy. The percentage ofdioecious angiosperm species has been reported to be as low as 4% (Richards, 1986) and as high as 10% (Lloyd, 1982). A more recent survey of 240 000 angiosperm species by Renner and Ricklefs (1995) found that 6% were dioecious. Many ecological (e.g., Bawa and Opler, 1975; Bawa, 1980; Freeman, Harper, and Ostler, 1980; Muenchow, 1987; Bawa, 1994; Renner and Ricklefs, 1995, and references therein) andgenetic (Bawa and Opler, 1975; Thomson and Barrett, 1981; Lloyd, 1982; Charlesworth and Charlesworth, 1987) factors have been suggested as likely to lead to the development of the dioecious condition. Since dioecy has evolved in such a variety of plant groups and under such a wide range of habitats, Baker (1984) and Anderson and Stebbins (1984) have recommended that each case of dioecy be investigatedindividually. Many of the current ideas on the role of specialization in the evolution of dioecy can be traced back to Darwin (1877). Dioecy results in a division of labor between the sexes and may eventually lead to greater reproductive efficiency (Lloyd, 1982). Male and female reproductive functions may be optimized through different selective pressures (Lloyd and Webb, 1977; Willson, 1979).This sex-specific selection may lead to phenological differences and dimorphism in male and female plants (Cox, 1981; Meagher, 1984). Sex-specific selection may
1 Manuscript received 9 September 1999; revision accepted 1 February 2000. The authors thank A. Bram, S. Handel, M. Leck, T. Meagher, and two anonymous reviewers for advice and comments on earlier versions of the manuscript; T. Shelley, K.Anderson, and B. Beal for permission to work at field sites; and K. Stevens for greenhouse assistance. 2 Author for reprint requests, current address: Patrick Center for Environmental Research, Academy of Natural Sciences of Philadelphia, 1900 Benjamin Franklin Parkway, Philadelphia, Pennsylvania 19103-1195 USA.

also lead to differential resource use by the sexes, sometimes called ‘‘sexual niche...