2601 Ecology, 82(9), 2001, pp. 2601–2616  2001 by the Ecological Society of America ECOLOGICAL DIFFERENTIATION OF COMBINED AND SEPARATE SEXES OF WURMBEA DIOICA (COLCHICACEAE) IN SYMPATRY ANDREA L. CASE 1 AND SPENCER C. H. BARRETT Department of Botany, University of Toronto, 25 Willcocks Street, Toronto, Ontario, Canada M5S 3B2 Abstract. The evolution and maintenance of combined vs. separate sexes in flowering plants is influenced by both ecological and genetic factors; variation in resources, partic- ularly moisture availability, is thought to play a role in selection for gender dimorphism in some groups. We investigated the density, distribution, biomass allocation, and physi- ology of sympatric monomorphic (cosexual) and dimorphic (female and male) populations of Wurmbea dioica in relation to soil moisture on the Darling Escarpment in southwestern Australia. Populations with monomorphic vs. dimorphic sexual systems segregated into wet vs. dry microsites, respectively, and biomass allocation patterns and physiological traits reflected differences in water availability, despite similarities in total ramet biomass between the sexual systems. Unisexuals flowered earlier at lower density, and they allocated sig- nificantly more biomass below ground to roots and corms than did cosexuals, which al- located more biomass above ground to leaves, stems, and flowers. Females, males, and cosexuals produced similar numbers of flowers per ramet, but unisexuals produced more ramets than cosexuals, increasing the total number of flowers per genet. Contrary to ex- pectation, cosexuals had significantly higher (more positive) leaf carbon isotope ratios and lower leaf nitrogen content than unisexuals, suggesting that cosexuals are more water-use efficient and have lower rates of photosynthesis per unit leaf mass despite their occurrence in wetter microsites. Cosexuals appear to adjust their stomatal behavior to minimize water loss through transpiration while maintaining high investment in leaves and reproductive structures. Unisexuals apparently maximize the acquisition and storage of both water and nitrogen through increased allocation to roots and corms and enhance the uptake of CO 2 by keeping stomata more open. These findings indicate that the two sexual systems have different morphological and physiological features associated with local-scale variation in water availability. Key words: carbon isotope ratios; cosexuality; geophyte; habitat specialization; leaf nitrogen content; plant gender; resource allocation; sexual systems; sympatry; unisexuality; water limitation; Wurmbea dioica. INTRODUCTION The evolution of separate sexes from combined sexes has occurred many times among angiosperm families. The ecological and genetic conditions favoring these contrasting sexual systems therefore represent a central question in evolutionary biology (reviewed in Geber et al. 1999). Theoretical models have invoked three key factors affecting selection for gender dimorphism, including the genetic control of sex expression, the fitness consequences of selfing and outcrossing, and the optimal allocation of resources to female and male function (Charnov 1982, Lloyd 1982, Charlesworth 1999). Because ecological context can affect the rel- ative importance of these factors, recent work on gen- der dimorphism has focused on the environmental con- ditions that contribute to the relative success of com- bined vs. separate sexes. Manuscript received 22 February 2000; revised 8 September 2000; accepted 26 September 2000; final version received 25 Oc- tober 2000. 1 Present address: Department of Biological Sciences, Uni- versity of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA. E-mail: acase+@pitt.edu Numerous empirical studies have implicated ecolog- ical conditions, particularly harsh environments, in the evolution and maintenance of gender dimorphism (Webb 1979, Hart 1985, Arroyo and Squeo 1990, Delph 1990a, b, Barrett 1992, Costich 1995, Weller et al. 1995, Wolfe and Shmida 1997, Ashman 1999, Sakai and Weller 1999). These studies have reported in- creased gender specialization and a greater incidence of gender dimorphism in resource-limited habitats. For example, all of the dimorphic species of Schiedea (Car- yophyllaceae) in Hawaii are found in dry habitats, whereas all but four monomorphic species occur in wet habitats (Weller and Sakai 1990). In the Iberian pen- insula, the sexual systems of Ecballium elaterium (Cu- curbitaceae) are geographically segregated, with the monoecious subspecies restricted to the wetter northern region and the dioecious subspecies occurring in the drier south (Costich 1995). In several dimorphic (gyn- odioecious and subdioecious) species, increased gender specialization (e.g., higher frequencies of female plants and reduced fruit production by hermaphrodites) is as- sociated with more stressful environments (Webb 1979, Arroyo and Squeo 1990, Delph 1990a, Barrett 1992, Wolfe and Shmida 1997, Ashman 1999, Case 2000).