SHORT COMMUNICATIONFluctuating selection by water level on gynoecium colour polymorphism in an aquatic plantXiao-Xin Tang and Shuang-Quan Huang* College of Life Sciences, Wuhan University, Wuhan 430072, China *For correspondence. E-mail sqhuangwhu.edu.cnReceived: 8 June 2010Returned for revision: 12 July 2010Accepted: 19 July 2010Published electronically: 27 August 2010Background and Aims It has been proposed that variation in pollinator preferences or a fluctuating environmentcan act to maintain flower colour polymorphism. These two hypotheses were tested in an aquatic monocotButomus umbellatus (Butomaceae) with a pink or white gynoecium in the field population.Methods Pollinator visitation was compared in experimental arrays of equivalent flowering cymes from both colour morphs. Seed set was compared between inter- and intramorph pollination under different water levelsto test the effect of fluctuating environment on seed fertility. Key Results Overall, the major pollinator groups did not discriminate between colour morphs. Compared with the white morph, seed production in the pink morph under intermorph, intramorph and open pollination treat-ments was significantly higher when the water level was low but not when it was high. Precipitation in July was correlated with yearly seed production in the pink morph but not in the white morph. Conclusions The results indicated that the two colour morphs differed in their tolerance to water level. Ourstudy on this aquatic plant provides additional evidence to support the hypothesis that flower colour polymorph- ism can be preserved by environmental heterogeneity.Key words: Butomus umbellatus, colour polymorphism, environmental heterogeneity, pleiotropic effects, pollinator preference, water stress.INTRODUCTIONPollinators are usually attracted to flowers initially by floralcolour or scent, although most flowers reward pollinators with nectar or pollen as food (Proctor et al., 1996). Pollinators may use these signals as cues of quality or quan- tity of the reward (Weiss, 1991; Mele ndez-Ackerman et al., 1997; Armbruster et al., 2005, and references therein).Therefore, variation in flower colour may affect pollinator visitation rates and, in turn, plant reproductive success (e.g. Mogford, 1974; Waser and Price, 1981; Stanton, 1987; Rausher and Fry, 1993; Comba et al., 2000; Gigord et al., 2001; Jones and Reithel, 2001). For example, an experimental study in Delphinium nelsonii indicated that a reduction ofseed set in white-flowered plants compared with pigmented plants was caused by pollinator discrimination (Waser and Price, 1981). Other studies have not observed pollinator discrimination against certain petal colours (Mogford, 1978; Hannan 1981; Miller 1981; Levin and Brack, 1995; Jones, 1996; Jersa kova et al., 2006). However, flower colourpolymorphism could be maintained by fluctuating selection from variation in pollinator species (Brown and Clegg, 1984; Mele ndez-Ackerman et al., 1997; Subramaniam and Rausher, 2000; Turelli et al., 2001; Eckhart et al., 2006; StreisfeldandKohn,2007).Areciprocaltransplantexperiment of coastal red- and inland yellow-flowered races in Mimulus aurantiacus showed that hummingbirds preferredred flowers (.95 % of visits) but hawkmoths preferredyellow flowers (.99 % of visits) (Streisfeld and Kohn,2007). Strong preferences for alternative floral morphs inM. aurantiacus suggest that pollinators play a direct role in colour divergence.On the other hand, flower colour polymorphism could be caused by pleiotropic effects or indirect selection on plant per- formance in contrasting physical environments (Mlgaard, 1989; Schemske and Bierzychudek, 2001, 2007; Chittka et al., 2001; Warren and Mackenzie, 2001; Conner, 2002; Coberly and Rausher, 2003, 2008; Jorgensen and Andersson, 2005; Lacey and Herr, 2005; Strauss and Whittall, 2006).Studies have shown that floral pigments are associated with tolerance to environmental stress. For example, Warren andMackenzie (2001) compared plant fitness in five flower colour polymorphic species under dry and well-watered con- ditions. They revealed that the anthocyanin-pigmented plants performed relatively better in the dry conditions, while theunpigmented (white-flowered) plants performed relatively better in the watered treatment (e.g. producing more seeds).A long-term study of field populations in Linanthus parryaefound that blue-flowered morphs performed better than white-flowered morphs in drought years while white morphs performed better in years of high rainfall (Schemske and Bierzychudek, 2001, 2007). Gynoecium colour polymorphism was recently discovered inanemergent,aquaticmonocotButomusumbellatus (Butomaceae). The species, native to Europe and north tem- perate Asia, usually has pink gynoecia, but plants with white gynoecia were observed in some populations in northeastern China (Huang and Tang, 2008). This gynoecium colour poly- morphism provides a unique opportunity to understand the mai