Abstract
●Many angiosperms are hermaphroditic and produce bisexual flowers in which male (pollen export) and female (stigma receptivity) functions are separated temporally. This sequential hermaphroditism may be associated with variation in flower size, color, or pattern, all of which may influence pollinator attraction. In this study, we describe variation in these traits across discrete functional sex stages within and between 225 greenhouse‐grown individuals of Clarkia unguiculata (Onagraceae). In addition, to identify the effects of floral phenotype on pollinator attraction in this species, we examine the effects of these floral traits on pollen receipt in ~180 individuals in an experimental field array.●Petal area, ultraviolet (UV)‐absorbing nectar guide area, and blue and green mean petal reflectance differ significantly across the functional sex stages of C. unguiculata. Male‐ and female‐phase flowers display significantly different pollinator attraction traits. Petal and UV nectar guide area increase as flowers progress from male phase to female phase, while blue reflectance and green reflectance peak during anther maturation.●In field arrays of C. unguiculata, female‐phase flowers with large UV nectar guides receive more pollen than those with small nectar guides, and female‐phase flowers with high mean blue reflectance values are more likely to receive pollen than those with low blue reflectance. Female‐phase flowers with green mean reflectance values that differ most from background foliage also receive more pollen than those that are more similar to foliage. These findings indicate that components of flower color and pattern influence pollen receipt, independent of other plant attributes that may covary with floral traits. We discuss these results in the context of hypotheses that have been proposed to explain sex‐specific floral attraction traits, and we suggest future research that could improve our understanding of sexual dimorphism in sequentially hermaphroditic species and the evolution of features that promote outcrossing.
Highlights
Sexual reproduction occurs via a wide variety of reproductive modes, including unisexuality and hermaphroditism (Desjardins & Fernald, 2009; Heule, Salzburger, & Böhne, 2014)
This is among the first studies to examine sexual dimorphism between the functional sex stages of a bisexual flower (Davis et al, 2014; Jabbari et al, 2013)
Two mechanisms have been proposed to promote the evolution of sexual dimorphism in flowering plants by natural selection: the avoidance of self-fertilization and the maximization of sex-specific fitness (Delph & Ashman, 2006)
Summary
Sexual reproduction occurs via a wide variety of reproductive modes, including unisexuality and hermaphroditism (sequential, serial, and simultaneous) (Desjardins & Fernald, 2009; Heule, Salzburger, & Böhne, 2014). Most flowering plant species produce bisexual flowers (~90%), with pollen-producing (male) and ovule-producing (female) structures contained in the same flower (Barrett & Hough, 2013). The evolution of dioecy is commonly associated with morphologically distinct male and female flowers (Charlesworth, 1999; Darwin, 1877). Such sexual dimorphism is thought to evolve in response to sex-specific differences in the strength or direction of selection on individual traits or in the cost of reproduction (Barrett & Hough, 2013; Delph & Ashman, 2006)
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