Pollinators are a primary driver of floral trait evolution and traits including flower colours, scents, and morphologies frequently reflect selection from pollinator behaviour and morphology (Darwin 1877; Barrett 2010; Johnson and Anderson 2010; Schiestl and Johnson 2013). For instance, the flowers of many plant species have evolved ultraviolet-absorbing centres that contrast strongly with the rest of the flower and are thought to reflect selection by pollinators with strong preferences for these colour patterns (Silberglied 1979; van der Kooi et al. 2019). Likewise, a variety of flowering plant taxa have evolved specialized flowers with long nectar spurs, reflecting selection by insect pollinators with long proboscises (Hodges et al. 2004; Vlašánková et al. 2017). Accordingly, floral traits evolving via pollinator-mediated selection affect pollination and foraging success. In particular, pollinator-mediated selection on flowering plants favours the evolution of floral traits that enhance the dispersal of pollen to pollinators and maximize the transfer of pollen to conspecific stigmas, while simultaneously minimizing pollen wastage (e.g., pollen lost or consumed as food by the pollinator) (Harder and Wilson 1994; De Kock et al. 2018). Yet while the functional effects of floral traits should depend on how plant and pollinator interact (e.g., Fukuda et al. 2014; Hopkins et al. 2014; Hazlehurt and Karubian 2016), consequences for pollination and foraging success are poorly understood (but see Betts et al. 2015; De Kock et al. 2018; Lichtenberg et al. 2018).
Modifications of anther architecture (i.e., sizes, degree of fusion, and spatial/functional connections) are common and thought to significantly affect flower function (Endress 2012; Nevard et al. 2021). For example, anthers frequently vary in the degree to which they are joined in the flowers of buzz pollinated plants, yet the functional significance of this variation is imperfectly understood (Faegri 1986; Vogel 1978; Glover et al. 2004; Vallejo-Marín et al. 2022). Such plants are pollinated by bees capable of generating powerful vibrations (‘floral buzzing’; performed by an estimated 58% of bee species; Cardinal et al. 2018), which expel pollen from the terminal pores of the tube-like poricidal anthers (Macior 1968; Vallejo-Marín 2019; Brito et al. 2020). Many of these buzz pollinated plant species possess flowers whose anthers are spread apart and capable of relatively independent movement (‘free anther architecture’) (Glover et al. 2004; Vallejo-Marín et al. 2022). Yet many other buzz pollinated species across taxonomically diverse families have independently evolved a joined anther architecture, in which poricidal anthers are arranged and joined in the centre of the flower, resembling a cone (Fig. 1; found in at least 21 plant families, see supplementary; Vogel 1978; Faegri 1986; Glover et al. 2004; De Luca and Vallejo-Marín 2013; Russell et al. 2016). Both joined and free anther architectures are widely distributed among buzz pollinated species, and floral buzzing is expected to affect pollen expulsion differently depending on the degree to which the anthers are joined (Glover et al. 2004; Nevard et al. 2021; Vallejo-Marín et al. 2022). Despite this, the functional significance of both anther architectures for flower and bee remain unclear (see Vallejo-Marín et al. 2022).
The degree to which anthers are joined could affect pollination and foraging success in complementary or opposing ways. For instance, joined anther architecture could simultaneously enhance pollination (benefitting the plant) and pollen collection (benefitting the bee). Given that simulated bee vibrations are propagated more effectively and pollen release is increased when anthers are joined (Nevard et al. 2021; Vallejo-Marin et al. 2022), a bee might be able to collect more of the released pollen. Increased pollen release might also translate to more pollen deposited on the stigma. Alternatively, joined or free anther architecture could enhance either pollination or pollen collection, but not both. For instance, increased pollen release from joined anther architecture might enhance collection by the bee, without resulting in more pollen transferred to conspecific stigmas (e.g., Russell et al. 2021). This would be expected, for instance, if the anther cone more consistently deposited pollen in a readily groomed location on the bee. In contrast, loosely held, sprawling anthers of free anther architectures may more readily distribute pollen to so-called safe-sites on the bee, which are accessible to plant stigmas, but protected from bee grooming (Herrera 1987; Harder and Barclay 1994; Huang et al. 2015; Koch et al. 2017; Tong and Huang 2017). Similarly, if free anther architecture releases less pollen (Vallejo-Marin et al. 2022), more pollen may remain in the anthers for subsequent pollinators, resulting in more opportunities for pollination. Reduced pollen release might even entice a given pollinator into spending more time on the flower, thereby enhancing pollen transfer to the stigma.
How anther architecture affects pollination and foraging success also likely depends on pollinator characteristics. Pollination effectiveness is considered to be generally influenced by the physical fit between flower morphology and pollinator body, which can, for instance affect the removal of pollen and contact with floral reproductive structures (Herrera 1987; Minnaar et al. 2018; Moreira-Hernandez and Muchhala 2019; Russell et al. 2021). Given that body size frequently varies both within and among bee species (Cariveau et al. 2016; Cullen et al. 2021), physical fit may strongly influence how a given anther architecture affects pollination and foraging success. For instance, relative to larger bees, smaller bees might be less effective pollinators on flowers with free anther morphology, because their smaller bodies would be less likely to contact the stigma as they move among anthers (Li et al. 2015; Solis-Montero and Vallejo-Marin 2017; Mesquita-Neto et al. 2021). Likewise, given that joined anthers vibrate together (Nevard et al. 2021), relatively larger bees might be less effective pollinators on flowers with joined anther morphology, because their more powerful vibrations (De Luca et al. 2013; 2019; Switzer et al. 2019) could enable them to collect more pollen or deplete anthers more completely, leaving less pollen to be transferred to conspecific stigmas by subsequent visitors.
In this laboratory study, we assessed how pollination and foraging success in a plant-pollinator mutualism were influenced by anther architecture and pollinator body size. To control for differences among species, we modified the anther architecture of a single plant species (Solanum elaeagnifolium) and used a single species of bumble bee (Bombus impatiens), which varies substantially in body size, even among individuals within a given colony (e.g., by a factor of 3.2 in size and 10-fold in body mass; Harder 1985; Couvillon et al. 2010; Kelemen et al. 2022). We hypothesized that pollination and foraging success would differ between anther architectures, with foraging bees leaving less pollen within joined anther architecture (the experimentally modified condition), and pollen being deposited more precisely on the bee, resulting in more pollen collection by the bee, and in less pollen ultimately transferred to stigmas. We also hypothesized that bee body size would substantially affect these patterns, with smaller bees leaving more pollen, collecting less pollen, and spending more time on joined anther architecture, and transferring less pollen to stigmas of flowers with free anther architecture (the natural condition).