Connective appendages in Huberia bradeana (Melastomataceae) affect pollen release during buzz pollination

Abstract

Floral structures, such as stamen appendages, play crucial roles in pollinator attraction, pollen release dynamics and, ultimately, the reproductive success of plants. The pollen‐rewarding, bee buzz‐pollinated flowers of Melastomataceae often bear conspicuous staminal appendages. Surprisingly, their functional role in the pollination process remains largely unclear. We use Huberia bradeana Bochorny & R. Goldenb. (Melastomataceae) with conspicuously elongated, twisted stamen appendages to investigate their functional role in the pollination process. We studied the effect of stamen appendages on pollinator behaviour and reproductive success by comparing manipulated flowers (appendages removed) with unmanipulated flowers. To assess bee pollinator behaviour, we measured three properties of buzzes (vibrations) produced by bees on Huberia flowers: frequency, duration and number of buzzes per flower visit. We measured male and female reproductive success by monitoring pollen release and deposition after single bee visits. Finally, we used artificial vibrations and laser vibrometry to assess how flower vibrational properties change with the removal of stamen appendages. Our results show that the absence of staminal appendages does not modify bee buzzing behaviour. Pollen release was higher in unmanipulated flowers, but stigmatic pollen loads differ only marginally between the two treatments. We also detected lower vibration amplitudes in intact flowers as compared to manipulated flowers in artificial vibration experiments. The presence of connective appendages are crucial in transmitting vibrations and assuring optimal pollen release. Therefore, we propose that the high diversity of colours, shapes and sizes of connective appendages in buzz‐pollinated flowers may have evolved by selection through male fitness.

Anther cones increase pollen release in buzz‐pollinated Solanum flowers

“…2019; Bochorny et al. 2021), we selected a single test frequency based on previous empirical work in Solanum , which has established that, within the range of frequencies produced by bees (∼100–400 Hz), frequency has a relatively minor contribution to pollen release compared to the amplitude and duration of the buzz (De Luca et al. 2013; Rosi‐Denadai et al.…”

“…The choice of a single frequency, duration, and amplitude enabled us to focus on the comparison between the two contrasting anther architectures studied here while controlling for buzz type. Although spectral properties of bee vibrations, such as their fundamental frequency, vary between bee species, between individuals of the same species, and even between buzzes by the same bee (Burkart et al 2011;Arroyo-Correa et al 2019;De Luca et al 2019;Switzer et al 2019;Bochorny et al 2021), we selected a single test frequency based on previous empirical work in Solanum, which has established that, within the range of frequencies produced by bees (∼100-400 Hz), frequency has a relatively minor contribution to pollen release compared to the amplitude and duration of the buzz Rosi-Denadai et al 2020; see also the theoretical model of Hansen et al 2021). Future work might benefit from exploring the effect of other combinations of parameters on pollen release and vibration transmission (principally amplitude and duration that are linked to pollen release Vallejo-Marín 2019;Rosi-Denadai et al 2020], but also other potential sources of variation including frequency, harmonic structure, different modes of the bee holding the flower while buzzing, humidity variation, etc.…”

Examining the Role of Buzzing Time and Acoustics on Pollen Extraction of Solanum elaeagnifolium

“…Even though buzz pollination is known for more than 100 years [15,16], only recently, the field has received a rejuvenated interest [12,13,17,18]. In the past decade, exceptional work has been done on understanding buzz pollination in different plant and bee species, their evolutionary and ecological consequences, as well as both plant and pollinator fitness [3,6,12,13,17,[19][20][21][22][23][24]. For example, although bee vibrational acceleration (instantaneous changes in amplitude, ms −2 ) and frequency (number of cycles per second, Hz) are independent of bee mass and flower mass and vary between bee species, floral characteristics have been found to significantly affect their transmission to the flowers [3].…”

Transmission of bee-like vibrations in buzz-pollinated plants with different stamen architectures

“…Few studies have examined the effect of specific morphological traits on vibration transmission in buzz-pollinated flowers, but closely-related species of Solanum with similar morphologies can differ in their vibration transmission properties 35 . Moreover, partial removal of stamen structures, such as the connective appendages in Huberia bradeana (Melastomataceae), can affect the relative amplitude of vibrations 55 . Although the species studied here differed in anther architecture and the transmission of vibrations through the androecium, all of them have stamens positioned relatively closely together, more or less forming a cone.…”