Morphological specialization of heterantherous Rhynchanthera grandiflora (Melastomataceae) accommodates pollinator diversity

Abstract

The tropical Melastomataceae are characterized by poricidal anthers which constitute a floral filter selecting for buzz‐pollinating bees. Stamens are often dimorphic, sometimes with discernible feeding and pollinating functions. Rhynchanthera grandiflora produces nectarless flowers with four short stamens and one long stamen; all anthers feature a narrow elongation with an upwards facing pore. We tested pollen transfer by diverse foraging bees and viability of pollen from both stamen types. The impact of anther morphology on pollen release direction and scattering angle was studied to determine the plant's reproductive strategy. Medium‐sized to large bees sonicated flowers in a specific position, and the probability of pollen transfer correlated with bee size even among these legitimate visitors. Small bees acted as pollen thieves or robbers. Anther rostrum and pore morphology serve to direct and focus the pollen jet released by floral sonication towards the pollinator's body. Resulting from the ventral and dorsal positioning of the short and long stamens, respectively, the pollinator's body was widely covered with pollen. This improves the plant's chances of outcrossing, irrespective of which bee body part contacts the stigma. Consequently, R. grandiflora is also able to employ bee species of various sizes as pollen vectors. The strategy of spreading pollen all over the pollinator's body is rather cost‐intensive but counterbalanced by ensuring that most of the released pollen is in fact transferred to the bee. Thus, flowers of R. grandiflora illustrate how specialized morphology may serve to improve pollination by a functional group of pollinators.

Low bee visitation rates explain pollinator shifts to vertebrates in tropical mountains

“…In connection with this, we observed marked differences in buzzing behaviours of bees, with bees buzzing the entire androecium at once in flowers with small appendages, but sequentially buzzing single stamens in flowers with large appendages (Fig. 1b; also compare Mesquita‐Neto et al ., 2018; Konzmann et al ., 2020). We argue that allocating pollen to 10 separate buzzing units (two whorls of five stamens each in pentamerous flowers), as in large‐appendage flowers, is an adaptive strategy to optimize pollen dispensing and delivery.…”

Minimum size threshold of visiting bees of a buzz‐pollinated plant species: consequences for pollination efficiency

“…However, not all visitors of buzz‐pollinated flowers are, in fact, pollinators. Flower visitors may contribute to plant reproduction to different degrees, depending on a set of behavioral and morphological components, such as body size, frequency of contact with stigmas, duration of visits, and visitation rates of flowers and of conspecific plant individuals (Ne’eman et al, 2010; Mesquita‐Neto et al, 2017, 2018; Solís‐Montero and Vallejo‐Marín, 2017; Pinheiro‐Costa et al, 2018; Rego et al, 2018; Konzmann et al, 2020).…”

“…The overall size of visitors seems to be a particularly strong determinant of pollinator efficiency. For example, some visiting bees are too small to contact stigmas of poricidal flowers and are therefore poor pollinators of such plants (Mesquita‐Neto et al, 2017, 2018; Solís‐Montero and Vallejo‐Marín, 2017; Pinheiro‐Costa et al, 2018; Rego et al, 2018; De Luca et al, 2019; Konzmann et al, 2020). Furthermore, pollen removal varies among flower‐visiting bees in flowers with poricidal anthers, and the capacity of bees to sonicate is taxon dependent (Cardinal et al, 2018).…”

Evolutionary lability in floral ontogeny affects pollination biology in Trimezieae

“…Pollen‐pollinated lineages of Melastomataceae, such as Microlicia and Cambessedessia , are particularly diverse and endemic to the campo rupestre (Bochorny et al, 2019; Versiane, 2019). In addition, many plant species from several families that display variable degrees of specialization to pollen bees have recently been described in this habitat (Mesquita‐Neto et al, 2018; Nogueira et al, 2018; Konzmann et al, 2020). Some of these lineages also show an increased diversification in the campos rupestres , particularly Chamaecrista and Microlicia (Vasconcelos et al, 2020).…”