Pollen packaging and dispensing: adaption of patterns of anther dehiscence and flowering traits to pollination in three Epimedium species

Pollen dispensing schedules in buzz-pollinated plants: experimental comparison of species with contrasting floral morphologies

PREMISE

Plants can mitigate the fitness costs associated with pollen consumption by floral visitors by optimizing pollen release rates. In buzz-pollinated plants, bees apply vibrations to remove pollen from anthers with small pores. These poricidal anthers potentially function as mechanism staggering pollen release, but this has rarely been tested across plant species differing in anther morphology.

METHODS

In Solanum Section Androceras, three pairs of buzz-pollinated species have undergone independent evolutionary shifts between large- and small-flowers, which are accompanied by replicate changes in anther morphology. We used these shifts in anther morphology to characterize the association between anther morphology and pollen dispensing schedules. We applied simulated bee-like vibrations to anthers to elicit pollen release, and compared pollen dispensing schedules across anther morphologies. We also investigated how vibration velocity affects pollen release.

RESULTS

Replicate transitions in Solanum anther morphology are associated with consistent changes in pollen dispensing schedules. We found that small-flowered taxa release their pollen at higher rates than their large-flowered counterparts. Higher vibration velocities resulted in quicker pollen dispensing and more total pollen released. Finally, both the pollen dispensing rate and the amount of pollen released in the first vibration were negatively related to anther wall area, but we did not observe any association between pore size and pollen dispensing.

CONCLUSIONS

Our results provide the first empirical demonstration that the pollen dispensing properties of poricidal anthers depend on both floral characteristics and bee vibration properties. Morphological modification of anthers could thus provide a mechanism to exploit different pollination environments.

Optimal pollen stickiness to pollinators for maximizing paternal fitness: Increased number of recipient flowers or increased pollen deposition on recipient flowers?

A plant can sire more seeds by increasing the number of pollen recipient flowers or the amount of pollen deposited on recipient flowers. We theoretically analyzed how pollen stickiness contributes to paternal fitness through changing the pattern of pollen dispersal including both the number of recipient flowers and overall pollen deposition (the overall amount of pollen deposited on recipient flowers) in animal-pollinated plants. We developed a numerical model in which pollen stickiness to pollinators increases with production of expensive materials on pollen surfaces, and a high level of stickiness diminishes the proportions of pollen lost from a pollinator body during a flight and pollen deposited on a stigma during a visit. We found that the number of recipient flowers monotonically increased with increasing pollen stickiness allocation while overall pollen deposition was maximized at a certain amount of stickiness allocation. We demonstrated that evolutionarily stable pollen stickiness attained many recipient flowers at the expense of overall pollen deposition in most cases while it merely favored maximization of overall pollen deposition in all other cases. Sticky pollen evolved if pollinators were highly likely to drop pollen during flights and did not diffuse well. In this situation, the evolutionarily stable pattern of pollen dispersal was acquisition of many pollen recipient flowers rather than maximization of overall pollen deposition. Sticky pollen also evolved if additional sticking elements were moderately effective in increasing the force of adhesion to pollinators. Pollen stickiness has a significant effect on the pattern of pollen dispersal via the extent of pollen carryover, and our results suggest that plants maximize paternal fitness by giving pollen the optimal stickiness, which varies with pollinating partners.

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

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.

Nectar foragers contribute to the pollination of buzz-pollinated plant species

PREMISE OF THE STUDY

Pollination performance may depend on the type of floral resource (pollen or nectar) foraged by visitors. In buzz-pollinated plant species, the poricidal anthers release pollen during active pollen collection that induces flower vibrations. These buzz-pollinated species generally do not produce nectar. Nevertheless, several Ericaceae are buzz-pollinated and produce nectar. We estimated the relative effectiveness of visitors according to the type of resource collected, nectar or pollen (buzzing).

METHODS

We compared the relative performance of pollen removal, transport, and deposition (effectiveness) of the main insect visitors on four ericaceous species: three buzz-pollinated species with different pore sizes, Erica tetralix, Vaccinium myrtillus, and V. vitis-idaea; and one non-buzz-pollinated species, Calluna vulgaris.

KEY RESULTS

Bumblebees were the main pollinators for the three buzz-pollinated species, whereas hoverflies were the main pollinators for the non-buzz-pollinated generalist C. vulgaris. For the studied plant species, we observed no difference in pollination effectiveness among bumblebee species. Buzzing bumblebees were the most effective visitors for pollination per flower visit for the two Vaccinium species, whereas nectar foragers were the most effective visitors for pollination of E. tetralix. In the case of Vaccinium myrtillus, nectar foragers contributed the most to pollination success because they were more abundant than pollen foragers.

CONCLUSIONS

We showed that consideration of the resource collected by visitors and their behavior is necessary to compare their relative performance. The combination of visitation rate and effectiveness per visit reveals that nectar foragers make a substantial contribution to pollination of the buzz-pollinated ericaceous species.

Sequential stamen maturation and movement in a protandrous herb: mechanisms increasing pollination efficiency and reducing sexual interference

Successive stamen movement directly controls pollen presentation schedules through sequential stamen maturation and changes the extent of herkogamy by altering the positions of sexual organs. However, the implications of such movements in terms of pollination are not well understood. Pollen presentation theory predicts that staggered pollen presentation should be favoured when plants are subject to diminishing returns on pollen transfer. Herkogamy on the other hand, has been interpreted as an adaptive trait that reduces sexual interference in hermaphrodite flowers. In this study, we conducted floral manipulations to determine the function of successive stamen movement in pollen transfer. By artificially manipulating the flowers to present two anthers simultaneously in the floral centre, we attempted to investigate whether changes in the anther presentation strategy affect pollen removal, deposition and the efficiency of pollinators. Compared with the natural treatment, the pollen transfer efficiency of halictid bees decreased significantly when the flowers were manipulated to present two anthers simultaneously. Although the presentation of two anthers simultaneously led to a similar pollen removal rate, there was a significant reduction in pollen deposition on neighbouring stigmas. To evaluate the effect of movement herkogamy on pollen export and deposition and seed set, the flowers were manipulated with or without the movement of stamen bending out from the floral centre. Pollen export decreased significantly when the central anther was moved away from the pistil, and pollen deposition and seed set declined significantly when the five spent anthers were retained on the pistil. Our study provides good support for the pollen presentation theory and provides direct experimental evidence that successive stamen movement could increase pollen transfer efficiency by sequential stamen maturation. Moreover, movement herkogamy promotes pollen export, deposition and seed set, and could therefore be regarded as an effective mechanism to reduce interference between male and female functions.

Food Limitation Affects Parasite Load and Survival of Bombus impatiens (Hymenoptera: Apidae) Infected With Crithidia (Trypanosomatida: Trypanosomatidae)

Bumble bees (genus Bombus) are globally important insect pollinators, and several species have experienced marked declines in recent years. Both nutritional limitation and pathogens may have contributed to these declines. While each of these factors may be individually important, there may also be synergisms where nutritional stress could decrease pathogen resistance. Understanding interactions between bumble bees, their parasites, and food availability may provide new insight into the causes of declines. In this study, we examined the combined impacts of pollen and nectar limitation on Crithidia, a common gut parasite in Bombus impatiens Cresson. Individual worker bees were inoculated with Crithidia and then assigned in a factorial design to two levels of pollen availability (pollen or no pollen) and two nectar sugar concentrations (high [30%] or low [15%] sucrose). We found that lack of pollen and low nectar sugar both reduced Crithidia cell counts, with the most dramatic effect from lack of pollen. Both pollen availability and nectar sugar concentration were also important for bee survival. The proportion of bees that died after seven days of infection was ∼25% lower in bees with access to pollen and high nectar sugar concentration than any other treatment. Thus, nectar and pollen availability are both important for bee survival, but may come at a cost of higher parasite loads. Our results illustrate the importance of understanding environmental context, such as resource availability, when examining a host-parasite interaction.

Specialist pollinators deplete pollen in the spring ephemeral wildflower Claytonia virginica

Pollinators that collect pollen – and specifically, pollen‐specialist bees – are often considered to be the best pollinators of a (host) plant. Although pollen collectors and pollen specialists often benefit host plants, especially in the pollen that they deliver (their pollination “effectiveness”), they can also exact substantial costs because they are motivated to collect as much pollen as possible, reducing the proportion of pollen removed that is subsequently delivered to stigmas (their pollination “efficiency”). From the plant perspective, pollen grains that do not pollinate conspecific stigmas are “wasted”, and potentially costly. We measured costs and benefits of nectar‐collecting, pollen‐collecting, and pollen‐specialist pollinator visitation to the spring ephemeral Claytonia virginica. Visits by the pollen‐specialist bee Andrena erigeniae depleted pollen quickly and thoroughly. Although all pollinators delivered roughly the same number of grains, the pollen specialist contributed most to C. virginica pollen delivery because of high visitation rates. However, the pollen specialist also removed a large number of grains; this removal may be especially costly because it resulted in the depletion of pollen grains in C. virginica populations. While C. virginica appears to rely on pollen transfer by the pollen specialist in these populations, nectar‐collecting visitors could provide the same benefit at a lower cost if their visitation rates increased. Pollen depletion affects a pollinator's value to plants, but is frequently overlooked. If they lower the effectiveness of future floral visitors, visits by A. erigeniae females to C. virginica may be more detrimental than beneficial compared to other pollinators and may, in some circumstances, reduce plant fitness rather than increase it. Therefore, A. erigeniae and C. virginica may vary in their degree of mutualism depending on the ecological context.

The differential contributions of herkogamy and dichogamy as mechanisms of avoiding self-interference in four self-incompatible Epimedium species

Self-interference is one of the most important selective forces in shaping floral evolution. Herkogamy and dichogamy both can achieve reductions in the extent of self-interference, but they may have different roles in minimizing self-interference in a single species. We used four self-incompatible Epimedium species to explore the roles of herkogamy and dichogamy in avoiding self-interference and to test the hypothesis that herkogamy and dichogamy may be separated and become selected preferentially in the taxa. Two species (E. franchetii and E. mikinorii) expressed strong herkogamy and weak protogyny (adichogamy), whereas another two species (E. sutchuenense and E. leptorrhizum) expressed slight herkogamy and partial protandry. Field investigations indicated that there was no physical self-interference between male function and female function regarding pollen removal and pollen deposition in all species. Self-pollination (autonomous or facilitated) was greater in species with slight herkogamy than in those with strong herkogamy. Artificial pollination treatments revealed that self-pollination could reduce outcrossed female fertility in all species, and we found evidence that self-interference reduced seed set in E. sutchuenense and E. leptorrhizum in the field, but not in E. franchetii and E. mikinorii. These results indicate that well-developed herkogamy is more effective compared with dichogamy in avoiding self-interference in the four species. In genus Epimedium, herkogamy instead of dichogamy should be selected preferentially and evolved as an effective mechanism for avoiding self-interference and might not need to evolve linked with dichogamy.