The birds and the bees: pollinator behaviour and variation in the mating system of the rare shrub Grevillea macleayana

Reproductive biology and population structure of the endangered shrub Grevillea bedggoodiana (Proteaceae)

Narrowly endemic species are particularly vulnerable to catastrophic events. Compared to widespread species, they may also be less capable of adapting to shifts in environmental pressures as a result of specialisation on a narrow range of local condition and limited ability to disperse. However, life-history traits, such as preferential outcrossing and high fecundity can maintain genetic diversity and evolutionary potential, and boost species resilience. The endangered Grevillea bedggoodiana (Enfield Grevillea) is an understorey shrub restricted to an area of ca. 150 km2 in south-eastern Australia with a legacy of large-scale anthropogenic disturbance. Prior to this study little was known about its biology and population structure. Here, its breeding system was assessed through a controlled pollination experiment at one of its central populations, and eight populations were sampled for genetic analysis with microsatellite markers. The species was found to be preferentially outcrossing, with no evidence of pollination limitation. In most populations, allelic richness, observed heterozygosity and gene diversity were high (Ar: 3.8–6.3; Ho: 0.45–0.65, He: 0.60 − 0.75). However, the inbreeding coefficients were significant in at least four populations, ranging from Fi -0.061 to 0.259 despite high outcrossing rates. Estimated reproductive rates varied among sampled populations but were independent of gene diversity and inbreeding. Despite its small geographic range, the species’ populations showed moderate differentiation (AMOVA: FST = 0.123), which was largely attributable to isolation by distance. We interpret these results as suggesting that G. bedggoodiana is reproductively healthy and has maintained high levels of genetic diversity despite recent disturbance.

Pollen preferences of stingless bees in the Amazon region and southern highlands of Ecuador by scanning electron microscopy and morphometry

Stingless bees are effective pollinators of native tropical flora. Their environmental service maintains flow of pollen through pollination, increase reproductive success and influence genetic structure in plants. The management of stingless bees “meliponiculture”, is an activity limited to the countryside in Ecuador. The lack of knowledge of their managers about pollen resources can affect the correct maintenance/production of nests. The objective is to identify botanical families and genera of pollen grains collected by stingless bees by morphological features and differentiate potential species using geometric morphometry. Thirty-six pot pollen samples were collected from three Ecuadorian provinces located in two climatically different zones. Pollen type identification was based on the Number, Position, Character system. Using morphological features, the families and genera were established. Morphometry landmarks were used to show variation for species differentiation. Abundance, diversity, similarity and dominance indices were established by counting pollen grains, as well as spatial distribution relationships by means of Poisson regression. Forty-six pollen types were determined in two study areas, classified into 27 families and 18 genera. In addition, it was possible to identify more than one species, classified within the same family and genus, thanks to morphometric analysis. 1148 ± 799 (max 4211; min 29) pollen grains were counting in average. The diversity showed a high richness, low dominance and similarity between pollen resources. Families Melastomataceae and Asteraceae, genera Miconia and Bidens, were found as the main pollen resources. The stingless bee of this study are mostly generalist as shown the interaction network. The results of the present survey showed that stingless bees do not collect pollen from a single species, although there is evidence of a predilection for certain plant families. The diversity indexes showed high richness but low uniformity in the abundance of each family identified. The results of the study are also meaningful to the meliponiculture sector as there is a need to improve management practices to preserve the biodiversity and the environment.

The genetic mechanisms underlying the convergent evolution of pollination syndromes in the Neotropical radiation of Costus L

Selection together with variation in floral traits can act to mold floral form, often driven by a plant's predominant or most effective pollinators. To investigate the evolution of traits associated with pollination, we developed a phylogenetic framework for evaluating tempo and mode of pollination shifts across the genus Costus L., known for its evolutionary toggle between traits related to bee and bird pollination. Using a target enrichment approach, we obtained 957 loci for 171 accessions to expand the phylogenetic sampling of Neotropical Costus. In addition, we performed whole genome resequencing for a subset of 20 closely related species with contrasting pollination syndromes. For each of these 20 genomes, a high-quality assembled transcriptome was used as reference for consensus calling of candidate loci hypothesized to be associated with pollination-related traits of interest. To test for the role these candidate genes may play in evolutionary shifts in pollinators, signatures of selection were estimated as dN/dS across the identified candidate loci. We obtained a well-resolved phylogeny for Neotropical Costus despite conflict among gene trees that provide evidence of incomplete lineage sorting and/or reticulation. The overall topology and the network of genome-wide single nucleotide polymorphisms (SNPs) indicate that multiple shifts in pollination strategy have occurred across Costus, while also suggesting the presence of previously undetected signatures of hybridization between distantly related taxa. Traits related to pollination syndromes are strongly correlated and have been gained and lost in concert several times throughout the evolution of the genus. The presence of bract appendages is correlated with two traits associated with defenses against herbivory. Although labellum shape is strongly correlated with overall pollination syndrome, we found no significant impact of labellum shape on diversification rates. Evidence suggests an interplay of pollination success with other selective pressures shaping the evolution of the Costus inflorescence. Although most of the loci used for phylogenetic inference appear to be under purifying selection, many candidate genes associated with functional traits show evidence of being under positive selection. Together these results indicate an interplay of phylogenetic history with adaptive evolution leading to the diversification of pollination-associated traits in Neotropical Costus.

Population structure in Neotropical plants: integrating pollination biology, topography and climatic niches

Animal pollinators mediate gene flow among plant populations, but in contrast to well‐studied topographic and (Pleistocene) environmental isolating barriers, their impact on population genetic differentiation remains largely unexplored. Comparing how these multifarious factors drive microevolutionary histories is, however, crucial for better resolving macroevolutionary patterns of plant diversification. Here we combined genomic analyses with landscape genetics and niche modelling across six related Neotropical plant species (424 individuals across 33 localities) differing in pollination strategy to test the hypothesis that highly mobile (vertebrate) pollinators more effectively link isolated localities than less mobile (bee) pollinators. We found consistently higher genetic differentiation (F_ST) among localities of bee‐ than vertebrate‐pollinated species with increasing geographical distance, topographic barriers and historical climatic instability. High admixture among montane populations further suggested relative climatic stability of Neotropical montane forests during the Pleistocene. Overall, our results indicate that pollinators may differentially impact the potential for allopatric speciation, thereby critically influencing diversification histories at macroevolutionary scales.

The evolution of flower longevity in unpredictable pollination environments

Pollination requires a flower to remain open for long enough to allow for the arrival of pollinators. However, maintaining flowers costs energy and resources. Therefore, flower longevity, the length of time a flower remains viable, is critical for the outcome of plant reproduction. Although previous studies showed that the evolution of flower longevity depends on the rates of pollen deposition and removal, whether plants should increase or decrease flower life span when the pollination environment is unpredictable has not been explored. Moreover, the common hypothesis that an unpredictable pollination environment should select for increased flower longevity may be too simplistic since there is no distinction drawn between the effects of spatial and temporal variation. Adopting evolutionary game theory, we investigate the evolution of flower longevity under three types of variation: spatial heterogeneity, daily fluctuations within a flowering season and yearly fluctuations between flowering seasons. We find that spatial heterogeneity often selects for a shorter flower lifespan, while temporal fluctuations of fitness accrual rates at both daily and yearly time scales tends to favour greater longevity, although daily and yearly fluctuations have somewhat different effects. However, the presence of correlation between female and male fitness accrual rates seems to have no effect on flower longevity. Our work suggests that explicit measurements of spatial and temporal variation in both female and male functions may provide a better understanding of the evolution of flower longevity and reproduction.

Restored and remnant Banksia woodlands elicit different foraging behavior in avian pollinators

Pollinators and the pollination services they provide are critical for seed set and self-sustainability of most flowering plants. Despite this, pollinators are rarely assessed in restored plant communities, where their services are largely assumed to re-establish. Bird-pollinator richness, foraging, and interaction behavior were compared between natural and restored Banksia woodland sites in Western Australia to assess their re-establishment in restored sites. These parameters were measured for natural communities of varying size and degree of fragmentation, and restored plant communities of high and low complexity for three years, in the summer and winter flowering of Banksia attenuata and B. menziesii, respectively. Bird visitor communities varied in composition, richness, foraging movement distances, and aggression among sites. Bird richness and abundance were lowest in fragmented remnants. Differences in the composition were associated with the size and degree of fragmentation in natural sites, but this did not differ between seasons. Restored sites and their adjacent natural sites had similar species composition, suggesting proximity supports pollinator re-establishment. Pollinator foraging movements were influenced by the territorial behavior of different species. Using a network analysis approach, we found foraging behavior varied, with more frequent aggressive chases observed in restored sites, resulting in more movements out of the survey areas, than observed in natural sites. Aggressors were larger-bodied Western Wattlebirds (Anthochaera chrysoptera) and New Holland Honeyeaters (Phylidonyris novaehollandiae) that dominated nectar resources, particularly in winter. Restored sites had re-established pollination services, albeit with clear differences, as the degree of variability in the composition and behavior of bird pollinators for Banksias in the natural sites created a broad completion target against which restored sites were assessed. The abundance, diversity, and behavior of pollinator services to remnant and restored Banksia woodland sites were impacted by the size and degree of fragmentation, which in turn influenced bird-pollinator composition, and were further influenced by seasonal changes between summer and winter. Consideration of the spatial and temporal landscape context of restored sites, along with plant community diversity, is needed to ensure the maintenance of the effective movement of pollinators between natural remnant woodlands and restored sites.

Comparison of population genetic structures of the plant Silene stellata and its obligate pollinating seed predator moth Hadena ectypa

BACKGROUND AND AIMS

Population genetic structures and patterns of gene flow of interacting species provide important insights into the spatial scale of their interactions and the potential for local co-adaptation. We analysed the genetic structures of the plant Silene stellata and the nocturnal moth Hadena ectypa. Hadena ectypa acts as one of the important pollinators of S. stellata as well as being an obligate seed parasite on the plant. Although H. ectypa provides a substantial pollination service to S. stellata, this system is largely considered parasitic due to the severe seed predation by the Hadena larvae. Previous research on this system has found variable interaction outcomes across space, indicating the potential for a geographical selection mosaic.

METHODS

Using 11 microsatellite markers for S. stellata and nine markers for H. ectypa, we analysed the population genetic structure and the patterns and intensity of gene flow within and among three local populations in the Appalachians.

KEY RESULTS

We found no spatial genetic structure in the moth populations, while significant differentiation was detected among the local plant populations. Additionally, we observed that gene flow rates among H. ectypa populations were more uniform and that the mean gene flow rate in H. ectypa was twice as large as that in S. stellata.

CONCLUSIONS

Our results suggest that although the moths move frequently among populations, long-distance pollen carryover only happens occasionally. The difference in gene flow rates between S. stellata and H. ectypa could prevent strict local co-adaptation. Furthermore, higher gene flow rates in H. ectypa could also increase resistance of the local S. stellata populations to the parasitic effect of H. ectypa and therefore help to stabilize the Silene-Hadena interaction dynamics.

Effects of wildfire on floral display size and pollinator community reduce outcrossing rate in a plant with a mixed mating system

PREMISE OF THE STUDY

Wildfire changes the demography, morphology, and behavior of plants, and may alter the pollinator community. Such trait changes may drastically alter the outcome of pollination mutualisms on plants; however, the direct role of fire on these mutualisms is poorly known.

METHODS

Following a pair of fires in the northern California coast range chaparral, we censused floral visitor communities of Trichostema laxum (Lamiaceae), quantified visiting bee behavior, and estimated outcrossing rates using a widespread Mendelian recessive floral polymorphism across a matrix of populations in burned and unburned sites. We also compared pre- and postfire floral visitation in two populations.

RESULTS

Outcrossing rates were significantly lower in burned areas; however, our data suggest that the much larger size of plants in burned areas, not burn status itself, drove this pattern. Large-bodied bees dominated floral visitor communities after fire, likely recruiting to the abundant postfire floral resources. These bees visited more flowers per plant than did the smaller bees prevalent before fire and in unburned areas, likely increasing selfing through geitonogamy (within-plant pollination), an effect made possible by the far larger size of plants in burned areas.

CONCLUSIONS

Outcrossing rates dropped substantially after wildfires because of changes in the pollinators, plant display size, and their interactions. Reductions in outcrossing following fire may have important implications for population resilience and evolution in a changing climate with more frequent fires.

Habitat fragmentation leads to reduced pollinator visitation, fruit production and recruitment in urban mangrove forests

Mangrove forests worldwide undergo anthropogenic fragmentation that may threaten their existence, and yet there have been few tests of the effects of fragmentation on demographic processes critical for mangrove regeneration. Predicting the effects of habitat fragmentation on mangroves is problematic as pollinators may move more freely across water than terrestrial habitat, and propagules can be widely dispersed by water. Here, within each of two estuaries, we compared pollinator diversity and activity, reproductive effort and output, and rates of recruitment for sets of three large (>1500 trees), medium (300-500) and small (<50) stands. As predicted, most measures of reproductive activity and success were inversely related to stand size with large stands typically producing significantly more and larger fruit, and significantly more seedlings. Most strikingly, we found the effect of fragmentation on the abundance of pollinators (honeybees), the production and quality of fruit and the survival rate of seedlings to be similar, showing significant reduction of recruitment in small stands. This study provides the first rigorous evidence that recruitment of mangroves, like for many terrestrial plants, is negatively impacted by habitat fragmentation. From a management perspective, we argue that in the short term our data imply the importance of conserving the largest possible stands. However, additional work is needed to determine (1) the proportion of recruits within small stands that originate within large stands, (2) how seedling performance varies with fruit size and genotype, and (3) how seedling size and performance vary with the abundance and diversity of pollen.

Simultaneous inbreeding modifies inbreeding depression in a plant-herbivore interaction

Because inbreeding is common in natural populations of plants and their herbivores, herbivore-induced selection on plants, and vice versa, may be significantly modified by inbreeding and inbreeding depression. In a feeding assay with inbred and outbred lines of both the perennial herb, Vincetoxicum hirundinaria, and its specialist herbivore, Abrostola asclepiadis, we discovered that plant inbreeding increased inbreeding depression in herbivore performance in some populations. The effect of inbreeding on plant resistance varied among plant and herbivore populations. The among-population variation is likely to be driven by variation in plant secondary compounds across populations. In addition, inbreeding depression in plant resistance was substantial when herbivores were outbred, but diminished when herbivores were inbred. These findings demonstrate that in plant-herbivore interactions expression of inbreeding depression can depend on the level of inbreeding of the interacting species. Furthermore, our results suggest that when herbivores are inbred, herbivore-induced selection against self-fertilisation in plants may diminish.

Is the post-disturbance composition of a plant population determined by selection for outcrossed seedlings or by the composition of the seedbank?

Seedbanks are expected to buffer populations against disturbances, such as fire, that could alter the genetic composition of smaller, ephemeral adult populations. However, seedling genotypes may be influenced by the spatially heterogeneous nature of both the seedbank and the disturbance (for example, germination may vary with local disturbance) and also by selection acting on germination and post-germination performance. We used microsatellite-DNA surveys of seedlings emerging from the soil-stored seedbanks of Grevillea macleayana after wildfire to compare diversity and spatial structure in seedlings and adults, and through resampling of the seedling data set, to determine whether the resultant adult population reflected the effects of selection or random seedling mortality. The large post-fire seedling cohorts captured the full allelic diversity of the pre-fire adult population. However, we found a mismatch in the genotypic structure of adults and seedlings. Seedlings displayed larger heterozygous deficits than adults; however, over the ensuing 11 years, seedling heterozygosity eventually matched values for the pre-fire adults. Increasing heterozygosity among adults has generally been attributed to heterosis and/or reduction in Wahlund effects via self-thinning. Resampling of early post-fire seedlings to generate samples of equivalent size to survivors at 11 years showed that increases in heterozygosity must be driven by selection favouring outcrossed seed. This finding is important in an evolutionary context but also has implications for the restoration of natural or managed populations where a seedbank is a viable source of recruits.

Changes in pollinator assemblages following hurricanes affect the mating system of Laguncularia racemosa (Combretaceae) in Florida, USA

Hurricanes are major disturbance events in Neotropical mangrove communities, potentially affecting the reproductive success of mangrove species. This was the first investigation of changes in mangrove pollinator assemblages following hurricanes, and the effect of these changes on the mating system of Laguncularia racemosa. Insect pollinator assemblages were investigated in three Florida mangrove communities in 2001–2003, 2005 and 2009; two hurricanes affected the area in 2004. Visitation rates were estimated from 1445 insects observed during 272 10-min intervals; the number of flowers visited by each insect was also recorded. Pollinator diversity was estimated with the Shannon Index. Following the hurricanes, species richness was reduced by 43–65% and diversity declined by 36–70%. Significant declines in insect visitation to L. racemosa resulted in reduced outcrossing frequencies in 2005. Laguncularia racemosa flowers autogamously self-pollinate without insect visitors, so fruit set still occurred. Visitation rates returned to pre-hurricane levels by 2009, but foraging behaviours differed from pre-hurricane patterns; outcrossing was further favoured by reduced frequencies of long foraging bouts and increased frequencies of short foraging bouts. The mixed mating system of L. racemosa provides reproductive assurance following hurricane disturbances, when pollinator abundance is low.

Behaviour of pollinator insects within inflorescences of Scrophularia species from Iberian Peninsula

Several hypotheses have been proposed to explain the pattern of pollinator visits to vertical inflorescences. These range from a response to a pattern of resources to merely instinctive behaviour. In dichogamous plants, such behaviour has been associated with promoting outcrossing and avoiding geitonogamy. We here analyse behaviour of the principal pollinator groups in five protogynous species of Scrophularia with different flower sizes (S. sambucifolia, S. grandiflora, S. lyrata, S. scorodonia and S. canina), and the distribution of sexual phases along the inflorescences. The results in all cases show that pollinators follow a pattern of ascending visits accompanied by movements between flowers of the same whorl (horizontal movements). The relative frequency of these horizontal movements depends on the flower size, with a higher frequency in species with large flowers. In vertical movements of the three more common pollinator groups to several plant species (bumblebees, wasps and small bees), the behaviour was essentially independent of flower size, with bumblebees having the highest ratio of ascents to descents. Behaviour of the pollinators, together with the absence of a definite pattern of distribution of the sexual phases along the inflorescence, implies that geitonogamy is not avoided in any of the Scrophularia species studied.

The endangered Iris atropurpurea (Iridaceae) in Israel: honey-bees, night-sheltering male bees and female solitary bees as pollinators

BACKGROUND AND AIMS

The coastal plain of Israel hosts the last few remaining populations of the endemic Iris atropurpurea (Iridaceae), a Red List species of high conservation priority. The flowers offer no nectar reward. Here the role of night-sheltering male solitary bees, honey-bees and female solitary bees as pollinators of I. atropurpurea is documented.

METHODS

Breeding system, floral longevity, stigma receptivity, visitation rates, pollen loads, pollen deposition and removal and fruit- and seed-set were investigated.

KEY RESULTS

The main wild pollinators of this plant are male eucerine bees, and to a lesser extent, but with the potential to transfer pollen, female solitary bees. Honey-bees were found to be frequent diurnal visitors; they removed large quantities of pollen and were as effective as male sheltering bees at pollinating this species. The low density of pollen carried by male solitary bees was attributed to grooming activities, pollen displacement when bees aggregated together in flowers and pollen depletion by honey-bees. In the population free of honey-bee hives, male bees carried significantly more pollen grains on their bodies. Results from pollen analysis and pollen deposited on stigmas suggest that inadequate pollination may be an important factor limiting fruit-set. In the presence of honey-bees, eucerine bees were low removal-low deposition pollinators, whereas honey-bees were high removal-low deposition pollinators, because they removed large amounts into corbiculae and deposited relatively little onto receptive stigmas.

CONCLUSIONS

Even though overall, both bee taxa were equally effective pollinators, we suggest that honey-bees have the potential to reduce the amount of pollen available for plant reproduction, and to reduce the amount of resources available to solitary bee communities. The results of this study have potential implications for the conservation of this highly endangered plant species if hives are permitted inside reserves, where the bulk of Oncocyclus iris species are protected.

Insect visitation rates and foraging patterns differ in androdioecious and hermaphrodite-only populations of Laguncularia racemosa (Combretaceae) in Florida

Insect-pollinated Laguncularia racemosa has a variable breeding system; some populations are androdioecious, with male and hermaphroditic plants, while others lack male plants. We observed the foraging behaviours of insects in three androdioecious and three hermaphrodite-only populations of L. racemosa in Florida. In each population, insect visitation rates were estimated from 30–108 timed intervals. We recorded the number of flowers visited by 144–224 insects during foraging bouts made to 15–40 male and hermaphroditic plants. Male plants in androdioecious populations had significantly more visitors than hermaphroditic plants, increasing the number of vectors carrying pollen from male plants. Further, many insects visited few flowers during foraging bouts, which should increase outcrossing frequency. According to mathematical models, male plants benefit from these combined factors. Plants in hermaphrodite-only populations had significantly more visitors than hermaphroditic plants in androdioecious populations. Proportionately more insects visited many flowers during foraging bouts in hermaphrodite-only versus androdioecious populations. The increased likelihood of geitonogamous self-pollination could help explain the lack of male plants in hermaphrodite-only populations. Differences in pollinator assemblages and the relative abundances of several species were responsible for differences in foraging behaviours: Apis mellifera, Bombus sp., Melissodes sp., Xylocopa sp., Euodynerus sp. and a calliphorid species.

Ecology and evolution of plant-pollinator interactions

BACKGROUND

Some of the most exciting advances in pollination biology have resulted from interdisciplinary research combining ecological and evolutionary perspectives. For example, these two approaches have been essential for understanding the functional ecology of floral traits, the dynamics of pollen transport, competition for pollinator services, and patterns of specialization and generalization in plant-pollinator interactions. However, as research in these and other areas has progressed, many pollination biologists have become more specialized in their research interests, focusing their attention on either evolutionary or ecological questions. We believe that the continuing vigour of a synthetic and interdisciplinary field like pollination biology depends on renewed connections between ecological and evolutionary approaches.

SCOPE

In this Viewpoint paper we highlight the application of ecological and evolutionary approaches to two themes in pollination biology: (1) links between pollinator behaviour and plant mating systems, and (2) generalization and specialization in pollination systems. We also describe how mathematical models and synthetic analyses have broadened our understanding of pollination biology, especially in human-modified landscapes. We conclude with several suggestions that we hope will stimulate future research. This Viewpoint also serves as the introduction to this Special Issue on the Ecology and Evolution of Plant-Pollinator Interactions. These papers provide inspiring examples of the synergy between evolutionary and ecological approaches, and offer glimpses of great accomplishments yet to come.