Cuckoo male bumblebees perform slower and longer flower visits than free-living male and worker bumblebees

Cuckoo bumblebees are a monophyletic group within the genus Bombus and social parasites of free-living bumblebees, upon which they rely to rear their offspring. Cuckoo bumblebees lack the worker caste and visit flowers primarily for their own sustenance and do not collect pollen. Although different flower-visiting behaviours can be expected between cuckoo and free-living bumblebees due to different biological constraints, no study has yet quantified such differences. Here, we provide the first empirical evidence of different flower-visiting behaviours between cuckoo and free-living bumblebees. We recorded the flower-visiting behaviour of 350 individual bumblebees over two years in a wild population of the entomophilous plant Gentiana lutea, of which they are among the main pollinators. In cuckoo bumblebees (28.9% of the total), we only found males, while we found both workers and males in free-living bumblebees. Cuckoo bumblebees visited significantly more flowers for longer time periods than both free-living bumblebee workers and males within whorls, while differences at the whole-plant level were less marked. Free-living bumblebee males visited more flowers and performed slightly longer flower visits than workers. Behavioural differences between cuckoo male bumblebees and free-living bumblebee workers are likely related to different foraging needs, while differences between cuckoo and free-living bumblebee males may be caused by differences in colony development and a delayed mating period of free-living bumblebees. The longer visits made by cuckoo male bumblebees will likely negatively affect plant reproductive success through increased within-plant pollen flow.

Rapid genomic evolution in Brassica rapa with bumblebee selection in experimental evolution

Insect pollinators shape rapid phenotypic evolution of traits related to floral attractiveness and plant reproductive success. However, the underlying genomic changes and their impact on standing genetic variation remain largely unknown despite their importance in predicting adaptive responses in nature or in crop’s artificial selection. Here, based on a previous, nine generation experimental evolution study with fast cycling Brassica rapa plants adapting to bumblebees, we document genomic evolution associated to the adaptive process. We performed a genomic scan of the allele frequency changes along the genome and estimated the nucleotide diversity and genomic variance changes. We detected signature of selection associated with rapid changes in allelic frequencies on multiple loci. During experimental evolution, we detected an increase in overall genomic variance, whereas for loci under selection, a reduced variance was apparent in both replicates suggesting a parallel evolution. Our study highlights the polygenic nature of short-term pollinator adaptation and the importance of a such genetic architecture in the maintenance of genomic variance during strong natural selection by biotic factors.

The ecology and evolution of synchronized reproduction in long-lived plants

Populations of many long-lived plants exhibit spatially synchronized seed production that varies extensively over time, so that seed production in some years is much higher than on average, while in others, it is much lower or absent. This phenomenon termed masting or mast seeding has important consequences for plant reproductive success, ecosystem dynamics and plant–human interactions. Inspired by recent advances in the field, this special issue presents a series of articles that advance the current understanding of the ecology and evolution of masting. To provide a broad overview, we reflect on the state-of-the-art of masting research in terms of underlying proximate mechanisms, ontogeny, adaptations, phylogeny and applications to conservation. While the mechanistic drivers and fitness consequences of masting have received most attention, the evolutionary history, ontogenetic trajectory and applications to plant–human interactions are poorly understood. With increased availability of long-term datasets across broader geographical and taxonomic scales, as well as advances in molecular approaches, we expect that many mysteries of masting will be solved soon. The increased understanding of this global phenomenon will provide the foundation for predictive modelling of seed crops, which will improve our ability to manage forests and agricultural fruit and nut crops in the Anthropocene. This article is part of the theme issue ‘The ecology and evolution of synchronized seed production in plants’.

Testing for apomixis in an obligate pollination mutualism

Plants with a small number of specific pollinators may be vulnerable to fluctuations in the availability of those pollinators, which could limit plant reproductive success and even result in extinction. Plants can develop mechanisms to mitigate this risk, such as apomixis. Reproductive assurance mechanisms have been largely ignored in obligate pollination mutualisms (OPMs), that are some of the most specialised of plant-pollinator interactions. Furthermore, although OPMs are often referred to as obligate, this is rarely tested. We performed a flower-bagging experiment to test if the unisexual flowers of Breynia oblongifolia could set fruit in the absence of its highly specialised seed-eating moth pollinators. Surprisingly, many bagged female flowers developed fruits, suggesting apomixis. We therefore conducted a second series of experiments in which we 1) added or excluded pollinators from caged plants; and 2) surveyed a wild population for apomictic reproduction using mother-offspring genotyping. In the absence of pollinators, no fruits developed. In addition, we detected no genetic evidence for apomixis when comparing between mothers and their offspring or between adults in a wild population. We explain the production of fruits in bagged branches by our discovery that B. oblongifolia can retain pollinated female flowers over the winter period. These flowers develop to fruits in the spring in the absence of male flowers or pollinators. Our study thus shows that B. oblongifolia is unable to produce fruit in the absence of its specialist moth pollinators. Thus, the highly specific interaction between plant and pollinators appears to be truly obligate.

Effects of climate change and pollen supplementation on the reproductive success of two grassland plant species

Climate change has the potential to alter plant reproductive success directly and indirectly through disruptions in animal pollination. Climate models project altered seasonal precipitation patterns and thus the effects of climate change on available resources and pollination services will depend on the season. Plants have evolved reproductive strategies to minimize pollen and resource limitations, and therefore we expect that the disruption of climate change might cause plants to be more pollen limited in seasons that become wetter than they were historically. In this study, we conducted a pollen supplementation experiment within the Global Change Experiment Facility (GCEF) in Central Germany. The GCEF experimentally manipulates future climate based on a realistic scenario of climate change for the region (drier summers and wetter springs and falls) in a native grassland ecosystem. We quantified seed production of two perennial species Dianthus carthusianorum and Scabiosa ochroleuca in response to pollination treatments (control, supplement), climate treatments (ambient and future) and season (summer and fall). Dianthus carthusianorum produced more seeds in future climate conditions independent of the season, but only when given supplemental pollen. Both species showed an increased reproduction in summer compared to the fall. We did not find any evidence for our expectation of higher pollen limitation in the future climate and fall season (i.e. no three-way interaction pollination x season x climate), which might be explained by the high drought tolerance and generalized pollination of our focal plant species. We conclude that plant reproductive success might be limited by the services of animal pollinators in future climates, and have many suggestions for future studies that are necessary to understand the context-dependence and underlying mechanisms of plant reproductive responses to climate.

Defining the components of the miRNA156-SPL-miR172 aging pathway in pea and their expression relative to changes in leaf morphology

The timing of developmental phase transitions is crucial for plant reproductive success, and two microRNAs (miRNA), miR156 and miR172, are implicated in the control of these changes, together with their respective SQUAMOSA promoter binding-like (SPL) and APETALA2 (AP2)-like targets. While their patterns of regulation have been studied in a growing range of species, to date they have not been examined in pea (Pisum sativum), an important legume crop and model species. We analysed the recently-released pea genome and defined nine miR156, 21 SPL, four miR172, and five AP2-like genes. Phylogenetic analysis of the SPL genes in pea, Medicago and Arabidopsis confirmed the eight previously defined clades, and identified a ninth potentially legume-specific SPL clade in pea and Medicago. Among the PsSPL, 14 contain a miR156 binding site and all five AP2-like transcription factors in pea include a miR172 binding site. Phylogenetic relationships, expression levels and temporal expression changes identified PsSPL2a/3a/3c/6b/9a/9b/13b/21, PsmiR156d/j and PsmiR172a/d as the most likely of these genes to participate in phase change in pea. Comparisons with leaf morphology suggests that vegetative phase change is unlikely to be definitively marked by a change in leaflet number. In addition, the timing of FT gene induction suggests that the shift from the juvenile to the adult vegetative phase may occur within fourteen days in plants grown under inductive conditions, and calls into question the contribution of miR172/AP2 to the floral transition. This work provides the first insight into the nature of vegetative phase change in pea, and an important foundation for future functional studies.

Pollination and Plant Reproductive Success of Two Ploidy Levels in Red Clover (Trifolium pratense L.)

Plant reproduction in red clover requires cross-fertilization via insect pollination. However, the influences of visitation rate and timing on maximizing ovule utilization are yet to be determined. We aimed to study the influences of visitation rate, flowering stage, and self-incompatibility on reproductive success. We applied hand and honey bee pollination in the study of eight red clover cultivars with two ploidy levels released between 1964 and 2001. In hand pollination, increasing the visitation rates (from 10 to 80 pollinated florets per flower head) increased the seed number per flower head but reduced the seed number per pollinated floret. Different flowering stages (early, middle, and full flowering) did not influence the seed number per pollinated floret significantly. There was a marked difference in reproductive success depending on the ploidy level, with 0.52 seeds per pollinated floret in diploid and 0.16 in tetraploid cultivars. During the cultivar release history, seed number per pollinated floret seemed to decrease in diploid cultivars, whereas it increased in tetraploids. In honey bee pollination, diploid cultivars had more two-seeded florets than tetraploids. Different visitation rates and the stochastic nature of pollen transfer resulted in difficulties when the plant reproductive success between hand and bee pollination was compared. A maximum of 0.27 seeds per pollinated floret were produced in hand pollination compared to the 0.34 in honey bee pollination. In spite of this, hand pollination provided a valuable method for studying the pollination biology and reproduction of red clover. Future studies may employ hand pollination to unravel further aspects of the low reproductive success with the future perspective of improving seed number per pollinated floret in tetraploid red clover.

Floral Longevity of Paphiopedilum and Cypripedium Is Associated With Floral Morphology

Floral longevity (FL) is an important trait influencing plant reproductive success by affecting the chance of insect pollination. However, it is still unclear which factors affect FL, and whether FL is evolutionarily associated with structural traits. Since construction costs and water loss by transpiration play a role in leaf longevity, we speculated that floral structures may affect the maintenance and loss of water in flowers and, therefore, FL. Here, we investigated the slipper orchid Paphiopedilum and Cypripedium, which are closely related, but strongly differ in their FL. To understand the evolutionary association of floral anatomical traits with FL, we used a phylogenetic independent comparative method to examine the relationships between 30 floral anatomical traits and FL in 18 species of Paphiopedilum and Cypripedium. Compared with Paphiopedilum species, Cypripedium species have lower values for floral traits related to drought tolerance and water retention capacity. Long FL was basically accompanied by the thicker epidermal and endodermal tissues of the floral stem, the thicker adaxial and abaxial epidermis of the flower, and low floral vein and stomatal densities. Vein density of the dorsal sepals and synsepals was negatively correlated with stomatal density. Our results supported the hypothesis that there was a correlation between FL and floral anatomical traits in slipper orchids. The ability to retain water in the flowers was associated with FL. These findings provide a new insight into the evolutionary association of floral traits with transpirational water loss for orchids under natural selection.

Increased Arctic climate extremes constrain expected higher plant reproductive success in a warmer climate

The low reproductive success of Arctic plants is predicted to increase as the climate warms. However, climate extremes add complexity to these predictions. In the extremely cold year of 2018, multiple Arctic trophic levels experienced reproductive failure. We analysed a unique long-term record of seed viability from experimentally warmed and ambient plots at Alexandra Fiord, Ellesmere Island which has been running since 1992 and included 2018 and the extremely warm year of 2019. Positive and negative July temperature anomalies and summer temperatures have increased significantly by 0.5, -0.3 and 0.4°C/decade since 1977, resulting in greater extremes and increased inter-annual variation. Seed viability of some species has increased with climate warming. Across years, seed viability of woody species was consistently higher in warmed than ambient plots while the opposite was true for forbs. In 2018, seed viability of woody species in ambient plots was significantly lower than normal but comparable with past years for forb species. Not all species returned to normal seed viability levels in 2019. Our study highlights the potential for greater sexual reproductive failure in tundra plants with increasing climate extremes. We suggest that poor seed viability of woody species in cold years could constrain shrub recruitment and may aid forb species establishment on bare tundra.