Pollination by sexual deception promotes outcrossing and mate diversity in self-compatible clonal orchids

Effective population size in a partially clonal plant is not predicted by the number of genetic individuals

Estimating effective population size (N _e) is important for theoretical and practical applications in evolutionary biology and conservation. Nevertheless, estimates of N _e in organisms with complex life-history traits remain scarce because of the challenges associated with estimation methods. Partially clonal plants capable of both vegetative (clonal) growth and sexual reproduction are a common group of organisms for which the discrepancy between the apparent number of individuals (ramets) and the number of genetic individuals (genets) can be striking, and it is unclear how this discrepancy relates to N _e. In this study, we analysed two populations of the orchid Cypripedium calceolus to understand how the rate of clonal versus sexual reproduction affected N _e. We genotyped >1000 ramets at microsatellite and SNP loci, and estimated contemporary N _e with the linkage disequilibrium method, starting from the theoretical expectation that variance in reproductive success among individuals caused by clonal reproduction and by constraints on sexual reproduction would lower N _e. We considered factors potentially affecting our estimates, including different marker types and sampling strategies, and the influence of pseudoreplication in genomic data sets on N _e confidence intervals. The magnitude of N _e/N _ramets and N _e/N _genets ratios we provide may be used as reference points for other species with similar life-history traits. Our findings demonstrate that N _e in partially clonal plants cannot be predicted based on the number of genets generated by sexual reproduction, because demographic changes over time can strongly influence N _e. This is especially relevant in species of conservation concern in which population declines may not be detected by only ascertaining the number of genets.

An analytical pipeline to support robust research on the ecology, evolution, and function of floral volatiles

Research on floral volatiles has grown substantially in the last 20 years, which has generated insights into their diversity and prevalence. These studies have paved the way for new research that explores the evolutionary origins and ecological consequences of different types of variation in floral scent, including community-level, functional, and environmentally induced variation. However, to address these types of questions, novel approaches are needed that can handle large sample sizes, provide quality control measures, and make volatile research more transparent and accessible, particularly for scientists without prior experience in this field. Drawing upon a literature review and our own experiences, we present a set of best practices for next-generation research in floral scent. We outline methods for data collection (experimental designs, methods for conducting field collections, analytical chemistry, compound identification) and data analysis (statistical analysis, database integration) that will facilitate the generation and interpretation of quality data. For the intermediate step of data processing, we created the R package bouquet, which provides a data analysis pipeline. The package contains functions that enable users to convert chromatographic peak integrations to a filtered data table that can be used in subsequent statistical analyses. This package includes default settings for filtering out non-floral compounds, including background contamination, based on our best-practice guidelines, but functions and workflows can be easily customized as necessary. Next-generation research into the ecology and evolution of floral scent has the potential to generate broadly relevant insights into how complex traits evolve, their genomic architecture, and their consequences for ecological interactions. In order to fulfill this potential, the methodology of floral scent studies needs to become more transparent and reproducible. By outlining best practices throughout the lifecycle of a project, from experimental design to statistical analysis, and providing an R package that standardizes the data processing pipeline, we provide a resource for new and seasoned researchers in this field and in adjacent fields, where high-throughput and multi-dimensional datasets are common.

Orchid conservation: from theory to practice

BACKGROUND

Given the exceptional diversity of orchids (26 000+ species), improving strategies for the conservation of orchids will benefit a vast number of taxa. Furthermore, with rapidly increasing numbers of endangered orchids and low success rates in orchid conservation translocation programmes worldwide, it is evident that our progress in understanding the biology of orchids is not yet translating into widespread effective conservation.

SCOPE

We highlight unusual aspects of the reproductive biology of orchids that can have important consequences for conservation programmes, such as specialization of pollination systems, low fruit set but high seed production, and the potential for long-distance seed dispersal. Further, we discuss the importance of their reliance on mycorrhizal fungi for germination, including quantifying the incidence of specialized versus generalized mycorrhizal associations in orchids. In light of leading conservation theory and the biology of orchids, we provide recommendations for improving population management and translocation programmes.

CONCLUSIONS

Major gains in orchid conservation can be achieved by incorporating knowledge of ecological interactions, for both generalist and specialist species. For example, habitat management can be tailored to maintain pollinator populations and conservation translocation sites selected based on confirmed availability of pollinators. Similarly, use of efficacious mycorrhizal fungi in propagation will increase the value of ex situ collections and likely increase the success of conservation translocations. Given the low genetic differentiation between populations of many orchids, experimental genetic mixing is an option to increase fitness of small populations, although caution is needed where cytotypes or floral ecotypes are present. Combining demographic data and field experiments will provide knowledge to enhance management and translocation success. Finally, high per-fruit fecundity means that orchids offer powerful but overlooked opportunities to propagate plants for experiments aimed at improving conservation outcomes. Given the predictions of ongoing environmental change, experimental approaches also offer effective ways to build more resilient populations.

Pollen-mediated gene flow ensures connectivity among spatially discrete sub-populations of Phalaenopsis pulcherrima, a tropical food-deceptive orchid

BACKGROUND

Gene flow in plants via pollen and seeds is asymmetrical at different geographic scales. Orchid seeds are adapted to long-distance wind dispersal but pollinium transfer is often influenced by pollinator behavior. We combined field studies with an analysis of genetic diversity among 155 physically mapped adults and 1105 F1 seedlings to evaluate the relative contribution of pollen and seed dispersal to overall gene flow among three sub-populations of the food-deceptive orchid Phalaenopsis pulcherrima on Hainan Island, China.

RESULTS

Phalaenopsis pulcherrima is self-sterile and predominantly outcrossing, resulting in high population-level genetic diversity, but plants are clumped and exhibit fine-scale genetic structuring. Even so, we detected low differentiation among sub-populations, with polynomial regression analysis suggesting gene flow via seed to be more restricted than that via pollen. Paternity analysis confirmed capsules of P. pulcherrima to each be sired by a single pollen donor, probably in part facilitated by post-pollination stigma obfuscation, with a mean pollen flow distance of 272.7 m. Despite limited sampling, we detected no loss of genetic diversity from one generation to the next.

CONCLUSIONS

Outcrossing mediated by deceptive pollination and self-sterility promote high genetic diversity in P. pulcherrima. Long-range pollinia transfer ensures connectivity among sub-populations, offsetting the risk of genetic erosion at local scales.

Experimental examination of pollinator-mediated selection in a sexually deceptive orchid

BACKGROUND AND AIMS

Selection exerted by pollinators on flowers is predicted to occur along two distinct axes. While pollinator attraction to flowers is governed by pollinator preferences, pollen transfer efficiency is mediated by the mechanical fit of pollinators to flower morphology. Although pollinator attraction in sexually deceptive orchids is typically underpinned by floral odour, morphological traits are expected to play a vital role in mechanical fit during floral contact with pollinators.

METHODS

Here we utilize a comprehensive and novel procedure to test for pollinator-mediated selection through mechanical fit with the flower labellum in the orchid Chiloglottis trapeziformis. This approach combines detailed pollinator observations related to plant reproductive fitness with complementary experimental manipulation and phenotypic selection analysis.

KEY RESULTS

Experiments with virgin flowers revealed that pollen removal occurs only during vigorous pseudocopulation. This behaviour involves male wasps that grasp the insectiform callus structure on the labellum while probing the labellum tip in a forward orientation. Both orientation and duration of pseudocopulation were significant predictors of pollen removal, confirming a direct relationship between pollinator behaviour and plant fitness. Controlled floral manipulation that either shortened or elongated the distance between the callus and the labellum tip detected no change in pollinator attraction. The duration of pseudocopulation, however, was significantly reduced on flowers with shortened or elongated callus-tip distances, consistent with stabilizing selection. Phenotypic selection analysis confirmed this prediction in natural populations by uncovering evidence for stabilizing selection on the distance between the callus and the labellum tip.

CONCLUSIONS

Our experimental manipulations and selection analysis in natural populations thus demonstrate stabilizing selection on the distance from the callus to the labellum tip, and illustrate the utility of employing multiple approaches to confirm selection exerted by pollinators on floral form.

Mating consequences of contrasting hermaphroditic plant sexual systems

For hermaphroditic angiosperms with multiple flowers, the sex roles can be exclusively combined in bisexual flowers (monocliny), strictly separated among different flowers (monoecy), or arrayed in mixtures of bisexual flowers with female flowers (gynomonoecy) or male flowers (andromonoecy). The hypothesized benefits favoring the evolution of these contrasting hermaphroditic sexual systems are typically examined individually, usually by assessing success through only one sex role. We tested predictions of most hypotheses experimentally with an andromonoecious species, Anticlea occidentalis (Melanthiaceae), based on the performance of intact plants (andromonoecy) and those with emasculated bisexual flowers (functionally monoecious) or emasculated male flowers (functionally monoclinous with sterile peripheral flowers). Andromonoecy in this species enables efficient, size-dependent resource allocation, emphasizing female function in large plants. Emasculation revealed that anthers in male flowers promote female mating quality (outcrossing rate and mate diversity), whereas anthers in bisexual flowers promote male mating quantity (pollen dispersal distance and probability of any siring success). Thus, different hermaphroditic sexual systems likely evolve to capitalize on suites of benefits, rather than just one, and provide compromises between quantitative and qualitative reproductive components. These compromises apparently maximize an individual's combined genetic contributions through female and male functions, rather than separate contributions through each sex role.

Isotropic and anisotropic processes influence fine-scale spatial genetic structure of a keystone tropical plant

Limited seed or pollen dispersal enhances spatial genetic relatedness between individuals (fine-scale spatial genetic structure, FSGS), which usually decreases as a function of physical distance. However, such isotropic pattern of FSGS may not always occur when spatially asymmetric processes, for instance, wind direction during dispersal, are considered in wind-pollinated and -dispersed plants. This study assessed the pattern of FSGS in the keystone tropical wetland plant Cyperus papyrus (papyrus) as a function of these isotropic and anisotropic processes. We tested the hypothesis that the FSGS would be influenced by predominant wind direction during pollen and seed dispersal, as well as by the physical distance between individuals. We genotyped a total of 510 adults and 407 juveniles from three papyrus swamps (Ethiopia) using 15 microsatellite markers. In addition, the contemporary directional dispersal by wind was evaluated by seed release-recapture experiments and complemented with parentage analysis. Adults and juveniles differed in the strength of isotropic FSGS ranging from 0.09 to 0.13 and 0.12 to 0.16, respectively, and this suggests variation in dispersal distance. Anisotropic FSGS was found to be a function of asymmetric wind direction during dispersal/pollination that varied between sites. Historical gene dispersal distance was astoundingly low (<4 m), possibly due to localized seed rain. According to our contemporary dispersal estimates, mean pollen dispersal distances were longer than those of seed dispersal (101 and <55 m, respectively). More than two-thirds of seeds and half of pollen grains were locally dispersed (≤80 m). The difference in historical and contemporary dispersal distance probably resulted from the asymmetric wind direction due to change in vegetation cover in the surrounding matrix. We further concluded that, in addition to wind direction, post-dispersal processes could influence gene dispersal distance inferred from the FSGS.

Euglossine bees mediate only limited long-distance gene flow in a tropical vine

Euglossine bees (Apidae: Euglossini) have long been hypothesized to act as long-distance pollinators of many low-density tropical plants. We tested this hypothesis by the analysis of gene flow and genetic structure within and among populations of the euglossine bee-pollinated vine Dalechampia scandens. Using microsatellite markers, we assessed historical gene flow by the quantification of regional-scale genetic structure and isolation by distance among 18 populations, and contemporary gene flow by the estimation of recent migration rates among populations. To assess bee-mediated pollen dispersal on a smaller scale, we conducted paternity analyses within a focal population, and quantified within-population spatial genetic structure in four populations. Gene flow was limited to certain nearby populations within continuous forest blocks, whereas drift appeared to dominate on larger scales. Limited long-distance gene flow was supported by within-population patterns; gene flow was biased towards nearby plants, and significant small-scale spatial genetic structure was detected within populations. These findings suggest that, although female euglossine bees might be effective at moving pollen within populations, and perhaps within forest blocks, their contribution to gene flow on the regional scale seems too limited to counteract genetic drift in patchily distributed tropical plants. Among-population gene flow might have been reduced following habitat fragmentation.

Genetic variation and selfing rate in Lychnis flos-cuculi along an industrial pollution gradient

We studied nine populations of a meadow mixed-mating plant Lychnis flos-cuculi growing in a gradient of copper smelter emissions. We hypothesize that metal tolerant populations in the polluted areas have experienced a loss of genetic variation and are more selfing than the populations from the unpolluted areas. One hundred and thirty-five parental plants and 1059 offspring were genotyped with six microsatellite markers. Selfing rates were assessed manually, with Rmes, Mltr and Colony2. Soil toxicity, population density and pollinators' activity were estimated in the studied areas. Populations from the heavily polluted area have experienced a strong founder effect. However, at present, they are characterized by high densities. A recent genetic explosion was registered for the population from the most polluted site, probably due to forest thinning under pollution effects. Selfing rates estimated with different approaches agreed well only for populations with high genetic variation; they comprised 0-0.23 and were similar between polluted and clean areas. Self-fertilization in L. flos-cuculi hardly represents a mechanism for the fixation of advantageous alleles and a barrier for gene flow from non-tolerant populations. The employment of different methods of selfing rate estimation in populations with low genetic variation appears to be necessary, though not a guarantee of reliable conclusions.

Functional Significance of Labellum Pattern Variation in a Sexually Deceptive Orchid (Ophrys heldreichii): Evidence of Individual Signature Learning Effects

Mimicking female insects to attract male pollinators is an important strategy in sexually deceptive orchids of the genus Ophrys, and some species possess flowers with conspicuous labellum patterns. The function of the variation of the patterns remains unresolved, with suggestions that these enhance pollinator communication. We investigated the possible function of the labellum pattern in Ophrys heldreichii, an orchid species in which the conspicuous and complex labellum pattern contrasts with a dark background. The orchid is pollinated exclusively by males of the solitary bee, Eucera berlandi. Comparisons of labellum patterns revealed that patterns within inflorescences are more similar than those of other conspecific plants. Field observations showed that the males approach at a great speed and directly land on flowers, but after an unsuccessful copulation attempt, bees hover close and visually scan the labellum pattern for up to a minute. Learning experiments conducted with honeybees as an accessible model of bee vision demonstrated that labellum patterns of different plants can be reliably learnt; in contrast, patterns of flowers from the same inflorescence could not be discriminated. These results support the hypothesis that variable labellum patterns in O. heldreichii are involved in flower-pollinator communication which would likely help these plants to avoid geitonogamy.