Field Experiments of Pollination Ecology: The Case of Lycoris sanguinea var. sanguinea

Summary

To reveal the pollinator effectiveness of a given plant species, multiple methods of field experiments have been developed. This study demonstrates the basic methods of field experiments for pollination ecology using the case study of Lycoris sanguinea var. sanguinea and the novel pollination mechanism, breaking-bud pollination.

Abstract

Plant-pollinator interactions have been studied for approximately one hundred years. During that time, many field methods have been developed to clarify the pollination effectiveness of each pollinator for visited flowers. Pollinator observations have been one of the most common methods to identify pollinators, and bagging and cage experiments have been conducted to show the effectiveness of specific pollinators. In a previous study of Lycoris sanguinea var. sanguinea, its effective pollinators, the visitation frequencies of each floral visitor, and its reproductive strategies were not identified. This study reports the observation that small bees visited flowers that were partially opened (breaking buds). To the best of our knowledge, this phenomenon has not been reported previously. Further, this study investigates the hypothesis that small bees can pollinate at that flowering stage. This study demonstrates the basic methods of field experiments in pollination ecology using L. sanguinea var. sanguinea. Pollinator observations and digital video showed the visitation frequencies of each floral visitor. Bagging and cage experiments revealed that these flowers could be pollinated fully and that breaking-bud pollination could be important for the pollination of this plant species. The advantages and disadvantages of each method are discussed, and recent developments, including laboratory experiments, are described.

Introduction

Plant-pollinator interactions are prime examples for the study of evolutionary biology and ecology. The mutualistic relationships between flowers and pollinators are thought to have promoted the diversification of angiosperm^1,2 as a result of natural selection, although other biotic and abiotic factors have also exerted influence^3,4,5. It is also thought that floral traits have changed to adapt to the most effective pollinators and to produce more fruit and seeds⁶. These beliefs have been constructed though large studies based on different indices, such as pollination effectiveness, that involve various interpretations⁷.

Flowering plants that have generalized pollination systems are visited by various types of pollinators⁸. Herein, a flower visitor was defined as an animal species that visited to get a floral reward, and pollinators were defined as floral visitors that pollinate. Some of these visitors carry conspecific pollen grains to the stigmata of the flowers visited and can be classified as pollinators. Other visitors may also have some intraspecific pollen; they might conduct less pollination due to behavioral or morphological mismatches between the pollinators and the flowers. These comparable differences in the contribution to plant reproduction could produce varying degrees of selective pressure on floral traits⁹ and could cause the adaptive divergence of flowering plants. Therefore, although the composition of the pollinator community and the relative species abundance are important¹⁰, the accurate evaluation of each visitor's effectiveness is also critical to determine the adaptive and/or evolutionary processes of the plants.

In this study, quantitative estimations of pollinator efficiencies, defined as the fruit and seed production per visitation frequency, were determined¹¹. The species and frequency of each floral visitor were observed, and reproductive effects on the visited flowers were estimated. The recording of floral visits through human observations is a classical method in pollination ecology. However, this method imposes a large burden on observers, who are required to remain in front of the plants and to take careful, long-term measurements. Recently, the technologies of filming and recording have rapidly developed, and low-cost digital video cameras have enabled the introduction of video recording to pollinator observations^12,13. These methods can facilitate the gathering of basic information on floral visitors and could help to develop an understanding of the pollination ecology of a target plant species.

However, the visitation frequencies of the pollinators are not necessarily correlated to their pollination effectiveness^7,14, and it is important to evaluate the qualitative effects of each pollinator on flower fitness. Pollination effectiveness has been estimated through the number of pollen grains on the stigmata^15,16, pollen tube growth^17,18and fruit and/or seed production^19,20. Bagging experiments, conducted using visitor-exclusive bags, are the typical methods for testing self-compatibility, autogamy^21,22, and the presence of apomixes²³. Additionally, the evaluation of pollination effectiveness for a certain pollinator in the visitor assemblage has been frequently conducted in environments where other floral visitors have been restricted (i.e., a wire cage, net, or bag with a mesh small enough to exclude larger pollinators that is set on flowering plants). For example, bagging experiments with small mesh bags were conducted to reveal the pollination ability of ants or thrips^24,25. Moreover, bird exclusion experiments using a wire cage or net have shown the effective pollinators of the Aloe taxa^26-28.

The objectives of this study were: 1) to introduce the methods used in a previous paper and 2) to improve these methods for general use in other studies on floral visitors, their visiting frequencies, and their effects on plant fitness. Lycoris sanguinea var. sanguinea is one of the species included in the genus Lycoris, which is distributed broadly throughout Japan and narrowly in Korea²⁹and has funnel-shaped reddish-orange flowers (Figure 1a). A previous study revealed that L. sanguinea var. sanguinea was visited by multiple insect species, including an unidentified small bee species and the larger species Amegilla florea²⁹. However, the visitation frequencies and pollination effectiveness of these visitors were not identified. Pollinator observations for the identification of floral visitors were performed first. Visitation by small bees was observed on flowers that had not completely opened yet (breaking buds; Figures 1b, c). Small bees moved hurriedly around the undehisced anthers in the breaking buds and collected pollen using their mandibles. The hypothesis was that the small bees could be pollinators at the breaking-bud stage because the gaps between the anthers and the stigmata in the flowers were smaller than the body length of the bees. Therefore, bagging experiments were conducted to test the pollination ability of small bees at the breaking-bud stage, and additionally to examine the reproductive strategies of L. sanguinea var. sanguinea. These buds were bagged after the small bees visited, which allowed an estimation of the pollination ability of the bees. The individuals were caged with unopened buds. A small-mesh cage was used, through which only small bees could pass, allowing an estimation of the pollination efficiencies of small bees throughout the entire flowering stage.

Protocol

NOTE: This article is based on our previous work³⁰. Some parts are reprinted with permission from The Botanical Society of Japan and Springer Japan.

1 . Observation of Floral Visitors

1. Selection of the Observation Fields
   1. Search the areas where the plant materials are distributed, and select the candidate study sites using reliable resources, such as picture books, academic journals, etc. Adjust the number of study sites to fit the research objectives (e.g., a wide range of locations for the comparison of flower visitors throughout Japan).
   2. Check the locations of selected candidate populations and the distances from relevant starting points, such as research institutes and accommodations, if long-term research is needed.
   3. Roughly estimate each population size by counting the number of plant individuals in a unit area. Select large populations for manipulated experiments using many plant individuals.
   4. Pre-observation of floral visitors
      1. Decide which target individuals in each area will be observed during the same time interval, depending on the population densities. Select 5-10 freshly-opened flowers or breaking buds as target flowers, and reject degraded ones that have already opened.
      2. Start the visual observation for the period of an entire day, because the flowers could be visited by various pollinators in different time ranges^31,32.
      3. On the recording sheets, record the species name of floral visitors and the time of each visitation per hr per flower.
      4. Closely observe whether floral visitors touch the anthers and/or stigmata; if they do, record the visitors as pollinators. If the floral visitors only visit the flowers and do not touch the reproductive organs, record them as visitors.
      5. Using an insect net or hand-made aspirator, capture the floral visitors to identify and preserve them as specimens (Figure 1f). Kill the trapped visitors quickly with ethyl acetate or finger pressure to the thorax, and preserve them in a soft bag or in a plastic tube or case with 100% ethanol.
      6. After the observation, identify the specific names of specimens using morphological features or, in case of difficulties in identification, request the help of experts for each taxonomic group.
2. Observation in the Field
   1. Select research sites based on their accessibility, population size, and number of floral visitors. Choose time periods that include a representative sample of floral visitors in the flowering season, and determine observation periods based on research objectives (e.g., for research on visitation-frequency fluctuations by floral visitors, long time periods should be set).
   2. Prepare the appropriate recording equipment, such as digital video cameras (e.g., for night recording, video cameras with an infrared function should be used).
   3. Select the target flowers in the same way as for 1.1.4.
   4. Fix the video cameras to aluminum tripods. Set the cameras with tripods in front of the target individuals, approximately 50 cm away.
   5. Check and modify the quantity of light and the focus of the materials on the screen displays of the cameras before filming.
   6. Start the visual observation and video filming at the same time for the appropriate time zones estimated by the pre-observations (in the present case, observation periods were for more than six hours on average, from approximately 05:00 am to 13:00 pm).
   7. Identify the species names of the visitors and pollinators, and record their names and the times of their visits on the recording sheets.
   8. Capture and identify the visitors and pollinators in the same way as in 1.1.4.5.
   9. Repeat Steps 1.2.1 to 1.2.7 for each observation period. Set the observation periods based on the research objectives and pre-observation data.
3. Data Analyses After Observation
   1. Identify the species names of captured specimens after pollinator observation in the same way as in 1.1.4.6.
   2. Check the video clips and note the species names and their visit times in the same way as for the visual observation.
   3. Calculate the visitation frequencies per flower per hour of each floral visitor from the visual observation and video recording data. Compare these frequencies statistically between the sites and years of each visitor using appropriate statistical methods based on fundamental statistics and using suitable software, such as R, SPSS, and/or SAS^33-35 (e.g., a two-way analysis of variance (ANOVA) with Tukey's honest significant difference (HSD) test in R software).

2 . Bagging and Cage Experiments

1. Preparations for the Bagging and Cage Experiments
   1. For the bagging experiments, prepare bags that have small mesh sizes (~0.5-1 mm) and use them to completely prevent flower visitation (e.g., non-woven fabric bags were used for the bagging experiments in 2011 and 2012).
   2. For the cage experiments, prepare wire or plastic mesh boards with a diameter that allows the target smaller visitors to pass through but excludes the larger ones, and connect these mesh boards to form a cage. Ensure that there are no gaps larger than the mesh diameter. Adjust the size and shape of cages based on the target plant species and individual numbers.
2. Bagging Experiments
   1. Select 30 individuals per treatment that have no damage from herbivores or from a severe environment. For treatment, choose a single flower from each individual plant, or use individual flowers on a single plant.
   2. Bag the target flowers in the field after labeling them with tape (serial number and alphabet; Figure 1g). Be careful not to touch the anthers or the stigmata within the bags to avoid the possibility of self-pollination.
   3. Carefully fix bags to the flowers using a soft string or wire (Figure 1d).
   4. Carefully set individual flowers upright with supports using soft string or wire to support the individuals against tilt or collapse under the weight of the bags or the wind, as necessary. Twist or wind a string or wire softly around the stalk of a target individual, so as not to create damage.
   5. Treatments in the bagging experiments
      1. For the "Control" treatment, attach the labels to the target flowers and conduct no treatment. Allow floral visitors to freely visit.
      2. For the "Outcrossing" treatment, cover the buds until they flower, and then remove the anthers of the bagged flowers. Put some pollen grains from multiple individuals on the stigmata.
      3. For the "Selfing" treatment, cover the buds until the flowers open, and put pollen from the same flowers onto their stigmata. Cover these treated flowers again.
      4. For the "Auto-self" treatment, cover the buds with bags until the end of the flowering season.
      5. For the "Breaking bud" treatment, identify the breaking buds that have been visited and observe the entrance or exit of small bees (Figures 1b, c). Remove the anthers of the breaking buds to prevent repeated pollen depositions on the same flower after visitation by small bees, and bag these buds quickly to prevent further visitations.
      6. For the "Flowering" treatment, bag the buds until the opening stage to avoid visitations at the breaking-bud stage. After that, remove the bags and allow the visitors to collect nectar and pollen.
      7. For the "Bud" treatment, remove the anthers from unopened buds and outcross them artificially. Bag these buds quickly to prevent flower visitations.
3. Cage Experiments
   1. Cover the target plant individuals with prepared cages. Adjust the positions of caged flowers by hand to prevent contact and pollen depositions between different stigmata (Figure 1e).
   2. For the "Cage" treatment, select the individuals with unopened buds and put labels on them for the identification of selected flowers. Cage the individuals with labeled buds to reject the influences from before the establishment of cages (i.e., floral insects cannot visit unopened buds, and only the effects of insects that visit after the placement of cages can be estimated).
   3. Attach the cages firmly to the ground using iron poles to prevent entry by visitors between the cage base and the ground.
4. Data Analyses After the Bagging and Cage Experiments
   1. Collect all of the labeled flowers at the end of the flowering season (Figure 1g) by cutting and separating them from the maternal individuals. Preserve each labeled sample separately to prevent their contamination.
   2. Check for the presence or absence of fruit set on each labeled flower.Record the seed numbers of each fruit in case of setting fruit.
   3. Calculate the ratio of fruit numbers per flower (fruit-set ratio, defined as the number of fruits divided by the number of flowers) and of seed numbers per mature fruit (seed-set ratio, defined as the number of seeds divided by the number of ovules) using all of the recorded numbers.
   4. Statistically compare the fruit- and seed-set ratios between the treatments using appropriate methods and software, such as those listed in 1.3.3 (e.g., one-way ANOVA with Tukey's HSD or Fisher's exact test in R software³³).
   5. Investigations of the materials using the results of the bagging experiments
      1. To test the need for animal pollinators, statistically compare the results between "Control" and "Auto-self" treatments.
      2. To estimate the degree of pollen limitation, compare the "Control" and "Outcrossing" treatments.
      3. To test self-compatibility, compare the "Outcrossing" and "Selfing" treatments.
   6. Evaluations of the influences of breaking-bud pollination
      1. Compare the "Control" and "Bud" treatments to determine whether the stigmata of the breaking buds are reproductively mature and whether the value of the "Control" treatment can be used for the control at the breaking-bud stage.
      2. Compare the "Auto-self" and "Breaking bud" treatments to determine whether the reproductive success of flowers pollinated with small bees at the breaking-bud stage is statistically higher than that of plants performing autogamy (i.e., to test the existence of breaking-bud pollination). After that, compare these two treatments with the "Control" to estimate the pollination efficiencies of breaking-bud pollination.
      3. Compare the "Breaking bud," "Cage," and "Flowering" treatments to estimate the reproductive influences of breaking-bud pollination.

Results

Five populations were selected for pollinator observations. In the pre-observation phase, visitations of various insect species to opening flowers and small bees to breaking buds were confirmed. Floral visitor observations revealed that most of the visitors to all five study sites were individuals of the small bee species Lasioglossum japonicum. The total visitation record showed that the visitation ratios of this species were more than 90% at three sites (Figure 2

Discussion

Flower observations and bagging experiments were employed in this study to reveal the visitation frequencies and the female reproductive success of plants, respectively. In Dafni (1992)³⁸, the videotape method was effective because it could record the timing and duration of visitors for analysis and prevent observer bias. However, at the time, this method required expensive equipment, and the observation times were limited by battery life. Recently, the cost of equipment for producing video records has decline...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
recording sheet	any	NA
insect net	any	NA
pooter	any	NA
ethyl acetate	any	NA
100% Ethanol	any	NA
plastic tube	any	NA
plastic case	any	NA
soft bag	any	NA
digital video camera(s)	any	NA
tripod(s)	any	NA
bags	any	NA
wire or plastic mesh boards	any	NA
iron wires	any	NA
labeling tape	any	NA
stick supporters	any	NA
soft strings or wire	any	NA
pincette(s)	any	NA