Mutualism between plants and pollinators is commonly seen as a beautiful, beneficial relationship between two living organisms. However, let’s be clear, plants only want their flowers to be pollinated and pollinators are just interested in collecting resources, such as nectar, pollen and fragrances. This has developed into an evolutionary arms race between plants and pollinators, where each seeks to put its needs above those of the other and obtain the maximum possible benefit. As a result, plants have developed numerous life strategies and trait combinations to improve their fitness, including flowers.
During their flowering period, plants display a wide diversity of reproductive structures, determining how and with which pollinators they will interact. One would expect that with all the variety of traits available, evolution will lead plants to create a flower that could interact with all possible pollinators. However, there are morphological and physiological restrictions created by evolutionary and ecological processes that prevent such a thing from existing.
For instance, certain plants possess elongated tubular flowers that attract long-beaked hummingbirds, while flowers with narrow openings limit visits to insects with compatible proboscises, exemplifying morphological limitations. Additionally, some plants regulate the availability of nectar or pollen, either by offering limited quantities or releasing them at specific times, illustrating regulatory constraints. These sets of morphological and physiological traits, which allow or prevent the interaction between plants and specific pollinators, are the basis what are known as “pollination syndromes”.
Pollination syndromes are evident across various pollinator groups, including insects, birds, bats, and wind. They highlight the diverse evolutionary strategies plants employ to ensure successful reproduction through targeted interactions with specific pollinators. However, while extensive research has explored the relationships and constraints within vegetative organs like leaves and wood, our understanding of these associations in reproductive organs, particularly flowers, remains limited. Moreover, there is a shortage of knowledge regarding the combined effects of reproductive traits on plant reproduction.
Such gaps have attracted the interest of different researchers, including Dr José B. Lanuza (@barragan_lanuza on Twitter). Lanuza is a plant specialist who works with plant-pollinator interactions. He is particularly interested in plants and their reproductive strategies. His interest in plants and their reproductive strategies emerged during his second year of graduation in Biology when he participated in research on heterostyly, a phenomenon where the same plant species has flowers with pistils and stamens of different lengths. Since then, he has continued to research plant reproductive strategies, focusing on those that promote cross-pollination.
In an interview with Botany One, Lanuza told us why we know more about vegetative traits than reproductive traits He argues that vegetative traits are easier to measure, mainly because most plant species only produce flowers and fruits during certain times of the year. Additionally, reproductive traits can change within the same flower over time, with organs moving into different positions as they grow and mature. This means we often only get a snapshot that may not fully represent reality. If that wasn’t problematic enough, researchers still have to deal with the immense variety of plant species and the diversity of floral structures, which makes it difficult to obtain knowledge about reproductive traits, especially since each species has different life histories and reproductive mechanisms.
Aware of the lack of information on plant reproductive traits and our knowledge of how they shape interactions with pollinators, Lanuza and his collaborators conducted a study to identify the main reproductive traits of plants and to evaluate how the covariation of these traits influences the networks of plant-pollinator interactions on a global scale . When asked how the idea for this study came about and what was the biggest challenge in carrying it out, Lanuza replied that the idea for this study came up during his PhD.
Initially, the study aimed to assess how specialisation –the adaptation of plants and pollinators through specific physical and behavioural traits to work together efficiently and ensure pollination– varied along a latitudinal gradient. Still, the author questioned why not include plant traits, as these traits could explain possible differences in interactions. To do so, Lanuza and his team had to deal with a tremendous challenge: data quality, as much less information exists on reproductive traits than vegetative traits. In addition, species traits may vary according to the region, so we should not use data from regions that are very different from ours, as this may not reflect the reality of the study site.
Dealing with missing data, standardising them and ensuring that they reflect the reality of interaction networks were some of the issues the author had to resolve when analysing each line of his final database. In the end, they were able to compile and analyse data from 28 studies carried out in 18 different countries for a total of 64 plant-pollinator interaction networks.
The researchers observed a correlation between the number of flowers and several other plant characteristics. On the one hand, taller plants tended to produce more and smaller flowers with relatively fewer ovules and shorter styles than smaller, herbaceous species. For example, the flowering dogwood (Cornus florida) is a 7.5-metre-high shrub that produces about 10,000 flowers, each about 3 mm wide with 3.5 mm styles and two ovules. In contrast, the wild white petunia (Petunia axillaris), which is a herb with an average height of 0.5 m, produces approximately ten flowers per plant, each over 50 mm wide, featuring styles that extend up to 25 mm in length and more than 200 ovules per flower.
The researchers also noted that species with low self-pollination rates –meaning they receive more pollen from other plants than from their own flowers– tend to have more and larger flowers with long styles than those with high self-pollination rates. For example, Zuccagnia punctata is a self-incompatible species that relies entirely on animal pollination for seed production. It produces approximately 1500 flowers per plant and has 20 mm long styles. Contrastingly, the American speedwell (Veronica peregrina) is a self-compatible plant that requires minimal or no animal pollination. It has approximately 20 flowers per plant, reaches a height of 0.2 m, and possesses styles that are 0.25 mm long. Altogether, these observations highlight the intricate relationships between flower quantity, size, ovule count, style length, and the pollination mechanisms employed by different plant species.
Another intriguing finding from the study was the association between the number of flowers and the types of pollinators involved. Plants with a high flower count tended to interact more with beetles, non-bee Hymenoptera, and flies, while plants with fewer flowers had a higher frequency of interactions with bees and butterflies. These results illustrate the evolutionary strategies plants employ regarding using resources for building flowers and attracting specific pollinators. The findings also suggest that bees exhibit a stronger preference for plants with increased levels of autonomous selfing and larger styles, which can be attributed to the potential for more efficient pollen transfer and a reduced risk of pollen competition.
The intricate interplay between plant reproductive strategies and the preferences of specific pollinator groups shown by this research highlights the complex dynamics of plant-pollinator interactions and contributes to our understanding of the mechanisms that drive the evolution of floral traits and the coevolution between plants and their pollinators.
When asked about other essential messages in addition to those in his study, Lanuza points out that pollination syndromes are abstract concepts that should be used with caution, especially since plant-pollinator interactions can vary depending on the context. For example, red flowers in the tropical region are generally pollinated by birds, while in non-tropical areas, they are pollinated by bees. It is important to emphasise that other factors are involved in this relationship, but floral syndromes must be interpreted with care, as they have complex dimensions.
Lastly, Lanuza believes that, before answering new questions, it is essential to collect more information about the reproductive traits of plants in different locations, that is, to carry out more field studies. Significant emphasis should be placed on characteristics such as colour, phenology, and floral resources, as they have often been overlooked, treated as categorical variables, and, critically, can vary across different locations. By doing so, we will be contributing to obtaining more accurate and higher-quality data on the reproductive traits of plants.
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Lanuza, J. B., Rader, R., Stavert, J., Kendall, L. K., Saunders, M. E., & Bartomeus, I. (2023) Covariation among reproductive traits in flowering plants shapes their interactions with pollinators. Functional Ecology. https://doi.org/10.1111/1365-2435.14340
Victor H. D. Silva is a biologist passionate about the processes that shape interactions between plants and pollinators. He is currently focused on understanding how plant-pollinator interactions are influenced by urbanisation and how to make urban green areas more pollinator-friendly. For more information, follow him on Twitter: @another_VDuarte.