Victor H. D. SilvaPollination is a vital process for plant reproduction. It can occur in various ways, with, for example, insects, birds, or even the wind carrying pollen from one flower to another. When we think of pollinators, insects -and especially bees- usually, come to mind as the most important ones. However, animals, like birds and mammals, are also key to this process, particularly in tropical regions of the world.
All this information has been gathered over years of research, but two questions that still remain are: “How were flowers pollinated millions of years ago? And how did this change over time?” In a recent study published in the New Phytologist journal, Ruby E. Stephens and her collaborators investigated the main pollination types in the evolutionary history of flowering plants.
Insects were likely the first animals to assist in the pollination of plants. To understand why, let’s delve into the pollination process: Insects gather pollen from one flower and transfer it to another flower of the same species but a different individual. This is known as cross-pollination and is more efficient than wind pollination, resulting in less pollen wastage.
In an interview with Botany One, Stephens explained, “Insects make great pollinators, because, in contrast to bats, birds, and other vertebrates that also act as pollinators, insects possess specific traits that make them particularly suitable for this role. They are small, have more simple nutritional requirements, and don’t need abundant floral rewards –like nectar or oils– to be attracted. Additionally, the diversity and abundance of insects across various environments and their adaptability establishes them as key pollinators in a symbiotic relationship with flowers. This reinforces the notion that insects have played a crucial role in pollination over time”. Consequently, in Stephens’ own words, “insect pollination has been hugely advantageous for angiosperms”, shaping the interactions between these two groups of organisms throughout evolution.
In this study, the authors explored whether insect pollination is indeed the ancestral mode. In other words, they investigated whether the insect’s pollination is truly the oldest, original way. Additionally, they examined how temporal and environmental factors have shaped changes in pollination processes over time. To do this, they not only looked at past studies on plant evolution and pollination methods but also considered the entire evolutionary history of flowering plants. Understanding how this occurred in the past and what led to these changes in pollination modes could provide us with crucial information for protecting different types of plants and their habitats.
Stephens and her team selected 1,201 plant species and assigned a pollination type to each of them based on a variety of sources, including field observations, pollination ecology studies, and various botanical databases. They categorised these species into four pollination modes –wind, water, insects, or vertebrates– and constructed phylogenetic trees to model the evolution of pollination modes.
Unsurprisingly, the researchers found that the earliest angiosperms were likely pollinated by insect and that approximately 86% of the evolutionary history of angiosperm pollination was driven by insects. This role of insects as pollinators remained predominant in several key groups of angiosperms, including magnoliids, monocots, eudicots, asterids, and rosids. This discovery aligns with previous evidence from fossils and early angiosperm lineages, all pointing to the ancestral nature of insect pollination. The results of this research are also aligned with a previous study led by Stephens’s PhD supervisor, Dr Hervé Sauquet, which provides a reconstruction of an ancestral flower, which features different characteristics of insect pollination, such as flowers being bisexual and exhibiting radial symmetry.
Although most species have shown a preference for insect pollination, there have been some notable exceptions, with certain groups demonstrating adaptations to other pollination methods. For example, aquatic plants in Ceratophyllales and Alismatales are predominantly water-pollinated. In contrast, Zingiberales evolved to depend on vertebrate pollination, and Fagales along with Picramniales adopted wind pollination. According to Stephens, “in the case of aquatic angiosperms water pollination was probably a necessary evolutionary step in becoming flowering plants whose entire life cycle is completed underwater”. On the other hand, in the case of Zingiberales, this shift might have been influenced by the widespread distribution of this clade across tropical regions. This broader distribution could be connected to the prevalence of vertebrate pollination in this family, as tropical plants often have more resources to offer greater amounts of nectar to larger vertebrate pollinators.
The authors also observed that over time, some plants changed their pollination methods. Occasionally, plants that were pollinated by animals shifted to relying on the wind for pollination. This happened more often than the reverse, meaning plants that were wind-pollinated transitioned to relying on animals. However, it’s rare for plants to revert back to animal pollination after adapting to wind pollination, specially since flowers need to undergo several major changes in their flowers to adapt to wind pollination, such as reducing or even eliminating certain parts, such as altering the flower’s shape (making the unisexual flowers, with only male or female reproductive organs), increasing the pollen-to-ovule ratio, or changing the shape of the style (from solid styles to feathery styles). These changes are difficult to reverse, which explains why it’s rare for plants to return to animal pollination after adapting to wind pollination.
The study identified frequent transitions between insect and vertebrate pollination over angiosperm history, leading to specialized interactions. Remarkably, shifts from vertebrate to insect pollination were almost as common as the original transitions. Such adaptations illustrate dynamic mutualism, as both vertebrate and insect pollination offer advantages. This demonstrates effective mutualism, with floral rewards attracting vertebrates for repeated visits, promoting directed cross-pollination. Plants employ similar strategies to engage both vertebrate and insect pollinators, like nectar production, sticky pollen, and appealing floral appearances. Surprisingly, species mainly relying on vertebrate pollination can adapt to insects, showcasing reversibility. This underscores the adaptable nature of plant-pollinator interactions, shaping their dynamics over time.
Regarding the environmental factors that may have shaped changes in pollination modes, the authors found that wind-pollinated plants were common in open areas and higher latitudes, while animal-pollinated ones prevailed under dense canopies and lower latitudes. Wind pollination is believed to evolve when animal pollination is limited, and the abiotic environment allows wind flow. Significant climate and habitat changes throughout the history of angiosperms may have opened up habitats and disrupted access to pollinators. Therefore, plants adapted to use wind as an alternative means to convey their messages and ensure their reproduction.
Interestingly, the research transcends the realm of angiosperms, suggesting the possibility that the ancestor of all seed plants may have also been pollinated by insects. While many contemporary gymnosperms are wind-pollinated, there are lineages of both ancient and modern gymnosperms that are pollinated by insects, raising questions about whether insect pollination may have played a role in shaping the evolution of the first seed-bearing plants.
Overall, this study on angiosperm pollination modes provides valuable insights into the fundamental, long-standing relationship between flowering plants and the pollinator insects that accompany them. The research shows that the beneficial partnership between these plants and insects has strong origins in evolutionary history, with insect pollination being a consistent factor for the majority of the time that angiosperms have existed. The study reveals that the mutualistic bond between these plants and insects is deeply rooted in evolutionary history. However, in the face of challenges posed by the Anthropocene, a period characterized by environmental changes caused by human activity, the fate of pollination dynamics remains uncertain.
Understanding the complexity of past and present relationships between plants and their pollinators is crucial for conservation efforts, promoting sustainable agriculture, and preserving the diversity and stability of natural ecosystems, currently influenced by global changes. This research underscores the need to protect these invaluable ecological partnerships, ensuring the continued survival and prosperity of plants and pollinators in an ever-changing world.
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Stephens, R. E., Gallagher, R. V., Dun, L., Cornwell, W., & Sauquet, H. (2023). Insect pollination for most of angiosperm evolutionary history. New Phytologist. https://doi.org/10.1111/nph.18993
Portuguese translation by Victor H. D. Silva.
Spanish translation by Carlos A. Ordóñez-Parra.
Nigel Chaffey
Alun Salt
