Agapetes species native to the Himalayas exhibit red, tubular, pendulous flowers, characteristic of hummingbird pollination. However, these birds are not found on the Asian continent! Kriebel and colleagues reconstructed the evolutionary history of the family to test the hypothesis that now-extinct hummingbirds pollinated these plants.

Flowers present a set of characteristics, including shape, size, colour, scents, and resources that attract pollinators, such as bees, butterflies, or birds. Consequently, pollinators select the plants they will visit based on some of these features. Yet, in the Himalayas, there exist plants with floral traits to attract a pollinator currently absent in the region. Scientists are now studying them to determine whether its pollinator disappeared in prehistoric times.

Specific floral traits attract particular pollinators

Researchers working on pollination coined the term “pollination syndrome” to characterise specific plant pollination systems with a particular set of floral traits. A classic example is ornithophily: plant species often develop reddish, tubular, and unscented flowers to attract birds.

The primary pollinating birds are hummingbirds and nectar-feeding birds. Although the flowers they pollinate share common traits, some differences can be found. Indeed, flowers pollinated by hummingbirds are typically pendulous, as these birds can hover in flight without needing to land on floral structures to access the nectar. On the other hand, nectar-feeding birds, like the sunbirds of southern Africa, often need to perch on or near the flower to feed on the nectar; thus, flowers are usually in a horizontal position.

Ornithophily is well-known and studied in various families of flowering plants, such as the blueberry (Ericaceae). This family has around 4,500 species and is found on all continents. Their flowers commonly provide nectar as a reward to their floral visitors and have petals that are fused, forming elongated tubes or structures resembling pots or bells. However, the blueberry family displays a wide variety of flowers (Figure 1) that, accordingly, attract different types of pollinators, including insects, birds, and mammals.

Figure 1. Ornithophilous Ericaceae species from America (top-right) and Asia (The other ones). A) Cavendishia bracteata, pollinated by hummingbirds (Photo by Félix Uribe, Wikicommons). B) Agapetes serpens flowers (Photo by Björn S., Wikicommons). C) and D) Rhododendron species, pollinated by birds (Photos by Ravi Sangeetha and Dibyendu Ash, Wikicommons).
 

A closer look at ornithophilous species of the blueberry family around the world

The types of pollinators can also vary among continents. Ericaceae species found in Europe, Asia, and Africa engage with various groups, whereas hummingbird pollination prevails in the Americas, especially in tropical regions.  Despite the wealth of information about ornithophily in the Americas, the bird pollination syndrome outside this continent remains elusive.

In the Himalayas, Rhododendron species develop reddish, non-pendulous flowers pollinated by nectar-feeding birds. However, Agapetes species form red and pendulous flowers, but birds do not appear to access these flowers. Therefore, this plant might rely on another pollinator capable of hovering during flight. Surprisingly, botanists observed that Agapetes flowers resemble species from the tropical region of the Americas, characterised by nectar production, long brightly coloured corollas, and orientations that suggest accessibility to visitors like hummingbirds.

How can an Asian plant attract hummingbirds?

There is evidence that the ancestors of these graceful animals once existed in Eurasia and later dispersed to the American continent. The known age of hummingbird fossils (~30 million years old) predates the age of Agapetes (8.4 million years old). However, according to Kriebel and colleagues, for the hummingbird and the plant to coexist, we must assume that this bird survived in Eurasia before discovering any known fossils. With that, Kriebel and colleagues analysed the transitions in floral morphology and their correlations with changes in pollinators in Ericaceae. They highlighted ornithophily systems and tested Mayr’s hypothesis:

Some Old World Ericaceae are similar in floral morphology to Neotropical species known to be pollinated by hummingbirds because they have evolved to be pollinated by these birds.”

The authors combined data on corolla and anther morphology, along with information on pollination and the geographical distribution of species, to detect differences in pollination syndromes between continents in a phylogenetic context. They used image databases and pollinator data for over a thousand species within the family. They further subdivided ornithophilous species in categories based on pollinator group: American hummingbirds, supposedly Eurasian hummingbirds (e.g., Agapetes), and birds from Africa, Asia, and Australia (honeyeaters and sunbirds).

Which bird pollinates Agapetes species?

The authors successfully separated groups of Ericaceae species based on their pollination syndrome, and formally tested Mayr’s hypothesis for the first time. They observed that species pollinated by hummingbirds (including those supposedly pollinated by these birds) and those pollinated by other birds form corallas with similar length but different shapes. In fact, the floral morphology of Agapetes more closely resembled species pollinated by sunbirds, while the flowers actually pollinated by hummingbirds were morphologically more variable than other species. In addition, species pollinated by hummingbirds (including Agapetes) form extremely long and linear anthers, different from those pollinated by other birds.

By using morphological data, researchers could distinguished pollination syndromes within the Ericaceae family, but they couldn’t definitively identify the specific type of ornithophily for Agapetes . This was because the floral morphology share similarities to both species pollinated by hummingbirds (e.g., anthers, corolla length) and those pollinated by sunbirds (e.g., corolla shape). Therefore, authors could not affirm or refute Mayr’s hypothesis based on these results.

Interestingly, other studies revealed that honeyeaters primarily pollinate Ericaceae species in Southeast Asia and Australia, despite the presence of sunbirds in these regions. Therefore, researchers suggest further expanding knowledge about the ecology of ornithophilous Ericaceae in Asia to identify the possible occurrence of pollination by these birds in the region and potentially establish an evolutionary relationship with Agapetes.

Although evidence that prehistoric hummingbirds pollinate these species are not conclusive, this study greatly contributes to our understanding of the evolutionary history of Ericaceae’s pollination systems. Hopefully, this research will raise interest towards the natural history of these species and prmote investigation aimed at uncovering the mystery of the Agapetes‘ pollinator.

ABOUT THE AUTHOR

Mariana Duarte is a botanist passionate about the natural history of flowering plants. Her primary research focus is around the reproductive processes of these plants, with a particular emphasis on investigating of the mechanisms of self-incompatibility. Additionally, she also loves to explore the fascinating interactions between plants and pollinators.

Spanish and Portuguese translation by Mariana Duarte.

Cover image Agapetes serpens Paul venter / Wikipedia.

SUGGESTED READING

Kriebel R, Rose JP, Bastide P, Jolles D, Reginato M, Sytsma KJ. 2023. The evolution of Ericaceae flowers and their pollination syndromes at a global scale. American Journal of Botany https://doi.org/10.1002/ajb2.16220.

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