Mr Miguel Castañeda-Zárate, a Mexican TWAS (The World Academy of Sciences) funded PhD candidate in the School of Life Sciences (SLS), has published results from his research in Current Biology, a journal with an impact factor of 9.601.
In the paper, he and his two co-authors – also from the SLS – present a discovery that solves an evolutionary puzzle about how flowers adapt to a different pollinator.
Castañeda-Zárate’s PhD project focuses on the pollination and evolution of an African orchid species. Most populations of the species are pollinated at night by hawkmoths which are attracted to the white, sweetly-scented flowers with long nectar tubes from which they drink nectar. While researching this species in the Karkloof area of the KwaZulu-Natal Midlands, he discovered a population with slightly unusual flowers. After spending several fruitless evenings trying to observe moths in this population, he found that the tubes were devoid of nectar. It was during a day-time visit to the population that the real pollinator was encountered. Flowers were visited systematically by female bees, which collected oil instead of nectar from the flowers! Rather than inserting their tongues, they used their front legs to rub the flowers’ inside surface, resulting in orchid pollen packages being attached to their feet.
The surprise observation of a bee pollinating a flower that is, for all outward appearances, adapted for moth pollination, allowed Castañeda-Zárate to investigate how plants shift between different pollinators. It is known that this happened frequently during the evolutionary history of flowering plants, but the lack of intermediate flower types means that pollinator shifts usually cannot be studied as a process. However, here he encountered a case of “evolution in action”.
As expected, chemical analyses confirmed that the flowers pollinated by oil-collecting bees produced an oil-like substance, but much to Castañeda-Zárate’s surprise, he also found small amounts of oil in closely related moth-pollinated flowers. Moth- and bee-pollinated flowers were similar in colour, shape and scent. However, Castañeda-Zárate suspected that not all flower features still function in the same way. To test this idea, he experimentally shortened the floral nectar tubes by bending them upwards, effectively shortening them to half of their natural length. In a moth-pollinated population this dramatically decreased pollination success, but in the bee-pollinated population, spur shortening had no effect on pollination success. Reconstruction of evolutionary relationships based on DNA sequences revealed that bee pollination evolved from a moth-pollinated ancestor, which suggests that floral spurs are retained in the bee form, despite no longer being important for pollination.
The combined evidence from his study shows that in flowers which otherwise look and smell the same, a minor shift from a sugar-based reward to an oil-based one has led to a major shift between different pollinators. The presence of small amounts of oil in moth-pollinated flowers may have facilitated the shift to bee pollination, whereas the long spurs of the bee-pollinated flowers represent a stage of incomplete evolution in the transition from moth to bee pollination.
The results of his study were published online in Current Biology on 5 November: https://authors.elsevier.com/a/1c1793QW8Ru9gH
The research team consists of Castañeda-Zárate, Professor Steve Johnson (co-supervisor), and Dr Timo van der Niet (supervisor) from the Centre for Functional Biodiversity in the SLS at UKZN.
Words: Timo van der Niet