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| Rape, Brassica rapa, pollinated by bumblebees has more attractive flowers. Image: Florian Schiestl, UZH |
Evolution is a dynamic process in which a species responds to competing selection pressures in its environment. Some of these might push the species in one direction and others in the opposite direction. Changing environmental factors effectively redefine the term 'fitness' as it applies to relevant characteristics.
Brassica plants, for example, have large attractive flowers which attract insect pollinators. They are also monoecious or hermaphrodite and, like many plants, have both male and female parts in the same flower, so are capable of 'selfing' or self-pollination.
Pollination by pollen from other flowers, carried by pollinating insects, increases genetic mixing, so reducing the change of deleterious mutations occurring in the same individual. This pressure pushes the species towards the evolution of larger, more attractive flowers.
However, there is a price to pay in that large attractive flowers also attract herbivores that eat brassica leaves, so wasting the plant's resources and reducing their vigor. This pressure pushes the species towards less attractive flowers. This in turn reduces the likelihood of cross pollination, which pushes the species toward selfing and characteristic that make that more, not less likely.
To test how brassica rapa, the commercial plant used to provide rapeseed oil, respond to these competing pressures, scientists at the University of Zurich, Florian Schiestl, professor at the Department of Systematic and Evolutionary Botany, and doctoral candidate Sergio Ramos set up a two-year experiment in a greenhouse where the availability of pollinating bumblebees and herbivorous pests could be controlled.
They set up two groups; one with bumblebees only and one with both bumblebees and herbivorous caterpillars, and allowed pollination to occur naturally. They also set up two similar control groups where pollination was by hand. Then they let the groups cycle through six generations.
Their results were published today in Science.
Abstract
Pollination and herbivory are both key drivers of plant diversity but are traditionally studied in isolation from each other. We investigated real-time evolutionary changes in plant traits over six generations by using fast-cycling Brassica rapa plants and manipulating the presence and absence of bumble bee pollinators and leaf herbivores. We found that plants under selection by bee pollinators evolved increased floral attractiveness, but this process was compromised by the presence of herbivores. Plants under selection from both bee pollinators and herbivores evolved higher degrees of self-compatibility and autonomous selfing, as well as reduced spatial separation of sexual organs (herkogamy). Overall, the evolution of most traits was affected by the interaction of bee pollination and herbivory, emphasizing the importance of the cross-talk between both types of interactions for plant evolution.
Ramos, S. and Schiest, F.
Rapid plant evolution driven by the interaction of pollination and herbivory
Science 2019 vol: 364 (6436) pp: 193-196. DOI: 10.1126/science.aav6962
Copyright © 2019 The Authors
Reprinted by kind permission under licence #4566530148991
What they found was exactly what the Theory of Evolution predicts - the brassicas responded to the different environmental conditions differently. Those with bumblebees only, tended to have larger, more fragrant flowers while those with caterpillars tended to have smaller, less fragrant flowers and more toxic metabolites. The plants had diverted resources into defending themselves against the caterpillars.
Additionally, the plants subject to attack by caterpillars which were less attractive to the bumblebees evolved to spontaneously self-pollinate. At the start of the experiment, the pistil was longer than the long stamens. By the end of the experiment, this distance between pistil and stamens was unchanged in hand-pollinated plants without caterpillars; in plants with caterpillars, regardless of the mode of pollination, this distance had shortened, bringing the pistil closer to the stamens so facilitating selfing. In other words, after just six generations, there was a measurable morphological change to the flowers.
These evolutionary effects were measurable in just six generations. The importance of this experiment is that it shows how environmental change, for example, climate change, loss of habitat and use of pesticides can lead to rapid evolution in plants. The whole stability of ecosystems can depend on delicate balances such as these.
One nit: "The plants had diverted resources into defending themselves against the caterpillars" implies consciousness and/or purpose.
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