The catch-22 of being a carnivorous plant

Guest post by Sonja Dunbar (@PlantSciSonja)

Plants, like any other organism, want to reproduce. The usual way that plants achieve this is known as sexual reproduction, where an egg cell and sperm from two different individuals fuse and then develop into a new plant. However, since plants are generally anchored to one spot, they can’t meet up to reproduce. Instead, they rely on a variety of more indirect methods to transport sperm to other plants. For example, many flowering plants (also known as angiosperms) recruit insect messengers to carry their sperm, safely packaged in pollen grains, from one plant to another. They use colourful, sometimes scented, flowers to attract potential pollinators and often reward them with a sugary drink, nectar, while coating them in the pollen the plant wants them to carry. But what if you are a plant that also eats insects?

Pollinators S Dunbar[1]

Some of the most well-known pollinators; bees and butterflies. Image credit: S. Dunbar

Carnivorous plants obtain nutrients from trapped insects to help them cope with a lack of important nutrients in their environment, such as nitrogen, that they need to grow (1). There are several different trap types, from snap traps, to flypaper traps and pitfall traps. The fact that carnivorous species are found in multiple different plant families suggests this strategy has arisen several times.

 

Carnivorous plants

Examples of carnivorous plant trap types: A) Venus fly trap (snap trap), B) Pitcher plant (pitfall trap) and C) Sundew (flypaper trap). Image credit: S. Dunbar

The picture we have of a venus fly trap tends to be of its iconic snapping jaws, but carnivorous plants are all also flowering plants. Although some carnivorous plants can fertilise their eggs with sperm from their own pollen (self-pollination), many seem to use insect pollinators. This means that there is a potential conflict between using an insect for food or as a messenger to carry pollen. How do the plants tell the difference between their next meal and a potential pollinator?

Having looked at research on multiple different species, Jürgens et al. suggest several different solutions to this problem (2). One is to make sure pollinator species won’t be caught by the trap, but this is easier to achieve with some trap types than others. Alternatively the plant can make sure that its traps are not present while it is flowering. Spatial separation is also possible and is most clearly seen in the bladderworts which catch small aquatic creatures in their submerged bladder traps, but flower above water.

Utricularia_aureaRHu2

Pollinators are never at risk of being caught in a bladderwort’s aquatic trap. Image by Meneerke Bloem (CC BY-SA 4.0 via Wikimedia Commons)

A more elegant possible solution is to use the fact that pollinators have preferences for certain colours, smells or other features. These preferences are also the reason why we see so many different flowers. Plants don’t want their pollen to end up just anywhere; they need it to find another plant of the same species. These specific flower designs then act as a sort of address for the pollinator, making it more likely the pollen will arrive at the desired destination. Many carnivorous plants already use these preferences to attract prey by mimicking flowers. Jürgens et al. describe several studies showing that the red colour of many traps is important in increasing the number of insects caught, while others exude floral scents.

There are many other species of carnivorous plants that still need to be studied in more detail, but we can safely say that there are many ways a carnivorous plant can have its cake and eat it.

Sonja Dunbar is a PhD candidate in the laboratory of Dr Laura Itzhaki at the University of Cambridge. Her love of plants stems from her degree and she can be found talking about them on Twitter (@PlantSciSonja), on SciCam (https://www.youtube.com/user/SciCamVideos) and in her teaching and outreach.

 

 References:

  1. Thum, M. (1988). The significance of carnivory for the fitness of Drosera in its natural habitat. 1. The reactions of Drosera intermedia and rotundifolia to supplementary feeding. Oecologia 75: 472–480.
  2. Jürgens, A., Sciligo, A., Witt, T., El-Sayed, A. M. and Suckling, D. M. (2012). Pollinator-prey conflict in carnivorous plants. Biological Reviews, 87: 602-615. DOI: 10.1111/j.1469-185X.2011.00213.x http://onlinelibrary.wiley.com/doi/10.1111/j.1469-185X.2011.00213.x/abstract
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