A recent study, published in Nature Ecology and Evolution, has found that different species of the carnivorous pitcher plants located on different continents developed their defining characteristics in response to similarly stressful environments. The plants were also found to have similar strategies for handling the insects they capture, despite the carnivorism developing independently in the different species.
Pitcher plants are a type of carnivorous plants with a pitcher- or bucket-shaped leaves in which they capture insects. They lure the insects into the pitcher leaves using tempting colouring or nectar, and then entrap them. The insects are then digested and the plant can absorb nutrients like nitrogen and phosphorous from the resulting liquid.
Scientists believe that the pitcher plants turned carnivorous due to growing in nutrient-scarce soils. When getting enough nutrients from the soil was difficult, it was beneficial for the plants to develop the ability to get those nutrients from a different source – insects.
The way the pitcher plants did this is rather amazing in its simplicity. Genes that previously encoded enzymes used to protect the plants against fungi changed to now produce similar enzymes that aid in the digestion of insects. This works since the insects’ exoskeletons are made up of the same chemical as the fungis’ cell walls.
The plants’ stress-response hormones have also been repurposed. Originally, they would be present at the plant’s cell walls and used to signal damage when say, a plant is being chewed on by an animal. In carnivorous plants, these hormones are also used to transport nutrients from the pitcher leaves and as part of the digestion process.
Comparing pitcher plants from different continents, scientists found that the same chemicals are being repurposed in different species and the enzymes that make up the digestive fluid in their pitchers are the same to a great extent. They also found that the genes that provide the ‘recipes’ for these enzymes have common origins.
However, the carnivorous nature of the plants only evolved after the plants had been separated and became different species in different parts of the world. We can be certain of this because some of their genes have developed similar, but not identical, mutations since the separation. Upon comparing the complete set of genes of the Australian pitcher plant, cephalotus follicularis, to other species of pitcher plant, it was also found that c. follicularis has more genes in common with the star fruit than it does with the other species of pitcher plant.
The significance of this study is that different species of plants independently responded in the same way to poor conditions: they all became carnivorous, and developed the same tactic for carnivourism, down to the genetic and molecular level. Scientists already knew that convergent evolution, the process of different species independently developing similar characteristics for similar purposes, occurs in plants. What they didn’t know is that it occurs down to a molecular level, as it has in pitcher plants.
Pitcher plants all over the world are ‘choosing’ the same evolutionary path to become carnivorous, when in theory, they could have ended up developing very diverse solutions to the problem of scarce nutrition, implies that there might be a very limited number of ways for plants to become predators.
As evolution is a process of optimisation, it may also mean that the path these pitcher plants chose is the most efficient one, for example by taking the shortest time or requiring the fewest and simplest genetic mutations.
Image: Peter Häger