How the pitcher plant became predatory

An international research team has sequenced the genome of the Australian pitcher plant and, in doing so, discovered a key to the mystery of how the plant became carnivorous. Their study has been published in the journal Nature Ecology and Evolution.

All plants are photosynthetic organisms, ie, they transform the inorganic matter of the environment into organic molecules (glucose). But some plants live in habitats poor in nutrients, and so compensate for this with the ability to absorb nutrients from prey such as insects and other arthropods.

Seeking to understand this extraordinary evolutionary mechanism, a research team led by Kenji Fukushima analysed the nuclear genome of the Australian pitcher plant (Cephalotus follicularis), which evolved to include both carnivorous pitcher leaves and non-carnivorous flat leaves. It is this unique feature which allowed the scientists to analyse the plant’s genetic basis for carnivory.

The researchers found that in the plants’ adaptation to a carnivorous diet, the generation of new genes is not always necessary. Already existing genes in the plant genome have acquired new biological functions, a process known as co-option.

“Genes originally involved in the defence against certain diseases — or the response to biotic and abiotic stress — have acquired new functions related to the ability of feeding from animals,” said Pablo Librado from the University of Barcelona. “This is the case, for instance, of a specific set of proteins that evolved to act as digestive enzymes.

“The results of co-option, regarding both the digestive enzymes and the amino acid changes seen in these enzymes, show that evolution has acted on a limited number of evolutionary routes in the adaptive transition to the carnivorous diet.”

Julio Rozas, also from the University of Barcelona, added, “According to the results, leaves that catch insects have gained new enzymatic functions: basic chitinase, which breaks down chitin (the main component of insects’ exoskeleton); and purple acid phosphatase, which releases phosphate groups from molecules, and it contributes to the mobilisation of the prey’s phosphate.”

The scientists analysed the plant’s digestive fluid in three independently evolved pitcher plants and the sticky carnivorous plant Drosera. They concluded that in addition to convergent digestive physiology, the separately arisen digestive enzymes often incorporated similar genetic components, even though the lineages had split more than 100 million years ago — long before their respective carnivorous habits arose.

“It turns out that carnivorous plants are an amazing story of convergent evolution in systems at multiple scales,” said David Pollock from the University of Colorado School of Medicine. “The physiological convergence of leaf form and function is associated with convergent evolution in enzyme repertoires, gene expression patterns and, most strikingly, amino acid substitutions. Excess convergence is strong evidence of natural selection, especially when found on multiple scales.”

“The examples of parallel evolution at a molecular scale are not very common,” added Alejandro Sánchez-Gracia from the University of Barcelona. “Therefore, they are very interesting to understand the genetic causes of molecular adaptation and their study can help us to determine the relative role of the different evolutionary forces in biological diversification.”