Towards becoming a carnivore: evolution of carnivorous plants

“I care more about Drosera than the origin of all the species in the world”- wrote Charles Darwin in a letter to famed geologist, mentor and dear friend Charles Lyell. Drosera, a species of carnivorous sundew plants, has fascinated scientists that came both before and after Darwin, with even Carl Linnaeus having described it as “against the order of nature as willed by God”. Perhaps there is just something unsettling about the fact that plants, which we usually consider to be benign, even comforting, could at the same time be dangerous, or even deadly.

Whatever your feelings on carnivorous plant might be, it is hard not to be amazed at the evolutionary path that led to animal-preying plants. The famous Venus fly trap catches its pray by sensing the slightest movements of insects on its jaw-like leaves. Pitcher plants secrete sweet nectar on the slippery rims of their pitchers and any insect naïve enough to try and taste their sweet liquid inevitably slides down to be digested. Carnivorous plants have developed many strategies for catching their prey. Interestingly, however, similar strategies have evolved in many different lineages independently. As carnivorous plants are typically found in environments that lack nutrients such as nitrogen or phosphorus, it is thought that the use of an animal-based protein sources has evolved as a solution for the deficiency of these elements. Sequencing the genome of the pitcher plant Cephalotus follicularis is now finally giving some insight into the genetic basis of how these fascinating plants evolve.

Cephalotus contains two types of leaves- carnivorous pitcher leaves and flat, non-carnivorous ones. A recent study, led by Japanese scientists, sequenced the full genome of Cephalotus and analysed the gene expression and protein production patterns in the two leaf types. Their analysis showed that the Cephalotus genome is rich in genes that code for proteins involved in nitrogen recycling and contains many genes that code for proteins, which are known to be components of digestive fluids in related plants. Analysis of gene expression patterns from pitcher leaf samples showed that pitcher cells express many genes that control cell morphogenesis, starch metabolism and lipid biosynthesis. The morphogenesis genes are likely involved in molding the pitcher into the right shape, the starch metabolism genes provide instructions for making the nectar to attract insects, and the lipid metabolism genes are akin to those found in other plant species that code for wax and cutin biosynthesis, and probably are responsible for making the inner slippery surface of the pitcher. In contrast, the cells of the flat, non-carnivorous leaves express many genes that are involved in photosynthesis, reflecting the functional differences between the two leaf types.

Analysis of protein contents inside the digestive fluid of the pitcher traps indicated that many digestive enzymes found in the fluid are actually similar to the digestive enzymes found in other carnivorous plants. As these plants are not genetically related, the protein similarity was deduced to be a result of convergent evolution. It is likely that this has been due to similar evolutionary forces being present in the environments where these carnivorous plants evolved. Compared to non-carnivorous plants, the many proteins that are present in pitcher digestive fluids seem to be the same proteins that are important for survival under harsh environmental conditions. It appears therefore, that constitutive expression of stress-related proteins is an important step towards evolution of carnivory in plants.

Carnivory is a costly trade-off for plants. Many carnivorous plants perform poor photosynthesis and require a lot of energy to make their complex trap structures. The comparative analysis of the genes and proteins present in carnivorous and non-carnivorous plants in this research provides and important insight into evolutionary steps behind the emergence of this strange phenomenon of the natural world.

References:

Fukushima, Kenji, et al. “Genome of the pitcher plant Cephalotus reveals genetic changes associated with carnivory.” Nature Ecology & Evolution 1 (2017): 0059.

(featured image courtesy of Aaron Carlson, Wikimedia Commons CC BY-SA 2.0)