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Pitcher Plant Genomes Indicate Convergent Evolution

NEW YORK (GenomeWeb) – Carnivorous plants around the world have climbed a similar evolutionary staircase, according to a new study in Nature Ecology & Evolution that compared the genomes of pitcher plants from different continents.

Though separated by oceans throughout their natural history, Australian, Asian, and American pitcher plants have developed a similar set of evolutionary adaptations: a slippery leaf pouch that traps insects and a soup of digestive fluid at the bottom containing special enzymes to break down insects and extract precious nutrients.

"It suggests that there are only limited pathways for becoming a carnivorous plant," University at Buffalo biologist and co-senior author Victor Albert said in a statement. "In the end, they all come up with the same solution."

Led by scientists from Japan's National Institute for Basic Biology and BGI-Shenzhen, an international consortium sequenced and assembled the full genome of the Australian pitcher plant Cephalotus follicularis and compared it to genomic data from three other carnivorous plants. They annotated more than 36,000 protein coding genes as well as 72 microRNA loci and their potential targets. Using next-generation sequencing technology from both Illumina and Pacific Biosciences, the researchers showed that enzymes used to digest prey were co-opted from the same ancestral proteins, including stress-response proteins.

"Carnivorous plants often live in nutrient-poor environments, so the ability to trap and digest animals can be indispensable given the dearth of other sources of nourishment," co-first author Kenji Fukushima of Japan's Graduate University for Advanced Studies (SOKENDAI) said in a statement.

"Such parallel development often points to a particularly valuable adaptation," added SOKENDAI researcher and co-senior author Mitsuyasu Hasebe.

That the plants demonstrated evolutionary convergence wasn't necessarily surprising. Coffee and cocoa trees both evolved the ability to produce caffeine, an insect neurotoxin, drawing on closely related proteins.

Still, the similarities between C. follicularis and Nepenthes alata — a pitcher plant native to the Philippines — are striking, the authors said.

Chitinases — which break down an important protein component of insect exoskeletons — and purple acid phosphatases — which extract phosphorus from them — share many of the exact same or closely-related amino acid substitutions.

Analysis of RNase T2, an enzyme that breaks down RNA, in carnivorous plants provided several interesting tidbits. The researchers detected similarities in this enzyme in both C. follicularis and Drosera adelae, a carnivorous plant called the sundew that is not a pitcher plant but is closely related to N. alata.

Moreover, these mutations don't appear in non-carnivorous species, suggesting they're critical to the plants' ability to eat bugs. 

The authors noted that the Cephalotus RNase T2 and purple acid phosphatase genes are located adjacent to each other within a 40 kilobase interval.

The researchers also performed Illumina NGS-based gene expression analysis in the carnivorous plants, and wrote that they "found that various transporters were preferentially expressed in pitcher leaves." They also noted that a cytochrome P450 orthogroup related to wax biosynthesis genes in the model organism Arabidopsis thaliana and wax ester synthase orthologs were significantly expressed in pitcher plants.

"This result, together with the repeated co-option of digestive enzymes already described, indicates utilization of common genetic programs and evolutionary pathways in independently evolved carnivorous plant lineages," they noted.