Extra gene copies drive evolutionary innovation in pitcher plants

Possessing more than two complete sets of chromosomes can be a hindrance to long-term survival of a plant lineage, yet scientists are also finding evidence it's likely behind some evolutionary innovation.

Sudden inheritance of whole suites of extra gene copies can add redundancy to an organism's regular sets of functions, actually permitting some of those copies to evolve and express in entirely new ways.

In the case of the East Asian pitcher plant, this mutational freedom may have even fine-turned its ability to capture prey and satisfy its appetite for "meat."

That's just one of the findings in a new study that sequences the genome of Nepenthes gracilis, a species of carnivorous plant related to Venus flytraps, as well as sundews, beets and spinach.

Our findings not only provide key insights into the adaptive landscape of the Nepenthes genome, but also broaden our understanding of how polyploidy -; having multiple sets of chromosomes -; can stimulate the evolution of new functions."

Victor Albert, PhD, Empire Innovation Professor, University at Buffalo Department of Biological Sciences, within the College of Arts and Sciences

Albert is the co-senior author of the study, which was published Thursday (Nov. 23) in Nature Plants, along with Kenji Fukushima, PhD, of the University of Würzburg in Germany. Other contributors from UB include Charlotte Lindqvist, PhD, professor of biological sciences, and PhD students Emily Caroll and Michaela Richter.

Albert's work was supported in part by the National Science Foundation.

Recessive subgenomes are more free to change function

The idea of "man-eating plants" has long captured our macabre imaginations. Pitcher plants, however, require a much smaller meal.

They capture insects using their highly specialized pitcher-shaped leaves. The bottom of their pitchers are filled with digestive fluids that drown and eventually break down prey. This process releases nutrients, such as nitrogen and phosphate, that allow pitcher plants to thrive in nutrient-poor habitats.

"Plant carnivory is something of a hunt for fertilizer," Albert says.

In a 1992 study, Albert and colleagues discovered that the Asian, Australian and American pitcher plants possess similar features despite having evolved independently. Later research published in 2017 showed that each of these species co-opted many of the same ancient proteins.

In this new study, Albert and Fukushima's teams discovered that the specialized pitcher trap of the Asian pitcher plant, or Nepenthes, may have been promoted by polyploidy. Nepenthes' lineage had already evolved carnivory, so the duplicated genomes may have simply tweaked its mode of capture.

The team discovered that Nepenthes has a decaploid genome in its diploid state, a complex structure almost unprecedented in flowering plants that reflects possession of five whole-genome multiples, or "subgenomes."

The team found that the fifth subgenome is "dominant," retaining more gene copies and expressing them at higher levels than the other four older, "recessive" subgenomes. Yet it's the recessive subgenomes -; not the dominant one -; that may carry more of the key genes for Nepenthes' specialized carnivory.

"The dominant subgenome exhibits greater influence of natural selection pressure to maintain gene functions," Fukushima says. "Whereas the recessive subgenomes, impacted less by functional preservation, have become more free to vary over evolutionary time."

Some of Nepenthes' duplicate carnivory genes may have originally evolved for defending against what eventually became their prey. The enzymes that help Nepenthes break down insects' hard exoskeletons, for example, were repurposed from those that originally shielded plants from being eaten by these animals.

"This lineage of Nepenthes didn't evolve new genes to become carnivorous -; they grabbed collections, or toolkits, of genes that were already there," Albert says.

One hypothesis suggests that polyploidy has a negligible effect on long-term evolution, as species with multiplicated genomes may undergo extinction at rates higher than those like humans, which have just two sets of chromosomes. Yet the study's findings add to the evidence that ancient polyploidy events can sometimes underlie evolutionary jumps still visible among plants today.

Evidence found for the evolution of separate male and female plants

Nepenthes is part of the just 6% of flowering plant species that are dioecious, meaning each individual plant produces either male or female flowers. In fact, Nepenthes is the only dioecious carnivorous plant.

Albert and Fukushima's team also identified a male-specific region of the genome containing three genes potentially responsible for controlling these sex differences. One of these, called LEAFY, is a key gene expressed early in flower development that acts as a master regulator.

"LEAFY seems to have had a duplicate form and moved into the Y chromosome region of Nepenthes, thereafter diverging in its function. This use of LEAFY is unprecedented so far in flowering plants," Albert says. "The LEAFY gene is such a central regulator across flowering plants that, when artificially added or deleted through genetic engineering, it will change a plant's flowering time."

"Although we now have bioinformatic evidence pointing toward LEAFY being one of the key genes involved in the sex determination mechanism of Nepenthes, actually proving this is going to require further studies in living plants," he adds.