Hornworts Loan Genes to Ferns for Shady Environments

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How did ferns evolve the ability to utilize low levels of light? Image courtesy of Fay-Wei Li

If you are a hiker or woods-wanderer, and have ever wondered in your trip through a forest, “how do ferns keep themselves up in these dark patches?

Here is the answer for you. According to a study published today in PNAS, about 180 million years ago, a primitive plant called a hornwort gave those ferns a genetic gift so they could “see better” in dark forests. 

Phototropism

We know the “phototropism” concept from general biology: Plants tend to grow towards the direction of light. But how do they know about the surrounding light? There are antennas in plant cells called photoreceptors that can sense lights of different rainbow colours in their surroundings. These antennas dictate the growth of the plants accordingly.

In higher plants, the photoreceptors are well-evolved to adjust to the light in their surroundings. But primitive plants like liverworts, hornworts, mosses, and a lot of ferns, like the ones you saw in Jurassic Park, found a different way.

The primitive plants could easily get enough light when there was not much competition by trees around, but as trees started appearing around them, the mosses and others evolved a chimeric antenna – a photoreceptor gene called a neochrome.

The Chimeric Antennas

A neochrome can sense lights of colours, especially red light, which is more abundant than blue light, under the canopy. Scientists earlier thought that ferns were the only plants with a neochrome. However, Fay-Wei Li, a PhD student at the Duke University, Durham, the first author of this research, tells Decoded Science,

“When I was digging deeper into neochrome’s evolutionary history, I surprisingly found neochrome is also present in hornworts (a less-known moss relative).”

Initially, Li and his colleagues had no idea how ferns got neochrome, according to Li.

“Ferns and hornworts are obviously not related, like humans and sponges, so why neochrome is present only in ferns and hornworts?”

How Did Ferns Gain Neochrome?

The researchers began with three hypotheses, Li tells us,

“First, it could be that the ancestor of all land plants had neochrome, but then everybody threw it away, except that ferns and hornworts kept it. Or, ferns and hornworts could independently evolve neochrome. And finally, there could be horizontal transfer of neochrome between ferns and hornworts.”

Horizontal Gene Transfer?

Horizontal gene transfer (HGT) is a means through which nature shuffles genes without sexual reproduction. For instance, when pathogens infect us, they leave a portion of their genetic material in us that puts our immune system to work. In plants, however, until Li and colleagues conducted their research, there has been no reports of such a possibility of HGT of this nature.

The researchers had genome sequences of different plants to work with, including ferns and hornworts provided by the 1000 plants initiative, which “has sequenced exemplars for all the major lineages across the viridiplantae” (green plants), says Gane Ka-Shu Wong, the principal investigator of the initiative.

Li and colleagues did computational genetic analysis by sieving and mathematically picking the answers from those genomes that answered their hypotheses repeatedly. According to Li,

“We had no a priori belief as to which hypotheses might be more plausible before we carried out more lab works and analyses. It turned out all our results strongly point to the HGT hypothesis. What a surprise!”

“Ferns cannot mate with hornworts, like I cannot mate with sponges, so the fact that they share a very similar gene suggests horizontal gene transfer”

Mathematically, Li and colleagues did a “molecular clock analysis” and drew an evolutionary tree from the genomic data that explains how the gene-jumping could be possible.

  • The first knowledge was that, ferns bid a bye and diverted their route from the moss-relatives over 400 million years ago, to evolve separately.
  • The second was that the fern neochrome itself evolved only recently (180 million years ago) from the hornwort neochrome.

It is like a sponge and I share a gene that diverged only after, say last year, despite the fact that we (human and sponge) were separated hundreds of million years ago,” explains Li. He adds that, “the only way to explain this is that I got this gene horizontally from sponges last year, not by mating with them” says Li.

He adds, “similarly ferns got neochrome horizontally from hornworts 180 million years ago.

Interesting, but how did the gene-jumping actually happened in the forest?

A hornwort (Dendroceros validus) / Credit: Juan Carlos Villarreal

Ferns’ Exposed Genitals

It’s unbelievably simple! Ferns have “exposed genitals,” unlike many other higher or lower plants. If you have looked under the leaves of Maiden-Hair Fern (Adiantum sp.) you might have noticed some blackish brown spots. These are the fern’s “fruits” exposing their “spore-houses” to the thin air.

In the forests, the ferns are near the ground, as are the spores from small plants like hornworts, which also send their spores out to the thin air for reproduction. The cell-to-cell contact is the only possible way through which nature facilitated the mixing of the jumped genes in ferns. But how useful is this research for the future? According to Li,

“… there are actually more incidences of plant-to-plant HGT than we previously thought.”

He tells Decoded Science that “most of these HGT involve host-to-parasite transfers. For example, the parasitic plant Rafflesia  produces world’s largest flowers… and its host, a relative to grape… apparently have exchanged a lot of genes horizontally.

According to the press release, Li says, “there’s no definite answer as to what mediates it.

However Li thinks, “Neochrome HGT is an exciting discovery, because neochrome appeared to have facilitated the fern , suggesting that plant-to-plant HGT could have important evolutionary impacts.

He adds, for example, “ferns got a “genetic gift” from hornworts that helped them to “see better” under the low-light forest canopies.

Ferns and Humans

It is intriguing, whether such a research could be put to economic use. Li says,

“… neochrome indeed has potential economic value in increasing crop yields, given its pivotal role in helping plants locating light direction and thereby maximizing photosynthesis in low-light conditions.

So, can we see some work in agriculture or horticulture? Li speculates,

detailed characterization from an evolutionary perspective would be essential for assessing future GMO applications.

However he quickly adds,

one concern about GMO is that it’s unnatural to insert foreign genes into crops. But it seems that plants do, to some extent, naturally move genes around themselves.

Gane Ka-Shu Wong says their consortium is working in terms of understanding the weediness in plants. Regardless, we shall wait to see what Li and his colleagues discover for the future.

Ornamental Ferns

But one last curious question still remains: We humans grow ferns as ornamental plants in well-lit rooms. As we breed dogs and pigeons, are we also breeding ferns to lose their neochrome?

According to Li, this is unlikely:

“Humans have only begun to cultivate ornamental ferns since the 18th century, a very short stretch of time not enough to exert any selection force to tweak neochrome evolution [… and we] did not “domesticate” the ornamental ferns, but just pick those appropriate ones from the wild.”

“Some ferns do naturally grow in open habitats. For example, there is a group of ferns called cheilanthoids that grow mostly in the deserts (not much shade obvious)! Interestingly, neochrome might have been lost in those growing in open habitats.”

Ferns, Hornworts, and Genetic Transfer

Horizontal Genetic Transfer (HGT) – it’s nature’s GMO. Further research will tell us what lessons we can learn from the plants that share genetic material as they evolve.

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