How *Salvinia molesta* became an unstoppable invasive fern

Some weeds merely cause inconvenience. Salvinia molesta is far more like an occupying force. This floating fern is capable of doubling its biomass in only 36 hours. If left alone, it covers ponds and sluggish rivers with a dense green carpet.

That carpet stops sunlight reaching the water beneath. It also removes oxygen, causing the organisms below to suffer. The plant is now present in freshwater habitats in more than 60 countries. It is listed among the world’s 100 most invasive species. Its extraordinary pace of spread suggested that an unusual process was at work.

Salvinia molesta smothers lakes

For years, researchers had no answer to a straightforward question: why is Salvinia molesta so persistently effective at spreading?

Its sheer speed has frustrated water managers around the world. A pond may appear clear one week, then become clogged the following week.

The route to an explanation starts with a case of mistaken identity. For a very long time, scientists had been describing the wrong type of plant.

Decades of mistaken identity

A research team headed by Associate Professor Fay-Wei Li at the Boyce Thompson Institute (BTI) and Cornell University closely examined the plant’s genome. He was joined by Erin Sigel of the University of New Hampshire.

For decades, S. molesta had been labelled an allopentaploid. This term describes a hybrid with five chromosome sets inherited from several parent species.

Its genome revealed another explanation. The plant is in fact a diploid hybrid with only two chromosome sets. Each set originates from a separate parent species, and neither parent is known to science.

Finding those parents has become an open search. Somewhere, two unseen ferns left their genetic signature on this invader.

“The misidentification of S. molesta has stood for decades,” Li said. “Getting it right matters, not just for evolutionary biology, but for understanding how this plant became so successful as an invader.”

Cannot reproduce sexually

The plant’s two chromosome sets are incompatible with one another. That mismatch is central to the story.

“They carry different numbers of chromosomes and several major structural differences,” explained Yanã Rizzieri, first author of the study and a graduate student in Li’s lab.

“When the plant tries to undergo meiosis, those differences prevent proper chromosome pairing. No viable spores form. The plant cannot reproduce sexually.”

Its conventional reproductive pathway is therefore closed off. There are no spores, seeds or sexually produced offspring.

Cloning its way everywhere

Instead, the fern grows rapidly and breaks apart. Tiny pieces can detach from the parent plant and develop into entirely new plants.

Every resulting plant is genetically identical to its parent. Just one fragment reaching an untouched pond can begin a full-scale invasion.

Each subsequent plant is a clone of the original one. It is an unusual means of taking over waterways, but a disturbingly efficient one.

Almost no genetic difference

To examine this possibility, the researchers sequenced 100 individual plants. The samples represented five populations from across the southeastern United States.

There was almost no genetic diversity. Plants within each population were nearly indistinguishable from one another.

Most variation between individual plants appeared in just one plant at a time. This pattern is characteristic of a clonal population spreading outward from a single founder.

Fresh variation emerged solely through copying errors accumulating over time. Sexual reproduction never mixed the genetic deck again.

Salvinia molesta offers hope

This tendency to clone brings an important advantage for control efforts. The entire invasive population follows one genetic blueprint.

As a result, a treatment that consistently suppresses the plant in one location should be equally effective elsewhere. A successful method in Louisiana should also work in other places.

For those fighting invasive species, that is unusually promising. In effect, the opponent is always the same plant.

A control strategy shown to work in one waterway ought to transfer to the next. Managers can test an approach once and use it widely, rather than making site-by-site guesses.

Fern genomes surprise

For comparison, the team investigated a close relative, S. cucullata. This small aquatic fern has the smallest genome of any known fern, at just 250 million base pairs.

The comparison was intentional. Placing the invader alongside its most compact cousin offered a clear before-and-after perspective on changes in fern genomes.

The researchers combined long-read sequencing with Hi-C chromatin mapping. Together, these methods enabled them to produce chromosome-level assemblies for both ferns.

The findings then defied their expectations. Neither fern followed the pattern predicted by textbooks.

Genome size shifts

The genome of S. cucullata is about 14 times smaller than ours. Even so, it contains 68 chromosomes, almost four times more than had been expected.

Salvinia molesta shows the opposite pattern. Its genome is ten times larger than that of S. cucullata, yet it contains only 46 chromosomes.

Those 46 chromosomes are arranged in two separate subgenomes. The two diverged from one another approximately 25 million years ago.

What it means for ferns

“Our findings show that the evolution of Salvinia genomes is very dynamic, more like that in flowering plants than in most ferns that have large genomes,” said Sigel.

Most large fern genomes evolve extremely slowly. Salvinia, however, appears to operate under more active rules.

In this case, genome size and chromosome number have become disconnected. A larger genome did not correspond to more chromosomes, overturning a familiar assumption.

The research challenges a long-established model for fern genome evolution. It also makes Salvinia a valuable system for examining how reproduction influences a genome.

It further links the fern’s biology directly to the threat it poses. The same failed chromosome pairing that prevents sexual reproduction created such a relentless clone-and-spread invader.