Salvinia molesta can double its biomass in 36 hours. It spreads across ponds, lakes, and slow-moving waterways in a smothering green mat, blocking sunlight, consuming oxygen, and collapsing the ecosystems beneath it. Now present in freshwater bodies across more than 60 countries, it ranks among the top 100 most invasive species in the world. Scientists have long wanted to understand what makes it so relentlessly effective.
The answer, it turns out, starts with a case of mistaken identity.
Research led by associate professor Fay-Wei Li at the Boyce Thompson Institute (BTI) and collaborator Erin Sigel from the University of New Hampshire (UNH) revealed that S. molesta is not what scientists thought it was. For decades, the species was classified as an allopentaploid, a hybrid carrying five sets of chromosomes from multiple parent species. Upon close examination of S. molesta's genome, the team found that it is a diploid hybrid carrying just two sets of chromosomes, one from each of two parent species that are as yet unknown to science.
"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."
The two subgenomes inside S. molesta don't match. "They carry different numbers of chromosomes and several major structural differences," explained Yanã Rizzieri, first author of the study and a Cornell 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."
What it can do is grow fast and break apart. Fragments of S. molesta detach from a parent plant and develop into new, genetically identical individuals. In environments where it thrives, this is extraordinarily efficient. A single introduction can spark an invasion, with every subsequent individual a clone of the original.
To test this, Rizzieri and colleagues sequenced 100 individuals across five populations in the southeastern United States. Across all of them, genetic diversity was nearly absent. Most genetic differences found between individuals appeared in only a single plant, the signature of a clonal population expanding from one original founder, with mutations accumulating only through copying errors over time.
The clonal nature of S. molesta has a practical implication. Because the entire invasive population shares a single genetic blueprint, any treatment that suppresses it reliably in one location should work just as well anywhere else it has spread.
Rethinking fern genome evolution
The research team chose S. molesta as one of two Salvinia species to study in depth because of a striking contrast with its relative S. cucullata, a small aquatic fern with the tiniest genome of any known fern, just 250 million base pairs.
Using a combination of long-read sequencing and Hi-C chromatin mapping technology, they generated chromosome-level assemblies for both fern species.
What they found confounded expectations on both counts.
S. cucullata's genome is tiny, roughly 14 times smaller than the human genome, yet it contains 68 chromosomes, nearly four times more than scientists had predicted. And S. molesta, with a genome ten times larger than S. cucullata, has fewer chromosomes: 46, arranged in two distinct subgenomes that diverged from each other roughly 25 million years ago.
"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.
The results challenge a prevailing model of fern genome evolution and position Salvinia as a critical study system for understanding how reproductive biology shapes the genome.
The research was published in the Proceedings of the National Academy of Sciences .
