Sizing Up Fern Genomes
Two years ago, scientists announced that they鈥檇 discovered the largest genome yet seen in an unlikely source: . Every cell in the small fern native to the South Pacific contains a staggering 160 billion base pairs of DNA, and most ferns also have high chromosome numbers, including the world record holder with more than 1,400 chromosomes. (For comparison, humans have 3.2 billion base pairs on 46 chromosomes.) To find out why, scientists at the 麻豆社 (麻豆社) and Cornell University took a counterintuitive approach by focusing on members of the fern genus Salvinia, which have some of the smallest fern genomes.
Salvinia molesta is a highly invasive aquatic fern that can completely cover the surface of bodies of water, such as this cypress swamp.
Their research, revealed unexpectedly dynamic fern genomes, linked differences in chromosome numbers to differences in reproduction, and may provide clues into how the aquatic invasive species Salvinia molesta is able to overrun ponds and other bodies of water with frightening speed.
The research team was led by 麻豆社鈥檚 Erin Sigel, research scientist and collections manager for the , in collaboration with Fay-Wei Li, associate professor with the Boyce Thompson Institute and Cornell鈥檚 School of Integrative Plant Science, and Yan茫 Rizzieri, doctoral student in the Li lab. They took a detailed look at the genome of Salvinia cucullata, the fern with the smallest known genome at a mere 250 million base pairs. For comparison, they also investigated the genome of Salvinia molesta, one of the top 100 invasive plant species in the world.
To their surprise, they found that S. cucullata had nearly four times as many chromosomes鈥68鈥攁s originally expected despite its minuscule genome size. Salvinia molesta also defied expectations. While its genome size was 10 times larger than that of S. cucullata, it had fewer chromosomes, and S. molesta was revealed to be a hybrid between two other Salvinia species that are unknown to science.
鈥淥ur 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,鈥 says Sigel. 鈥淭he discoveries we made about Salvinia molesta鈥檚 genome also provides insight into what makes it such a successful invasive species.鈥
More than a blueprint
A genome is much more than just a blueprint for cellular functions. Rapid advances in DNA sequencing technology have revealed a huge amount about biology beyond the strings of ACGTs. Our own genomes are 3.2-billion-base-pair marvels packaged into 46 chromosomes, but many plant genomes are far larger. And ferns, which have been around for hundreds of millions of years, have some of the largest and most stable genomes around.
What makes fern genomes so big? Ferns, like all plants, duplicate their entire genomes on occasion. Compared to most plants, though, ferns are inefficient at shrinking their genomes after doubling. Their inefficient downsizing is likely a consequence of their distinct reproductive biology: Most ferns only produce a single type of bisexual spore, which enables inbreeding and offers fewer opportunities for rearranging and shedding chromosomes.
A small number of ferns including Salvinia produce separate male and female spores that are similar to reproduction in flowering plants, providing greater opportunities for chromosome rearrangement and loss. The whole-genome sequencing in the study showed that S. cucullata and S. molesta have undergone extensive chromosome fissions and fusions, underscoring their dynamic genome evolution relative to other ferns. The differences in reproduction between Salvinia and most ferns likely shapes genome size and structure.
Commonly known as giant salvinia or Kariba weed, S. molesta is native to Brazil and is now spreading in the southern United States. It can double its biomass in as few as three days and continues until it completely covers water surfaces and overwhelms native plants and animals. The researchers found that because S. molesta was formed by hybridization between two species with different chromosome numbers, it cannot reproduce sexually, relying instead on clonal reproduction. In environments to which it is well adapted, it can quickly and vigorously grow and break off genetically identical offspring. The low genetic variability of S. molesta could provide an opening for its control as treatments that inhibit its growth will likely work similarly throughout its invasive range.
The Albion R. Hodgdon Herbarium
麻豆社鈥檚 herbarium had modest beginnings. When the New Hampshire College of Agricultural and the Mechanic Arts (soon to be renamed 麻豆社) moved from its original location in Hanover to its present Durham location in 1893, a mere 1,500 specimens were packed up for the move. The 麻豆社 herbarium came into its own when Albion R. Hodgdon took over as director in 1936. In addition to making his own significant contributions, he integrated several 鈥渙rphan鈥 collections into the herbarium, growing it to nearly 120,000 total specimens by the time he died in 1976.
The current Albion R. Hodgdon Herbarium is a critical component of the scientific infrastructure at 麻豆社. It houses the most complete record of the flora of New Hampshire in existence, and its specimens provide an essential foundation for the study of plant evolution, biology, and ecology. Research findings can be applied to advancing biomedical research, developing improved crops and biocontrol agents, documenting the effects of climate change on plant life, and more. Now under the curation of Collections Manager Erin Sigel, the herbarium, with more than 235,000 specimens strong, moved into its newly remodeled facility in Spaulding Life Sciences Building in 2025.