Humans, like many complex organisms, have large genomes that contain the code of life. Want to describe your dark hair, thin bones, and existential dread? Look at 46 chromosomes and 3 billion nucleotide base pairs.
However, these numbers are incomparable with genomes from other organisms containing twice as many base pairs and three times as many chromosomes. Are you an octopus? elephant? Orca? No, it’s a flying spider monkey fern.
A hardy plant native to Southeast Asia, the flying spider monkey tree fern, with a circular spread of flat leaves on the upper trunk, is just one of many spore-emitting plants with a huge genome. What Fei Wei Li, a botanist at the Boyce-Thompson Institute, calls “the biggest problem in fern genomics” is the one that requires or explains so much DNA.
In May, Dr. Lee led the team, arrayed We’re looking at the whole genome of the flying spider monkey tree fern to get answers.was only Third time The fern’s DNA has been fully mapped and this is the first time a fern with such a large genome has been sequenced.last week 2 more In a paper published in Nature Plants, Ginkgo biloba and ‘C-Fern’ (Ceratopteris richardii, a fern often used as a model organism in laboratories) have genomes comparable in breadth to the flying spider monkey tree fern. revealed to have.
This explosive study has taken years to come and challenges half-century-old assumptions about fern genes. And while the case for the fern genome can’t be closed, “it could tell us a lot about the evolution of the genome as a whole,” says a botanist at Stanford University, who said he’d like to sequence C-Fern. Blaine Marchant said he led.
Smithsonian Institution botanist Eric Schuetpeltz, who wasn’t involved in the recent study, said, “For decades we’ve been begging, ‘Fern, we gotta make this happen.’ “These are really exciting times.”
It is not entirely clear why some organisms have larger genomes or more chromosomes than others. Plants and animals with many genes are not necessarily physically or behaviorally complex. The current record holder for most base pairs (149 billion) is a flowering plant with the scientific name Paris japonica. The record holder for most chromosomes (1,440) is Adartan fern. Both plants are small and biologically simple.
One widely held explanation for large genomes is called polyploidy or whole-genome duplication. Normally, during reproduction, two gametes (cells with half the number of original chromosomes) come together to create a zygote with the full suite of genes. However, when these gametes first form, the pair of chromosomes may not be completely separated, resulting in a zygote that is twice as large as her in the parental genome. This seems to have happened early in the evolution of flowering plants, but most of the duplicated genes were removed after tens of millions of years of natural selection.
Although closely related to flowering plants, ferns have about 20% more base pairs in their genomes. For years, scientists have wondered why this is so. And in 1966, A paper was published in Science Ferns, many of which reproduce asexually, claim to have gained an evolutionary advantage through genome duplication. Essentially, the extra genes provide spare chromosomes that help prevent inherited diseases, the authors argued.
Pamela Soltice, a botanist at the Florida Museum of Natural History who helped sequence C-Fern, said it was “a truly influential and highly creative paper.” But did the fern genomes actually contain signs of mass duplication, or were they simply larger? Confirming the theory would require sequencing portions of these large genomes. had.
Finally, this year it happened — and sequencing showed no evidence of polyploidy. “In fact, there is only evidence of two overlaps in this lineage, potentially dating back hundreds of millions of years.”
C-Fern seems to have acquired its large genome primarily from repetitive DNA and transposable elements (“jumping genes” that often move within the chromosome), but their function is poorly understood. For Dr. Soltis, sequencing has put an end to the long-held hypothesis of fern polyploidy. “I think this is the coffin nail for that,” she said.
However, Dr. Lee was not so convinced. His DNA from the flying spider monkey tree fern contains evidence of whole-genome duplication about 100 million years ago, and the genome has been remarkably stable ever since. Although this is an isolated case, it seems to reinforce the idea that polyploidy gave plants an evolutionary leg. ‘ he said.
Dr. Schuettpelz said: Things are going to get exciting really fast as more and more genomes accumulate, and genomes that are more representative of the whole fern. ”
This is what all fern scientists agreed on. “The more fern genome assemblies to compare with, the more informative the inferences will be,” Dr. Marchant said.
Dr. Soltis said: And these he didn’t really have a reference until the genomes of the two large ferns were published. ”
Dr. Soltis has been involved in recent genome sequencing efforts. every known life form on earthThe project is ambitious, she admitted, but so is science. We need to find out what happened and what the situation was before taking on the current function,” she said.
Dr. Lee added: We need more genomes. ”