Ferns are rare. They are green and leafy like other forest plants, but they reproduce more like fungi, releasing clouds of spores. Many species do not require a mate for fertilization, unlike most of their seed-producing cousins. Recent studies estimate that ferns diverged from seed plants about 400 million years ago.
And fern genomes are disconcertingly large. Yet despite the unique physiology of ferns and their relationship to seed plants, these strange genomes have been largely neglected by researchers. Until recently, only two (relatively small) fern genomes were fully sequenced, compared to more than 200 flowering plant genomes. Now, the first complete tree fern genome has been successfully sequenced, that of the flying spider monkey tree fern, suggesting how these peculiar plants amassed such a massive set of genes.
“If you want to understand the origin of seeds or flowers, ferns are a very important comparison,” says Fay-Wei Li, a fern biologist at Cornell University’s Boyce Thompson Institute and a co-author of the new study, published today. in Nature plants. “But what I really want to know is why are fern genomes so big?”
Li’s team found that the palm-shaped fern has more than six billion DNA base pairs, one billion more than the average flowering plant genome (humans, by comparison, have about three billion). of pairs). The new analysis suggests that more than 100 million years ago, an ancestor of this fern duplicated its entire genome, a replication error that is common in plants, Li says.
But it’s not clear why tree ferns would maintain so much genetic material; most flowering plants return to slimmer genomes after duplications. This species could be accumulating chromosomes, Li says: “I call this the Marie Kondo hypothesis. Chromosomes bring joy to ferns, but not to seed plants.” For plants that reproduce asexually, he says, a large genome can add opportunities for beneficial mutations to occur while buffering undesirable ones. Ferns are also long-lived, so thus they evolve more slowly, which may have contributed to the retention of genetic material.
Using the fully sequenced genome, the researchers also found which genes build the fern’s unusual trunk-like stalk, valuable insight into how key traits in stem-bearing plants evolved, says Jan de Vries, a plant evolutionary biologist at the University from Göttingen in Germany, who was not involved in the study. “Evolution is a bungler. Shedding light on what viable molecular programs have evolved tells us what is biologically possible and where the limitations are,” he says. “Using this knowledge, we can start tinkering with synthetic biological ends.”