Avant Champlain...: juillet 2021

(...) Angiosperms evolved about 125 million years ago and now dominate many of Earth’s landscapes. They reproduce via seeds, as do the evolutionarily older gymnosperms, which include pine trees, ginkgoes, and others. But angiosperms evolved some key innovations for seed production that likely enabled their success. Their seeds form in the carpel, a tubular structure that sticks up from the center of a flower and matures into a pod that holds seeds—peas or beans, for example—inside.

The carpel grabs the pollen and transfers it to a chamber called an ovary, where seeds develop. Angiosperm seeds are encased in an inner and outer layer; the outer layer helps form the hard coat of a pea or the colored surface of a bean, for example.

Angiosperms evolved from gymnosperms, but how carpels and the second seed coat arose has been a big mystery. Fossils have yielded a few clues, but “nobody has been able to show where carpels come from,” says Michael Donoghue, a plant evolutionary biologist at Yale University who was not involved with the work.

(...) So Shi was intrigued in 2015, when a local from Inner Mongolia showed him a piece of rock with swamp plant fossils containing cupules with exquisitely preserved details.

(...) After comparing their new finds with other fossils with cupules, the researchers concluded that all belonged to a group of plants characterized by cupules of different types, they report today in Nature. Their existence suggests not only where the second seed coat came from, but also how carpels came to be, as some of these cupules appear to have modified leaves that could have evolved into carpels.

“It is very exciting to see a new link forged in the evolutionary chain from early seed plants to angiosperm,” says Charles Gasser, an emeritus plant developmental biologist at the University of California, Davis, who was not involved with the research. “The overall path [to flowering plants] is now much clearer.”