A new tree of flowering plants? For spring? unprecedented

Veronique Greenwood

New Delhi: Almost every plant we eat has a flower in it, and flowering plants are found in every corner of the planet. But many questions remain about how and when in the history of life on Earth this giant group emerged.

Now, after a heroic DNA sequencing effort, a new family tree for flowering plants has been created in collaboration with hundreds of scientists. Comparing the gene sequences of more than 9,500 species – many of them dried specimens preserved in museums – scientists have outlined important branching points in the evolution of flowering plant life. In a study published in April in the journal Nature, the data they presented shows that more than 80 percent of the lineages of major modern flowering plants arose as a result of sudden invention, starting at the end of the Jurassic period, about 150 million years ago. Had happened.

Previous evolutionary trees of plants created by scientists often used the genome of the chloroplast, the organelle that allows plants to photosynthesize. These genomes can be sequenced using older methods. But scientists could not be sure that the patterns they showed were the same as those that could be revealed by the plant's primary genome, which is stored in the cell's nucleus and is more difficult to study.

Then, five years ago, another scientific collaboration published detailed information about the nuclear genomes of more than 1,100 plant species. This allowed the team behind the Nature paper to design new tools for nuclear gene sequencing from a variety of flowering plants, said William Baker, who leads the Kew Gardens Tree of Life Initiative and is an author of the new paper.

They used the instruments on living plants, but the team also reached out to institutions in 48 countries with dried plant collections to obtain samples of rare specimens. Four plants included in the analysis are already extinct, including the Guadalupe Island olive, which was sequenced using a dry twig dated 1875. In the end, the team used data from about 60 percent of all modern species of plants.

As they put together the new evolutionary tree, they found that it confirmed many of the relationships suggested by trees made from chloroplasts. However, there were surprises: new data reshuffled the relationships of many plant groups, and some individual species were reclassified.

A discovery that has surprised plant experts belongs to a group of flowers that are so common that it is easy to take them for granted. Asteraceae, a family that includes daisies and sunflowers, did not fit into the new evolutionary tree as well as researchers expected. The researchers found that depending on how the new data was used to create the tree, the daisies' relationships with surrounding flower families would change. “In the past, when similar results were found, we blamed lack of data,” said Alexander Zuntini, a Kew Gardens biologist and author of the paper.

But now, with data less scarce than before, such anomalies in the natural history of flowering cannot be easily dismissed. Although no one can say what might have caused this anomaly, Dr. Zuntini suggests that one possibility is that there was rapid or disturbed development of the flowering branch at the time.

The researchers also attempted to link their evolutionary tree to known geological eras. In itself, the network of relationships shown by DNA has no date. Again, it is difficult to say how many years ago a pair of species began to diverge.

But many flowering plants have been observed in fossils that can be dated. Using 200 fossils of flowers to add dates to the lineage, the team pointed to a huge explosion in flowering plant diversity in the late Jurassic and early Cretaceous periods, when dinosaurs lived about 150 million years ago. This supports estimates made in the past, Dr. Baker said. The new tree shows that another increase in the number of species occurred about 40 million years ago, amid declining global temperatures.

The team is sharing their sequencing tools, and hopes other researchers will use them. They also hope to add more species to this evolutionary lineage in the future, Dr. Baker said, because more data means a higher-resolution look at what happened in the past. Slowly, petal by petal, the history of flowering plants comes into focus.

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