New Family Tree Reveals Surprising Evolution of Flowering Plants

Almost every plant we eat has flowers, and flowering plants spread to all corners of the planet. But there are still many unanswered questions about how and when this broad group appeared in the history of life on Earth.

Now, after a heroic DNA sequence effort, a collaboration involving hundreds of scientists has created a new family tree for flowering plants. By comparing the gene sequences of more than 9,500 species – many of them dried specimens preserved in museums – scientists have revealed an important branching point in the evolution of flowering plant life. In a study published in April in the journal Nature, the data they present shows that more than 80 percent of the main lineages of flowering plants come from a sudden burst of innovation that started from about 150 million years ago, at the end of the Jurassic period.

Plant evolutionary trees previously constructed by scientists often used the genomes of chloroplasts, organelles that allow plants to carry out photosynthesis. This genome can be sequenced by older methods. But scientists cannot be sure that the patterns shown are the same as what might be revealed by the plant’s main genome, which is stored in the cell nucleus and is more difficult to study.

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 a new tool for sequencing a large array of flowering plants, said William Baker, who heads Kew Botanic Gardens’ Tree of Life Initiative and is an author of the new paper. .

They used these tools on live plants, but the team also collaborated with institutions in 48 countries with collections of dried plants to obtain samples from rare specimens. Four of the plants included in the analysis are extinct, including the Guadalupe Island olive, which has been classified using dry branches since 1875. At in the end, the team used data from about 60 percent of modern plant genes.

When they built the new evolutionary tree, they found that it confirmed many of the relationships suggested by the tree built with chloroplasts. However, there were surprises: the new data led to a reorganization of the relationships of several plant groups, and some individual species were reclassified.

One discovery that surprised plant experts concerned a group of flowering plants that is so common, it’s easy to take it for granted. Asteraceae, the family that includes flowers such as daisies and sunflowers, did not fit into the new evolutionary tree as the researchers expected. Depending on how the new data is used to build the tree, the daisy’s relationship with the surrounding flower family will shift, the researchers found.

“In the past, when similar findings were found, we usually blamed the lack of data,” said Alexandre Zuntini, a biologist at Kew Botanic Gardens and author of the paper .

“But today, with more abundant data than ever before, anomalies in the natural history of flowers cannot be ignored. Although no one can say what could be causing the anomaly, Dr. Zuntini suggests that one possibility is that a faster or worse evolution of the floral branch occurred at that time.

The researchers also tried to relate their evolutionary tree to known geological periods. By itself, there are no dates in the network of relationships revealed by DNA. It is difficult to say, therefore, how many years ago a pair of species began to diverge.

But many flowering plants are fossilized and can be dated. Using 200 flower fossils to date the genealogy, the team estimated a huge explosion in the diversity of flowering plants in the late Jurassic and early Cretaceous periods, when dinosaurs lived, beginning about 150 million years ago. This supports estimates made in the past, said Dr. Baker. Another explosion in the number of species occurred around 40 million years ago, according to the new evolutionary tree, amid falling global temperatures.

The team is sharing their sequencing tools, and they hope other researchers will take advantage of them. They also hope to add more species to this evolutionary genealogy in the future, said Dr. Baker, because more data means seeing with more resolution what happened in the past. Little by little, petal by petal, the history of flowering plants is beginning to become clear.