Flowering plants owe their success to downsized DNA

Somewhere between 120 and 150 million years ago, flowering plants started taking over the world, leading to the dazzling diversity of flowers we have today. To figure out why, according to new research by a San Francisco State University biologist, you need to think small. Now even smaller.

At a critical period in history, flowering plants downsized their genomes — the collection of DNA in each cell — allowing them to make smaller cells and build leaves with more delicate and complicated structures. That likely gave them an edge over their nonflowering cousins. “Flowering plants are the most important group of plants on Earth, and now we finally know why they’ve been so successful,” said Assistant Professor of Biology Kevin Simonin, one of two authors on the study published today in PLoS Biology.

Before the rise of flowers, the plant world was ruled by ferns and another group that includes modern pine trees. But flowering plants took root quickly and now make up almost 90 percent of all plant species. Charles Darwin himself was perplexed by their rapid diversification, and scientists ever since have been offering potential reasons for it, ranging from the influence of pollinators to a reshuffling of genes. What’s been missing is an explanation of just how flowering plants became so successful in the first place. 

Pulling together data from throughout the published literature, Simonin and his collaborator, Adam Roddy at Yale University, showed that flowering plants went through a major genome downsizing as they evolved, while the genomes of their more primitive relatives remained about the same. Smaller genomes means the option to make smaller cells. And with those smaller building blocks, the team showed, flowering plants can construct more complicated networks of veins to keep their cells hydrated and more pores in their leaves to draw in the carbon dioxide they need to make food.

The researchers then lined up those innovations with the plant family tree. Sure enough, flowering plants started downsizing their genomes just around the time they started taking over the world. Having a greater variety of cellular building blocks apparently gave them the edge they needed to rise above the ferns and pines. It’s a good thing, too. Flowering plants give us much of our food and also feed many of the animals around us.

Simonin began his path to an answer from an odd direction. While he was teaching an introductory biology course at San Francisco State, a student asked: “Do whales have big genomes?” They don’t — but the student’s question got Simonin thinking about his research on plant physiology in a different way. “It sent me down this whole path of genome-size research,” he said. “It reinvented the research in my lab in many ways.”

That question is still driving the research in Simonin’s lab. He plans to explore some groups of flowering plants that haven’t downsized their genome — it may be that in some environments where photosynthesis is difficult, there’s no pressure for plants to create smaller cells. If he’s right, it would mean more support for the relationship between genome size and the success of flowers and another nail in the coffin for what Darwin once called “an abominable mystery.”