Dividing cells. Photo by George Von Dassow. Source: http://www.cell.com/cell_picture_show-cellcycle
Dividing cells. Photo by George Von Dassow. Source: http://www.cell.com/cell_picture_show-cellcycle

Another Path For Evolving Bodies

This post is an unexpected sequel to a post I published last month about how single-celled microbes can evolve into multicellular bodies.

Here’s a quick recap of that story. Life became multicellular at least a couple dozen times over the past few billion years. To explore the factors that drove life through these transitions, scientists at the University of Minnesota ran experiments with single-celled yeast. They gave the yeast time to settle in a flask and then drew out some fluid from the bottom. Repeating this many times created conditions in which the yeast quickly evolved  into snowflake-like clumps. Bigger clumps fell faster, providing a reproductive advantage over single-celled yeast, which drifted slowly to the bottom of the flask.

This week a team of scientists at Harvard published a study in the journal eLife on their own evolutionary experiments on yeast. They got to the same destination, but by a different route.

Yeast feed on sugar, but they start digesting their meal before it’s even inside them. Their cell walls are loaded with enzymes that break down sucrose into two smaller kinds of sugar, glucose and fructose. The yeast cell can then pump those small sugars into its interior. But a lot of that sugar diffuses away from the cell wall and away from the yeast.

This waste doesn’t matter much when yeast can gorge themselves on a dense soup of sugar. But when sucrose is scarce, losing so much sugar makes it very hard for yeast to grow. The Harvard scientists wondered what course evolution would take if they reared single-celled yeast on a such a meager diet.

They allowed yeast to grow in a flask supplied with only a little sucrose, and then they drew a little fluid to seed a fresh flask. The scientists repeated this for dozens of rounds, and ran a dozen separate trials on different populations of yeast. In eleven trials, the yeast evolved to form clumps.

The clumps typically took on the same snowflake-like structure seen in the University of Minnesota experiment I wrote about last month. And they developed in much the same way. When a cell budded off a new cell, its daughter remained attached rather than making a clean break.

To investigate these clumps, the Harvard scientists put them in a flask with their single-celled ancestors and let them compete for the sucrose. Every time the researchers ran the experiment, the multicellular clumps won, swiftly eliminating their ancestors. Their victory strongly suggests that natural selection was responsible for their evolution to clumps. But their transformation only gave them an advantage when they fed on sucrose. If the scientists added fructose or glucose to their diet, they lost their competitive edge.

The scientists then took a close look at the biochemistry of the evolved yeast. They gained an advantage partly from an improvement in how they fed. The evolved yeast produced more sucrose-digesting enzymes. They also made more proteins to transport the smaller sugars into their interior.

But multicellularity also helped them survive on scarce food. The reason a body provided an advantage  has to do with how yeast eat. As yeast cells break down sucrose, they release a lot of sugar into their immediate neighborhood. If another yeast cell is nearby, it can enjoy this meal for free–in other words, without using energy to make the enzymes to prepare the sugar. And if a lot of yeast cells live next to each other, they can collectively create a bigger buffet of sugar that they can all enjoy.

So in the space of a month, we have two studies that see the origin of multicellularity in the same species–but for two separate reasons. In one experiment, the advantage of falling fast provided the push. In another, it was surviving on scarce food.

It stands to reason that bodies might have provided different advantages to animals and plants and other multicellular organisms. For the ancestors of animals, a body might have made them better predators. For the ancestors of plants, a body might have offered them protection from those predators. It’s hard to study those different factors in this major transformation of life, since they took place hundreds of millions of years ago. So it’s a delight to find that scientists can observe different forces at work a single species, right under their noses.

(Update: I corrected the text to indicate the research took place at Harvard, not Oxford. One of the authors moved on from Harvard to Oxford.)