Unicellular Green Algae Captured in Real-Time Evolution, Confirming Theory on Multicellular Origins

One of the persisting questions in evolutionary biology is the transition of unicellular organisms into multicellular forms - and a new experimental study might have an answer.

Researchers led by Professor Lutz Becks from the Limnological Institute of the University of Konstanz in Germany, in a new breakthrough, were able to show how the unicellular green algae Chlamydomonas reinhardtii, were able to develop mutations that led to the first steps toward its transition to becoming a multicellular life form. They were able to achieve this with help from a collaborator from the Alfred Wagner Institute.

From being a unicellular organism, the green algae managed to make the first steps toward becoming a multicellular organism in just 500 generations. Details of the new study are published in the latest issue of Nature Communications, in an article titled "The evolution of convex trade-offs enables the transition towards multicellularity."

Confirming a Theory on the Origin of Multicellular Life

There are various theories on how multicellularity first evolved. According to online biotech portal Bionity, the most common theories involve those of symbiosis, development of colonies, or specialized cells developing divisions in their membrane that later evolved for specific functions. Other theories look at a combination of any of the three theories.

One proposed explanation for the emergence of multicellular organisms is that the evolution of cell groups and their evolution towards multicellularity is only possible when both cell groups are better at reproduction, having better survivors than single cells. This same theory, proposing multicellularity as an evolution of unicellularity, inspired Professor Becks' team in designing their experiment, with the first step having colonies of identical daughter cells that form without separating after division.

A critical aspect of the theory remains untested until now: that in the initial conditions, the colony of unicellular life with a higher chance of survival emerges. Next, these surviving colonies further adapt to increased production. Once these two conditions are met, the next steps toward specialized somatic and germ cell functions start to develop. The University of Konstanz team experimentally tested which conditions cause the transition from unicellularity to the formation of colonies.

Observing Cell Specialization and Evolution toward Multicellularity on the Genome Level

To check which colonies have a higher chance of survival and a higher reproduction rate, researchers created selection pressure on the populations by adding a predator, which in this case, is a multicellular rotifer - a microscopic aquatic animal. In this experimental setup, a lone cell of the green algae is left unprotected while those that develop mutations that cause them to grow in colonies allow them to stick together and have higher rates of survival.

The green algae C. reinhardtii, which is used in this study, comes from a group of algae where different stages of its evolution toward multicellularity can be found. Additionally, these green algae in different stages all come from a common unicellular ancestor. The pre-requisites for verifying the evolution theory were met, and that the colonies were observed in real-time.

As the researchers examined the properties of the green cell after 500 generations of the species, they found that colonies were more likely to grow in the media with predators and had significantly higher rates of reproduction compared to the colonies in the media with no predators. In a press release from the University of Konstanz, Becks explains that the distribution of surviving and rapidly reproducing colonies fits with the evolution theory they tested. "Not only have we shown that they exist, but also that they evolve repeatedly under certain conditions."