Let’s stick together
How did animals evolve from single-celled organisms?
All animals descended from a common ancestor that made the leap from living as a single cell to becoming something more complicated, with many cells working together. At first, such a creature would likely have been made from clusters of cells that all had the same function. Eventually, however, different cells took on different roles, and today animals have many organ systems, each made up of specialized cell types. How the ancestors of animals transitioned from being single celled to multicellular, however, is poorly understood.
It is now possible to reconstruct key steps in the evolution of ‘multicellularity’ by comparing modern animals with their closest living relatives — the choanoflagellates. These are a group of aquatic micro-organisms that can either live as single cells, or develop into multicellular colonies. The genes that allow choanoflagellate cells to form colonies are thought to be similar to the genes that the very first animals used to become multicellular organisms.
Now, Tera Levin and colleagues have studied a choanoflagellate called S. rosetta. This species is a good choice, as its genome sequence has been decoded and because it is relatively easy to induce S. rosetta cells to switch between living on their own and living in spherical colonies called rosettes.
Using a technique known as “forward genetics”, Levin and colleagues bombarded S. rosetta cells with chemicals and X-rays to introduce genetic mutations into the cells. The mutated cells were then grown in conditions that would normally cause S. rosetta to form rosette colonies; the cells that continued to live in isolation in these conditions were then studied further as this meant that mutations had occurred in the genes responsible for colony formation.
Levin and colleagues identified several mutant S. rosetta strains that cannot form rosettes. One of these mutant strains had an altered copy of a gene that was given the name rosetteless. The protein produced by the rosetteless gene is similar to proteins that connect animal cells to one another in tissues and organs. Normally in S. rosetta, this protein is found on the outside of cells, at the points where the cells join to each other to form a colony. In the Rosetteless mutants, the protein is often incorrectly made, and typically ends up on the wrong part of the cell. Levin and colleagues further confirmed the importance of the rosetteless-encoded protein by creating antibodies that stick to, and interfere with, the protein’s function. This also prevented normal S. rosetta cells from forming colonies.
Unraveling the role of the rosetteless gene is an important step towards understanding which genes made it possible for single-celled organisms to evolve into complex multicellular animals. Future genetic screens in S. rosetta promise to reveal whether rosetteless is part of a network of genes and proteins which regulate animal development, and could thus illuminate the molecular machinery behind multicellularity in the long-extinct predecessors of animals.
To find out more
Read the eLife research paper on which this story is based: “The rosetteless gene controls development in the choanoflagellate S. rosetta” (October 9, 2014).
Read a commentary on this research paper: “Multicellularity: Forward genetics for back-in-time questions”.
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