Navigate like a choanoflagellate
The closest living relatives of animals actively move toward sources of oxygen.
Most animals are made up of millions of cells, yet all animals evolved from ancestors that spent their whole lives as single cells. Today the closest single-celled relatives of animals are a group of aquatic organisms called choanoflagellates. Certain species of choanoflagellates can also form swimming colonies. This kind of multicellularity might resemble that seen in the earliest of animals. As such, studies into modern-day choanoflagellates can give insights into how the first animals to evolve might have behaved.
Many organisms can find their way towards favorable areas using different strategies. For instance, bacteria can bias their tumbling to gradually swim towards food, and algae can turn and move directly toward light. While choanoflagellates require oxygen, it was not known if they could also actively navigate towards it, or any other resource.
Now, Julius Kirkegaard and co-workers find that the choanoflagellate Salpingoeca rosetta can indeed navigate towards oxygen — an ability called aerotaxis. This was true for both individual cells and for colonies made up of many cells. This discovery suggests that the transition from living as a single cell to living as a simple multicellular organism could still have allowed the earliest animals to seek out and move towards resource-rich areas.
Aerotaxis requires cells to both sense oxygen and react appropriately to changes in its concentration. Kirkegaard and co-workers watched choanoflagellate colonies swimming under controlled conditions and varied the oxygen concentration in the water over time. These experiments revealed that the colonies navigate based on the logarithm of the oxygen concentration, so that at low oxygen levels the cells were even more sensitive to small changes in oxygen concentration. This type of ‘logarithmic sensing’ is similar to how our ears sense sounds and our eyes sense light. Kirkegaard and colleagues went on to conclude that the colonies were not actively steering in the correct direction. Instead, the colonies appeared to choose directions at random and later decide whether such a turn was correct.
It remains unclear whether the common ancestor of animals and choanoflagellates could also perform aerotaxis, and if so what mechanisms this involved. Further studies to compare aerotaxis and aerotaxis-related genes in simple animals and other single-celled relatives of animals would be needed to illuminate this. Future studies could also explore the maximum and minimum oxygen concentrations that choanoflagellates can detect, and how well they navigate at these upper and lower limits.
To find out more
Read the eLife research paper on which this eLife digest is based: “Aerotaxis in the closest relatives of animals” (November 24, 2016).