A recent study reported in the journal Science described witnessing a case of spontaneous association of yeast and algae cells that seems to be an example of free-living species becoming obligate mutualists, which is an example of the beginning of cooperation in evolution. This case of mutualistic symbiosis was of interest because it was not based on previous co-evolution between the two species. An associated article was published in the same issue of the journal offering a historical perspective on how this discovery may be a view of the birth of cooperation in evolution and how mutualistic symbiosis can shape the course of evolution.
Mutualistic symbiosis are mutually beneficial associations between individual organisms of different species and the evolution of mutualistic symbiosis are thought to have contributed to some of the major transitions that have occurred in the evolution of life on earth. The evolution of chloroplasts in plants, which are the cellular components that are responsible for photosynthesis, and the movement of plants from the seas to the land are considered to have been largely dependent on symbiotic activities.
In this observed case of the development of a symbiotic relation between yeast and algae cells, the relationship was formed according to “ecological fitting.” Ecological fitting means that there was no prior co-evolution between the two species and they formed their relationship because of both taking advantage of ecological opportunity. In the particular case that was observed, the two species of cells established a reciprocal carbon and nitrogen exchange. The yeast could use glucose as a carbon source and the algae could use the waste product of glucose metabolism, which was carbon dioxide, in the process of photosynthesis.
Regarding nitrogen, the algae used nitrite as a nitrogen source and the yeast used the conversion product of nitrite metabolism (ammonia). When both types of cells were cultured in a dish with glucose, the algae depended on carbon dioxide provided by the yeast and the yeast depended on the ammonia, which contained nitrogen, provided by the algae.
What is particularly interesting about this experiment with yeast and algae cells is the two species became completely dependent on each other, whereas when living in separate flasks they were independent. In addition, a surprising outcome was the morphology of the algae that formed the mutual relationship differed from that of the free-living forms of algae.
Another hypothesis for how mutualistic symbiosis events can arise is through co-evolution. The fact that this study showed ecological fitting as the basis for the event of cooperation in evolution does not rule out the possibility of mutualistic symbiosis events developing through co-evolution. It is highly likely that the complexity of ecosystems has evolved by many mechanisms, some of which may not even be imagined at this point. A question that was pointed out in the perspective article is how often ecological conditions promote the spontaneous appearance of mutualistic symbiosis versus enslavement by a host with or without a previous history of co-evolution. This question will need to be answered with further study.
Cases of mutualistic symbiosis can shape the course of evolution both on the level of species and ecosystems and are important in the development of biological complexity. This is an exciting time in the study of evolution because scientists are beginning to place more value on the study of cooperation in evolution, which certainly must be important in the development of ecosystems in life on earth.
Opinion by Margaret Lutze