The origin of multicellular life is one of the most important milestones in earth’s history. And despite it happening independently nearly two dozen times in the past, very little is known about the way the initial evolution from unicellular to multicellular life had taken place. This is because these transitions occurred some 200 million years ago.
Contrary to the general perception that this important transition was challenging, and took a long time to happen, scientists have experimentally proved the ease with which this can take place. They achieved the transition in a yeast species in very short span of time — 60 days.
The multicellular yeast showed many key characteristics of a truly many-celled organism.
“The first crucial steps in the transition [can take place] remarkably quickly under an appropriate selective condition,” the scientists write in their paper published recently in the Proceedings of the National Academy of Sciences .
Organisms, both unicellular and multicellular, have to adapt to changing conditions like temperature, pressure, nutrient supply, oxygen content etc to survive. For instance, failure to adapt to changing climatic conditions resulted in the extinction of dinosaurs.
In this case, the scientists used gravity as a selection pressure as it was easy to observe, study and replicate in a lab using test tubes. Such a selection pressure is however not seen in nature. They used gravity to select for primitive multicellularity by allowing clusters of unicellular yeast to settle at the bottom. Clustering yeast settles faster than single cells, and bigger clusters settle faster than smaller clusters.
At the end of the day, those clusters that had settled at the bottom were separated and transferred to a new test tube. After repeating the cycle for two weeks, the researchers could see yeast forming into snowflake-like clusters.
Clusters do tend to form in nature by adhesion of cells. While cells in such clusters are genetically distinct, the clusters formed in the lab were found to be genetically identical. Genetically identical cells in a cluster could have formed only by division of mother cells into daughter cells.
Proof of division
The proof that the clusters were formed by the division of individual cells came through 16 hours of microscopic examination for growth. Cells taken from the clusters proved their hallmark characteristic — each cell giving “rise to a new snowflake-like cluster [cell].”
Cells did not divide at random. While cells in the juvenile stage grew rapidly to multiple cells, and hence helped in increasing the size of the cluster, the fully-grown adult stage was marked by division of the matured cells into daughter cells. The presence of both juvenile and adult stages is a mark of true multicellularity.
The fact that single-celled yeast “sacrifices” its ability to reproduce for the good of a collection of cells makes the transition very challenging. It goes against the grain of Darwinian principles.
The scientists also investigated the most vital and crucial question that has been dogging science — transition from unicellular to multicellular life. The most important difference between unicellular and multicellular life lies in the size of the daughter cells. While unicellular yeast divides into two daughter cells of similar size as the parent cell, the daughter cells of multicellular yeast “were consistently half the size of their parental clusters [cells].”
Division of labour
Division of labour between individual cells — another important characteristic of higher order organisms — was seen in the yeast snowflakes. Such is the importance of this characteristic that higher-order organisms have clearly demarcated functions carried out by a specific set of cells. In fact, as the authors write, “cellular differentiation is a hallmark of complex multicellularity.”
Similarly, apoptosis or programmed cell death (where old cells die after a point of time) was witnessed. Though apoptosis is seen even in single-celled yeast and other species, the end purpose of apoptosis witnessed in snowflakes was quite different.
It was in response to selective pressure — apoptotic cells breaking off from the snowflakes and allowing the rest of the flake to produce greater number of cells within a given time. Bigger clusters settle faster at the bottom and hence become eligible for repeated studies.
For instance, apoptosis had evolved so quickly between selection 14 and 60 that the snowflakes at selection 60 were much bigger than that of at 14. This kind of apoptosis has never before been seen in unicellular yeast.
All these characteristics seen in the snowflakes “demonstrate that multicellular traits readily evolve as a consequence of among-group selection [selective pressure],” the researchers write.