When a cell divides, the genome has to be duplicated, then the two copies separate, with one going to each daughter cell. But there have been whole genome duplication events in organisms in which both copies of the genome have stuck around. Whole genome duplication (WGD) events are usually associated with evolution. Some research has suggested that in the early evolution of vertebrates, there were two cases of WGD. But scientists still don't know much about why WGD happens, what makes it persist, and how it promotes evolution. Now, scientists have gained insight into some of those mysteries. Reporting in Nature, researchers have revealed more how WGD happens and how it stabilizes over thousands of generations.
The Multicellular Long-Term Evolution Experiment (MuLTEE) began in 2018 in the lab of William Ratcliff, a Professor at Georgia Tech. The investigators used Saccharomyces cerevisiae, or snowflake yeast, to explore the mechanisms of evolution in an organism that went from a single cell to form complex, multicellular organisms. The researchers encouraged the evolution by selecting the largest yeast cell every day.
When the yeast was about 1,000 days old, the researchers noticed that it may have transitioned from a diploid organism, which has two sets of chromosomes, to a tetraploid organism, which has four sets. But tetraploidy is not usually stable and within several hundred generations, it tends to revert to diploidy, so the scientists were skeptical. But work showed that the duplication of the yeast genomes happened very early, within only about fifty days of the start of MultEE. So the tetraploid state had lasted for far longer than thought possible – about 1,000 days.
WGD seems to have happened because the yeast with the extra genetic material could grow much bigger cells and make larger multicellular groups. This was all favored when the researchers selected the larger cells.
"We set out to explore how organisms make the transition to multicellularity, but discovering the role of WGD in this process was completely serendipitous," said Ratcliff, who is senior study author. "This research provides new insights into how WGD can emerge, persist over long periods, and fuel evolutionary innovation; that’s truly exciting."
Although WGD does not usually persist in snowflake yeast, the survival advantage it conferred upon cells changed that, and made it stable. Then, more changes happened in the yeast, and the unusual number of chromosomes seemed to be crucial to the generation of multicellularity.
This type of research highlights the unexpected findings that can come from experimentation; researchers often do not anticipate the answers that will come from different projects. Sometimes those answers are to questions that were not originally asked. This is one reason why funding basic research science is so important, even though the value or potential applications of the work may not be readily apparent.
“Scientific progress is seldom a straightforward journey. Instead, it unfolds along various interconnected paths, frequently coming together in surprising ways. It's at these crossroads that the most thrilling discoveries are made,” noted co-corresponding study author Kai Tong, Ph.D., formerly of the Ratcliff lab and now a postdoctoral fellow at Boston University.
Sources: Georgia Institute of Technology, Nature
