Did Scientists Evolve a Multicellular Organism from a Single-Celled Organism?
by Rich Deem

Multicellularity?

Scientist from the University of Minnesota claim to have induced single-celled yeast to become multicellular organisms, simply through artificial selection. Are these yeasts really multicellular?

Rich Deem

Introduction

For billions of years, life on earth consisted solely of single-celled microorganisms. Originally these microorganisms were prokaryotes (bacteria and Archaea), followed then by eukaryotes (cells with nuclei). Multicellular eukaryotes did not appear until 1,000 to 650 million years ago. The fact that it took about three billion years to go from unicellular to multicellular organisms suggests that the transition was not a trivial one. However, scientists have announced that they have accomplished the same task in a mere 60 days.1 Or did they?

Yeast is unicellular?

Authors of the study used common brewer's yeast (Saccharomyces cerevisiae), which normally grows as individual cells. The cells reproduce by budding, so that two daughter cells are produced from one cell. What the authors only briefly mentioned, and cursorily dismissed as being irrelevant, was the fact that Saccharomyces cerevisiae will grow in a clumped manner, forming "pseudohyphae" when grown under adverse conditions. In fact, when grown on the ammonium-deficient medium SLAD, 56% of Saccharomyces cerevisiae strains showed a filamentous phenotype2 (left side of figure to right). So, it is pretty clear that these yeasts could already grow as "multicellular" structures even before scientists started selecting for that trait.

Experimental methods and results

Authors of the study took Saccharomyces cerevisiae and subjected them to a simple selection protocol, consisting of a requirement for rapid sedimentation. Cultures were grown as usual in broth with shaking (for aeration), but once a day cultures were allowed to settle (larger clumps settle first) and the bottom 1% of the culture was reseeded into a new culture. Within a few generations, the cultures consisted almost solely of "snowflakes" (branching clumps of cells that would settle quickly, see figure above). There are two basic mechanisms by which the cells might have become "multicellular." One is by increased clumping after budding and the other is a lack of separation after budding. Visual observation revealed that the mechanism was a lack of separation after budding. Their conclusion was that multicellularity could evolve in two months!

Differentiation?

The main hallmark of multicellularity is the differentiation of cells into divergent cell types that accomplish specialized functions. In the article, the authors claimed that "the snowflake phenotype exhibits juvenile/adult life stage differentiation." Figure 3b, which was purported to show this, merely showed cluster sizes as a function of settling selection protocol. The paper never demonstrated any biochemical or genetic/epigentic differences between so called "juveniles" and "adults" (obviously, the "juveniles" were smaller). In addition, their marker of "differentiation" was apoptosis (programmed cell death). In multicellular organisms, some cells are programmed to die as other differentiate into particular cellular phenotypes. However, apoptosis is not a marker of differentiation, since cells that undergo apoptosis die (death ≠ differentiation). In fact, the researchers never really even measured apoptosis. They merely stained the colonies with dihydrorhodamine 123 (DHR), which stains for the presence of reactive oxygen species. So they didn't even make sure that there was programmed cell death (by measuring chemical pathways that lead to apoptosis). There is more than one way in which cells within such "snowflakes" can become stressed and die. You will recall that all these cultures were shaking during their growth. Usually, this is done using an orbital shaker that "spins" the media in the cultures at speeds of at least 200 rpm. The supplemental methods3 say that the cultures were "shaking at 250 g" (Obviously, this is an error, most likely meaning 250 rpm). Such shaking results in sheer forces, which will damage cells near the middle of the "snowflake" as it is contorted by the forces of the rapidly moving culture medium. The larger the colonies become, the stronger the sheering forces that will act on it. The damaged cells will die, resulting in the release of a "juvenile snowflake." To test this hypothesis, the authors should have grown the cultures without shaking in containers that had a large surface to volume ratio (to enable aeration of the culture). Such cultures should show reduced "apoptosis" compared to shaken cultures. If so, they could be subjected to brief shaking to verify that such shaking was the cause of the cell death. So, I believe there was little, if any, real apoptosis happening in their "snowflakes."

Genotypes

The authors of the study used the word "genotypes" at least a few dozen times, even though they did zero genetic analysis. The proper term that describes their study would be "phenotypes" (observable traits that may or may not be due to genotypic differences), which they also used a couple dozen times. It's almost as if parts of the article were written by different people—one of whom did not understand what the word "genotype" really meant. Just because they isolated individual snowflakes does not mean that they had "genotypes." It is entirely possible that the original phenotype is genetically identical to the snowflake phenotype (differences could be due to epigenetic changes or changes in RNA expression). Without any kind of genetic analysis, it is impossible to conclude that any genetic changes were responsible for the phenotypic changes observed.

Alternative interpretations

Authors of the study were so blinded by their belief in macroevolution that they failed to consider the most obvious explanation for the results they observed. Wildtype Saccharomyces cerevisiae already have the ability to grow in a clumped manner by forming pseudohyphae. Granted, the morphology of the "snowflakes" is quite unlike that observed in pseudohyphae. However, this does not mean that Saccharomyces cerevisiae could not have appropriated at least some of the genes involved in pseudohyphae formation to form "snowflakes." Numerous studies are available in the literature that describe the genes involved in pseudohyphae formation in Saccharomyces cerevisiae.4 To conclude that Saccharomyces cerevisiae evolved new genes in a few days of selection in the laboratory is beyond credulity.

Conclusion Top of page

Scientists from the University of Minnesota want us to believe that multicellularity, which took billions of years to appear on earth, can evolve in a few days under simple laboratory selection. Instead of using modern techniques of genetic sequencing and gene array expression analysis, these scientists merely observed clumps of Saccharomyces cerevisiae to conclude that they had "differentiated" into "adult" and "juvenile" populations. They pretended to measure "apoptosis" through staining with DHR, which actually tests for the presence of reactive oxygen species. It is much more likely that sheering forces present during the shaking of the cultures was responsible for cell death near the middle of colonies as the branches were bent back and forth. Such a scenario would have resulted in cell death and eventual release of portions of the colony. These alternative explanations are easily testable and would likely invalidate the interpretation of the data offered by the scientists who published the study.



Related Materials Top of page

Origins of Life: Biblical and Evolutionary Models Face OffOrigins of Life: Biblical and Evolutionary Models Face Off by Fazale Rana and Hugh Ross. Probably the single most potent scientific argument against atheism is the problem with a naturalistic origin of life. This very problem led me to become a deist as a biology major at USC in the early 1970's. The problems for atheists have gotten no better since that time. In fact, the last 30+ years of research have turned up even more problems than those that existed when I first studied the theories. Fuz Rana (a biochemist) and Hugh Ross (an astrophysicist) have teamed up to write the definitive up-to-date analysis of the origin of life. The book examines the origins of life from the perspectives of chemistry, biochemistry, astronomy, and the Bible. A biblical creation model is presented along side the naturalistic models to help the reader decide which one fits the data better.


References Top of page

  1. Ratcliff, W. C., R. F. Denison, M. Borrello, and M. Travisano. 2012. Experimental evolution of multicellularity. PNAS: 1115323109v1-201115323..
  2. Casalone, E., C. Barberio, L. Cappellini, and M. Polsinelli. 2004. Characterization of Saccharomyces cerevisiae natural populations for pseudohyphal growth and colony morphology. Research in Microbiology 156: 191200.
  3. Supporting data for Ratcliff et al. 10.1073/pnas.1115323109.
  4. Gimeno, C. J. and G. R. Fink. 1994. Induction of pseudohyphal growth by overexpression of PHD1, a Saccharomyces cerevisiae gene related to transcriptional regulators of fungal development. Mol Cell Biol. 14:2100-2112.
    Wan-Sheng, L. and A. M. Dranginis. 1998. The Cell Surface Flocculin Flo11 Is Required for Pseudohyphae Formation and Invasion by Saccharomyces cerevisiae. Mol. Biol. Cell 9: 161-171.
    Gancedo, J. M. 2001. Control of pseudohyphae formation in Saccharomyces cerevisiae. FEMS Microbiology Reviews 25: 107123.
    H. Liu, C. A. Styles and G. R. Fink. 1996. Saccharomyces cerevisiae S288C Has a Mutation in FLO8, a Gene Required for Filamentous Growth. Genetics 144: 967-978.
    Gavrias, V., A. Andrianopoulos, C. J. Gimeno, and W. E. Timberlake. 1996. Saccharomyces cerevisiae TEC1 is required for pseudohyphal growth Molecular Microbiology 19: 12551263.
    M. Lussier, A. M. White, J. Sheraton, T. di-Paolo, J. Treadwell, S. B. Southard, C. I. Horenstein, J. Chen-Weiner, AFJ. Ram, J. C. Kapteyn, T. W. Roemer, D. H. Vo, D. C. Bondoc, J. Hall, W. Wei Zhong, A. M. Sdicu, J. Davies, F. M. Klis, P. W. Robbins and H. Bussey. 1997. Large Scale Identification of Genes Involved in Cell Surface Biosynthesis and Architecture in Saccharomyces cerevisiae. Genetics 147: 435-450.

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Last updated February 1, 2012

 

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