Selecting for multicellularity in the unicellular alga Chlamydomonas reinhardtii

Abstract

Background: Researchers have recently begun experimentally exploring the origins of multicellularity (4-6). Their studies have found that the transition to a multicellular state may have been surprisingly simple, considering its profound implications for the history of life (3). This study experimentally selected for multicellularity in the unicellular biflagellated alga Chlamydomonas reinhardtii. This organism is especially interesting because it is basal to the Volvocaceae—a family of biflagellates whose evolutionary transition from unicellular organisms to complex forms have been meticulously characterized (2). The present study aimed to recreate the early stages of this transition, starting from incomplete cytokinesis after cell division.

Methods: The procedure was modeled loosely on the experiment performed by Ratcliff et al. (4) in which the authors successfully selected for multicellular Saccharomyces Cerevisiae—unicellular baker’s yeast. Three experimental replicates and one control for nine strains of C. reinhardtii were cultured in round-bottom vials in shaking incubators. Prior to each transfer (every 3-4 days), each culture was slowly mixed, and selection lines were then gently and briefly centrifuged. This applied a selection pressure which rendered heavier (clustered) cells more fit. The very bottom ~2% of the tubes’ contents was transferred, and cell cultures were examined for multicellularity.

Results: Six of nine lines of C. reinhardtii demonstrated an increased frequency of C. reinhardtii existing in a two- to four-celled state (the paired cell state accounted for 88% of these clusters)—consistent with the first step toward multicellularity as outlined by Kirk (2). A close study of cell division in the line which exhibited the strongest shift towards the multicellular phenotype suggests that true multicellularity began to evolve in this experiment. A multicellular phenotype did not become fixed in any population.

Conclusion: Our findings suggest that an artificial selection pressure is capable of inducing the evolution of multicellularity. Expanding upon this study could help us understand the mechanisms underlying the evolution of multicellularity.

Limitations: Sporadic data, possibly the result of difficulties in the procedure, prevented us from rigorously examining the effect of selection through time, limiting our ability to describe the evolutionary response. In addition, the study of individual cells, due to its time-consuming nature, was limited to one replicate of one line exhibiting a pronounced multicellular response. Thorough replication would be required before drawing a strong conclusion from this assay.