Genetic predisposition towards multicellularity in Chlamydomonas reinhardtii
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Abstract
The evolution from unicellular to multicellular organisms facilitates further phenotypic innovations, notably cellular differentiation. Multiple research groups have shown that in the laboratory simple, obligate multicellularity can evolve from a unicellular ancestor under appropriate selection. However, little is known about the extent to which deterministic factors like ancestral genotype and environmental context influence the likelihood of this evolutionary transition. To test whether certain genotypes are predisposed to evolve multicellularity in different environments, we carried out a set of 24 evolution experiments each founded by a population consisting of 10 different strains of the unicellular green alga Chlamydomonas reinhardtii, all in equal proportions. Twelve of the initially identical replicate populations were subjected to predation by the protist Parmecium tetraurelia while the other 12 were subjected to settling selection by slow centrifugation. Population subsamples were transferred to fresh media on a weekly basis for a total of 40 transfers (~600 generations). Heritable multicellular structures arose in four of 12 predation-selected populations (6 multicellular isolates in total), but never in the settling selection populations. By comparing whole genome sequences of the founder and evolved strains, we discovered that every multicellular isolate arose from one of two founders. Cell cluster size varied not only among evolved strains derived from different ancestors but among strains derived from the same ancestor. These findings show that both deterministic and stochastic factors influence whether initially unicellular populations can evolve simple multicellular structures.
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We observed that multicellular cluster size varied among evolved strains over the course of 6-days.
Why do you think the multicellular cluster size decreased over time for 3 out of the 4 PS strains? Was this surprising or a sign of instability?
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COMBO necessarily brings about a different physiological state than TAP, and this difference may influence C. reinhardtii’s latent tendency to form clonal assemblages and/or extracellular matrices that cause bodies to settle rapidly in aqueous media.
It would be interesting to gain further insight into the physiological differences when grown in the two types of media.
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Next, images were manually screened to demarcate cluster boundaries and record cell number within each boundary using Cell Counter in ImageJ (https://imagej.nih.gov/ij/). Cell number was estimated by counting DAPI stained nuclei confined within cluster boundaries (e.g., Figure 2).
These cluster boundaries look hard to delineate by eye. I wonder if you considered using a stain (like Calcafluor white) to make the cell walls more obvious to the observer.
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Microscopic images
It's more accurate to call these micrographs rather than microscopic images.
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Multicellular structures evolved under predation-selection but not under settling selection
It would help orient the reader to have a visual representation of this part of the results. A schematic could show that multicellular phenotypes emerged at different times, but a subset of those that evolved under the predator-selective regime were stable. This could have the strain names with the stable multicellular ones highlighted so the reader can easily reference this figure later when looking at the results that follow up on those strains (like Figs. 3 & 4).
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