Eco-Evo-Devo: origins of multicellularity in Chlamydomonas reinhardtii
The evolution from single cells to multicellular organisms serves as the first step towards further phenotypic innovations, e.g. division of labor among cells in multicellular organisms. Recently the emerging field of ecological evolutionary developmental biology (Eco-Evo-Devo) is starting to reveal the importance of interactions among genes, environments and development. However, most origins of multicellular development are ancient and how environments influence this transition remains obscure.
Several microbial evolution experiments have shown that obligate multicellularity can readily evolve in the laboratory from single-celled organisms. Among them, two evolution experiments have used Chlamydomonas reinhardtii, the unicellular relative of the volvocine algae, to generate multicellular structures using selective regimes that favor increased size. Ratcliff et al. (2013) evolved multicellular clusters > 100 cells held together by transparent extracellular matrix by selecting clusters that settle rapidly in liquid medium. Herron et al. (2019) generated multicellular structures by the filter-feeding predator Paramecium tetraurelia. The structures evolved in response to predation are usually 22 - 25 cell clusters and similar to the ones in small colonial volvocine algae such as Pandorina.
Unfortunately, different outbred starting populations were used in these two selection experiments and this limits further interpretation regarding how environments versus ancestral genotypes influence the outcomes of multicellular development. I am currently conducting two selection experiments to directly investigate the roles environmental factors and genetic predisposition play in the transition to multicellularity. First, to examine how different environments influence the origins of multicellularity, an evolution experiment was started with a unicellular isolate of C. reinhardtii. Twelve initially isogenic replicate populations are being subjected to settling rate selection and 12 replicate populations being subjected to predation by P. tetraurelia. Subsamples of each population are transferred to fresh medium every week for a total of 48 weeks. Second, to broadly test whether certain genotypes are predisposed for multicellularity in different environments, I started the same
selection experiment with a heterogeneous population by equally mixing 10 different strains of C. reinhardtii. These strains include two lab strains and eight field isolates that are genetically and phenotypically diversified from each other, and contain identified polymorphisms that will be able to serve as markers to detect frequency changes of each genotype during the evolution experiment. The same experimental procedure is performed as described above. In both selection experiments, culture samples from each line are stored frozen every four weeks and phenotypes of isolates from each line are examined every eight weeks.
Recently I started to observe the trajectories to and incipient origins of multicellularity from these evolution experiments. I will present my preliminary data and discuss the relative importance of environment versus genetic background in determining the particular path taken in the evolution of multicellularity. Further, the experimental design will allow us to study life history trade-off between single-celled ancestors and evolved multicellular strains, and facilitate downstream analysis to identify underlying genetic mechanisms.