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The Herron Lab at Georgia Tech studies various aspects of the evolutionary origins of multicellularity through a combination of experimental, theoretical, and comparative approaches. Our main model systems are the volvocine algae (Volvox and kin) and their close unicellular relative, Chlamydomonas reinhardtii.

PLoS ONE

Figure 8 from Boyd et al. 2018
Figure 8 from Boyd et al. 2018. Analysis of algal movement due to light exposure where positive values indicate movement toward the light source and negative values indicate movement away from the light source.

Former undergraduate researcher Maggie Boyd has published her analysis of motility in experimentally evolved Chlamydomonas reinhardtii in PLoS ONE:

C. reinhardtii is capable of photosynthesis, and possesses an eyespot and two flagella with which it moves towards or away from light in order to optimize input of radiant energy. Motility contributes to C. reinhardtii fitness because it allows cells or colonies to achieve this optimum. Utilizing phototaxis to assay motility, we determined that newly evolved multicellular strains do not exhibit significant directional movement, even though the flagellae of their constituent unicells are present and active. In C. reinhardtii the first steps towards multicellularity in response to predation appear to result in a trade-off between motility and differential survivorship, a trade-off that must be overcome by further genetic change to ensure long-term success of the new multicellular organism.

Maggie is now a Ph.D. student in Northwestern University’s Biomedical Engineering program.

Boyd, M., Rosenzweig, F. and Herron, M.D. 2018. Analysis of motility in multicellular Chlamydomonas reinhardtii evolved under predation. PLoS ONE, 13: e0192184. doi: 10.1371/journal.pone.0192184

The lead author of the recent Evolution paper has posted a commentary on kudos that includes some ideas not included in the paper:

The life cycle and ecology of volvocine algae may be key to understanding the long-term persistence of self-fertilization. First, selfing in homothallic volvocine algae is facultative; in a genetically diverse population, most matings will be between genetically distinct strains. Second, volvocine algae have a haploid-dominant life cycle with a metabolically active, multicellular haploid stage and a dormant, unicellular diploid stage. Inbreeding depression may thus be less important than in species with diploid-dominant life cycles. Finally, the dormant diploid stage allows volvocine algae to overwinter, meaning that the ability to self-fertilize is crucial for the survival of colonists to new ponds. Thus facultative selfing might provide volvocine algae with the benefits of outcrossing (when other genotypes are around) without the cost of potentially being unable to find a mate.

Evolution logo

Ancestral character states
Ancestral state reconstruction of selfing (left) and monoecy (right). Left, the evolution of outcrossing (black) and selfing (green). Right, the evolution of dioecy (black, for this analysis, outcrossing heterothallic species were treated as dioecious) and monoecy (blue).

Hanschen ER, Herron MD, Wiens JJ, Nozaki H, Michod RE. 2017. Repeated evolution and reversibility of self-fertilization in the volvocine green algae. Evolution (pdf)

Pleodorina sphaerica

Figures 1-9 from Nozaki et al. 2017. Pleodorina sphaerica

Pleodorina sphaerica Iyengar was considered to be a phylogenetic link between Volvox and the type species Pleodorina californica Shaw because it has small somatic cells distributed from the anterior to posterior poles in 64- or 128-celled vegetative colonies. However, cultural studies and molecular and ultrastructural data are lacking in P. sphaerica, and this species has not been recorded since 1951. Here, we performed light and electron microscopy and molecular phylogeny of P. sphaerica based on newly established culture strains originating from Thailand. Morphological features of the present Thai species agreed well with those of the previous studies of the Indian material of P. sphaerica and with those of the current concept of the advanced members of the Volvocaceae. The present P. sphaerica strains exhibited homothallic sexuality; male and facultative female colonies developed within a single clonal culture. Chloroplast multigene phylogeny demonstrated that P. sphaerica was sister to two other species of Pleodorina (P. californica and Pleodorina japonica Nozaki) without posterior somatic cells, and these three species of Pleodorina formed a robust clade, which was positioned distally in the large monophyletic group including nine taxa of Volvox sect. Merrillosphaera and Volvox (sect. Janetosphaera) aureus Ehrenberg. Based on the present phylogenetic results, evolutionary losses of posterior somatic cells might have occurred in the ancestor of P. californica and P. japonica. Thus, P. sphaerica might represent an ancestral morphology of Pleodorina, rather than of Volvox.

Nozaki, H., W. Mahakham, S. Athibai, K. Yamamoto, M. Takusagawa, O. Misumi, M. D. Herron, F. Rosenzweig, M. Kawachi. 2017. Rediscovery of the species of “ancestral Volvox”: morphology and phylogenetic position of Pleodorina sphaerica (Volvocales, Chlorophyceae) from Thailand. Phycologia 56:469–475. doi: 10.2216/17-3.1

Postdoc Kimberly Chen and I will be presenting results from the experimental evolution of multicellularity project at the Astrobiology Science Conference in Mesa, Arizona April 24-28, 2017. Dr. Chen will present in the session I’m co-organizing with Eric Libby; her abstract is here.

I will present a poster on the results of the Paramecium predation experiment; my abstract is here.