Title

Predator-Induced Multicellularity in Chlamydomonas reinhardtii

Presentation Type

Presentation

Abstract

In Life’s nearly four billion year history, organizational transitions have occurred that fundamentally altered the course of evolution. One of these, the evolution of multicellularity, occurred independently in at least two dozen lineages, giving rise to a remarkable variety of complex forms. In the volvocine algae, many transitional structures are retained in extant lineages, ranging from the unicellular, flagellated Chlamydomonas reinhardtii to the extravagant Volvox barberi, which contains up to 50,000 differentiated cells. In addition to its normal unicellular state, C. reinhardtii is also capable of plastically forming amorphous multicellular clusters, called palmella, which may be triggered by the presence of grazing predators. The increased size of palmella relative to single cells offers C. reinhardtii protection from grazing predation, so we tested the hypothesis that the propensity to form palmella would increase due to this selective advantage. We performed experimental evolution by continuously co-culturing 8 replicate populations of Chlamydomonas reinhardtii with the predatory ciliate Paramecium tetrauerlia for ~350 generations. Response to selection is assayed via reaction norms for palmella formation, which capture shifts in phenotypic plasticity arising from predation pressure. Reaction norms are constructed by mixing experimental C. reinhardtii isolates with different concentrations of a cell-free filtrate of the predatory culture, and measuring the frequency of palmella formation at each concentration. Previous genomic comparisons show that gene families involved in the formation of extracellular matrix are expanded in Volvox relative to Chlamydomonas. Because of their expansion in the Volvox lineage, genes in these families are suspected of having a role in the evolution of multicellularity, and are likely candidates for control of palmella formation. We investigate changes in these gene families in the experimental populations, providing a mechanistic view into one possible route by which multicellularity can evolve.

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Predator-Induced Multicellularity in Chlamydomonas reinhardtii

UC 332

In Life’s nearly four billion year history, organizational transitions have occurred that fundamentally altered the course of evolution. One of these, the evolution of multicellularity, occurred independently in at least two dozen lineages, giving rise to a remarkable variety of complex forms. In the volvocine algae, many transitional structures are retained in extant lineages, ranging from the unicellular, flagellated Chlamydomonas reinhardtii to the extravagant Volvox barberi, which contains up to 50,000 differentiated cells. In addition to its normal unicellular state, C. reinhardtii is also capable of plastically forming amorphous multicellular clusters, called palmella, which may be triggered by the presence of grazing predators. The increased size of palmella relative to single cells offers C. reinhardtii protection from grazing predation, so we tested the hypothesis that the propensity to form palmella would increase due to this selective advantage. We performed experimental evolution by continuously co-culturing 8 replicate populations of Chlamydomonas reinhardtii with the predatory ciliate Paramecium tetrauerlia for ~350 generations. Response to selection is assayed via reaction norms for palmella formation, which capture shifts in phenotypic plasticity arising from predation pressure. Reaction norms are constructed by mixing experimental C. reinhardtii isolates with different concentrations of a cell-free filtrate of the predatory culture, and measuring the frequency of palmella formation at each concentration. Previous genomic comparisons show that gene families involved in the formation of extracellular matrix are expanded in Volvox relative to Chlamydomonas. Because of their expansion in the Volvox lineage, genes in these families are suspected of having a role in the evolution of multicellularity, and are likely candidates for control of palmella formation. We investigate changes in these gene families in the experimental populations, providing a mechanistic view into one possible route by which multicellularity can evolve.