Scholarship and Biography
In the 1960s my lab set out to use the power of genetics to dissect eukaryotic cell differentiation, those epigenetic changes that produce different cell types. Since animal cells seemed inaccessible, we chose as research organism a remarkable but little studied single-celled protozoan (“first animal”) that lives in freshwater, Naegleria gruberi, which can alternate between walking amoebae and streamlined, rapidly swimming flagellates. We developed controlled conditions such that when amoebae are transferred from their growth environment to a nutrient-free aqueous environment, they undergo synchronous, temporally reproducible differentiation to flagellates, a conversion completed within 100 minutes. We began to describe the visible and molecular changes that occur during differentiation, an investigation that has kept us busy for 60 years.
Early on, two discoveries came as major surprises. At the base of flagella are basal bodies, or centrioles, with a complex structure conserved throughout evolution of eukaryotes. At the time centrioles were universally accepted to arise only as daughters of preexisting centrioles. We found that Naegleria amoebae had no centriole, and that during differentiation the centrioles were assembled de novo. At first this discovery created controversy, until others found that animal cells could form centrioles de novo too.
The other surprise involved tubulin, the protein that builds the structure of the microtubules that assemble during mitosis as well as the microtubules of cytoskeleton and of flagella. Everyone believed that a single tubulin protein was used to build all these microtubules, but we showed that Naegleria synthesized new tubulins, encoded by different genes, to build its flagellate’s microtubules. This led to the discovery that most eukaryotes use multiple tubulin isotypes.
As Naegleria revealed new insights about eukaryotes, we recognized that Naegleria was informative because it was a far-out eukaryote, a member of a group, the Excavates, which separated from the common ancestors of eukaryotes well before plants and animals diverged (see Fulton, 2022). We discovered that Naegleria’s genome complexity rivals that of animals and other late-branching eukaryotes (Fritz-Laylin et al., 1990), to the extent we could conclude that, in evolution, “it was a giant step to an amoeba, yet a small step to man.”
Our experience so far indicates that Naegleria is likely to have more novel and instructive surprises to offer biologists. My long-term collaborator Dr. Elaine Y. Lai and I are currently focusing on a surprising mode of gene silencing in Naegleria. And, importantly, we are still seeking the elusive molecular genetic tools that are needed to facilitate deeper understanding of these remarkable organisms.