Ruth Sager


Ruth Sager (1918-1997) pioneered the now-thriving field of “cytoplasmic genetics” but it took decades before her theories were accepted. Sager was born in Chicago, Illinois in 1918. She received a degree in mammalian physiology from the University of Chicago in 1938, followed by a master’s degree in plant physiology from Rutger’s University in 1944 and a PhD in maize genetics from Columbia University in 1948. While at Columbia, she worked with another amazing female scientist, Barbara McClintock.

After graduating, she transitioned to the Rockefeller Institute, where she worked her way up from postdoctoral fellow to staff member. It was here that she made her first major discovery. At the time, it was widely believed that all genetic information in eukaryotes was stored and transferred via the nucleus. However, while performing research using an alga called Chlamydomonas reinhardtii, she found that genes were also being passed on in other “packaging” – chloroplasts.

Better known for their photosynthetic functions (they turn sunlight and carbon dioxide into sugar), chloroplasts also contain their own (though small) set of genes. This can be explained by Lynn Margulis’ endosymbiotic theory – that chloroplasts and other membrane-bound “organelles” called mitochondria evolved from an ancient cell engulfing a simpler cell. The engulfed cell had its own genome that, over time, got whittled down as it was repurposed for energy production.

By performing classical breeding experiments in the algae, Sager discovered that some genes were being passed on through these genomes. Animals don’t have chloroplasts, but we do have mitochondria, with their own small genomes. And, as Sager theorized, mutations in these organelles are responsible for a number of diseases.

“Cytoplasmic genetics” is now a thriving and well-accepted field, but when Sager initially proposed this nonchromosomal inheritance strategy, she was met with strong skepticism. This, combined with her gender, made it difficult for her to find faculty research positions, but  

she later held positions as research scientist (Columbia), professor of biology (Hunter College), and professor of cellular genetics (Harvard Medical School). At Harvard, she served as chief of the Dana-Farber Cancer Institute’s Division of Cancer Genetics where her research focused on the functions (and dysfunctions) of tumor suppressor genes.

Sager was elected to the National Academy of Sciences in 1977 and the American Academy of Arts and Sciences in 1979. She died from bladder cancer in 1997.

Photo credit: National Academy of Sciences