Decoding the Evolutionary Pathways of Transmissible Cancers in Tasmanian Devils

Excerpt / Peyton Smith

Cancer is a ubiquitous disease among multicellular eukaryotic organisms. However, the uncontrolled growth of cancerous cell lines is usually constrained within an individual organism. On rare occasions, cancerous cells can acquire the ability to transmit between individual organisms. Such is the case with Devil Facial Tumor (DFT). DFT is a transmissible cancer nearly 100% fatal for Tasmanian Devils (Sarcophilus harrisii) and has contributed to an overall 80% decline in the Tasmanian Devil population over the past 20 years.1 Though the emergence of transmissible cancer was thought to be rare, scientists discovered a second lineage of DFT in 2015.2 A new paper by Stammnitz et al. in Science explores the evolutionary histories of these two tumor lineages that threaten the Tasmanian Devil with extinction.3

Transmissible cancer has been documented a mere handful of times in nature. The most well-studied form, Canine Transmissible Venereal Tumor (CTVT), is sexually transmitted in domesticated dogs (Canis lupus familiaris). CTVT has been documented for over 200 years, can be found all over the world and originated thousands of years ago from a single founder.4 Transmissible cancer has also been observed in invertebrates. A disseminated leukemia-like neoplasia was first observed in soft-shelled clams (Mya arenaria), a marine bivalve, in the 1970s and was found to be a transmissible tumor in 2015.5 

Unlike these other tumors, Devil Face tumor has been thought to have emerged relatively recently. The first strain of DFT was first observed in 1996, discovered in 2006 to be a transmissible cell line and has since rapidly spread through Tasmania.2,6 The swiftness of DFT’s emergence compared to the other well-studied transmissible cancer CTVT puzzled scientists and plagued conservationists. The Tasmanian Devil’s face-biting behavior (while mating and feeding) and vagile nature partly explained DFT’s massive spread.4 Yet, scientists’ intrigue intensified with the discovery of a second strain of DFT (DFT2), visually indistinctive from the first, in 2015. Unlike the original strain (DFT1) that lacks sex chromosomes, DFT2 bears a Y chromosome providing strong evidence for an independent emergence.2 Transmissible cancer, the ultimate “Perfect Storm” thought to have only evolved in a select few organisms, somehow emerged twice in a single host. This discovery led scientists to hypothesize that transmissible cancer could occur more frequently in nature than previously expected and/or that Tasmanian Devils are particularly susceptible to developing transmissible cancers. 

Further research into marine bivalves in 2016 found neoplasia in three additional species, similar to the discovery of DFT2, attributable to multiple independent transmissible lineages.7 This insight increased support for the notion that transmissible cancer could arise more regularly than previously expected and that certain species may be predisposed to developing transmissible cancer. Still, the genetic and mutational pathways that these cancers shared to become transmissible remained unclear. 

Author Commentary / Peyton Smith 

I wrote this piece for my fall JP in the department of Ecology and Evolutionary Biology. Our Junior Independent work task is uniquely short in comparison to other Princeton Junior Papers; Instead of emphasizing experimental design and analysis, our assignment instead focuses on conducting background research and refining our scientific writing. Consequently, we were instructed to write a short review of a recent paper for a larger audience that contextualizes the paper.

I found this paper on Transmissible Tasmanian Devil Cancer while browsing through the archives of Science and became instantly curious about the system after reading the abstract. I began informally looking up information about transmissible cancers, Tasmanian Devils, and the researchers behind the project. After seeing the rich background and context behind this 2023 paper, I knew that I had to cover it in my fall JP.

Yet with this glut of information (all of which I found very interesting), how was I supposed to approach actually presenting the paper? I could have approached the research from a conservationist perspective, highlighting the threat to Tasmanian Devil’s survival as a species posed by these cancer lineages. I could have also come from an epidemiological standpoint and explicated the difficulty in predicting and preventing the spread of this novel disease. Ultimately, I chose the angle that most aligned with the central question in the back of my mind: How can cancers even become transmissible? I then focused on the hypotheses and research surrounding this central question. That meant I didn’t limit myself to Tasmanian devils as a part of my literature review but delved into transmissible cancers in other species.

Still, these other angles naturally emerge throughout my paper as necessary to fully orient the reader to this system. When I first presented my topic to my EEB junior tutorial with my fellow students and advisor, my peers and professor were similarly curious about the system and asked many questions. At the time, I didn’t have answers to some of these questions, as I hadn’t yet finished my background research; once I began drafting my JP, however, I tried my best to integrate answers to their questions and curiosity into my writing to orient those who may be first encountering Tasmanian Devil transmissible cancer.

Scientific papers generally orient to a small audience, which can reduce the amount of people who are able to appreciate the interesting and consequential scientific work conducted in paper. Hence, I appreciate the purpose of this task which deemphasized methods in favor of exploring the paper’s implications. In science writing, it can feel as if there is a continuous trade-off between accessibility and detail, yet a coherent background motivated by a concise question can be appreciated and understood by anyone.

Editor Commentary / Molly Taylor

Finding a topic for junior independent work, or any open-ended writing assignment, is a daunting task. Peyton’s process is particularly instructive: after reading widely, she decided to analyze the study that stuck with her. In her scientific review, the junior paper of the Department of Ecology and Evolutionary Biology, Peyton establishes the significance of her chosen study in the context of transmissible cancer research. Her work illustrates how a strong scholarly motive—and an intriguing paper topic—follow from personal motive. 

Written for a public audience, the review first orients the reader with a concise introduction to transmissible cancer and Devil Facial Tumor (DFT). Rather than providing a highly technical definition, Peyton presents DFT to non-specialist readers by explaining why it matters. The first paragraph references its global impact (“DFT… has contributed to an overall 80% decline in the Tasmanian Devil population), as well as the surprise it poses to scientists (“Though the emergence of transmissible cancer was thought to be rare, scientists discovered a second lineage of DFT in 2015”). By infusing orientation with motive, the first paragraph conveys to readers why understanding research on DFT is important. 

The following paragraphs present a chronological account of research on transmissible cancer, covering the tumors before DFT, the puzzle of DFT’s swift emergence, and theories on DFT.  This section succinctly establishes the scholarly motive of Steinmetz et. al, who look to identify the mechanisms of DFT emergence. It is also accessible and compelling to readers. Peyton only uses technical terms when necessary and, after doing so, explains their significance in plain language. For example, after discussing the difference in chromosomes between the two DFT strains, Peyton ensures readers understand its implication: “Transmissible cancer, the ultimate ‘Perfect Storm’ thought to have only evolved in a select few organisms, somehow emerged twice in a single host.” Further, by including details such as DFT “puzzled scientists and plagued researchers”, and that “scientists’ intrigue intensified” upon the discovery of a second strain, Peyton invites readers to embrace the mystery of DFT. 

Reviewing a scientific study for a general audience is not easy, but the success of Peyton’s piece can be attributed to her thorough research, clarity in orienting, and enthusiasm for the topic. Just as Peyton was curious about transmissible cancer, it turns out, other researchers were too—allowing for not only a rich scholarly motive but also a global motive that leaves readers finding themselves just as fascinated by DFT. 

The author

Peyton Smith

Peyton Smith ’25 is a junior concentrating in Ecology and Evolutionary Biology from Colorado Springs, Colorado. She enjoys helping students as a fellow at the Writing Center as well as contributing her own writing to the Nassau Weekly. In her free time, Peyton loves competing with Princeton Women’s Club Volleyball, knitting garments, and reading Ursula K. Le Guin novels. She wrote this piece as a Junior.

Molly Taylor ’25 is a junior studying computer science and history from Chicago, Illinois. Outside of the Writing Center, she serves as head editor for the features section of the campus newspaper, The Daily Princetonian. In her free time, Molly enjoys going on long bike rides and making hot chocolate.