Tasmanian devil genomes offer some hope, few answers

Two new sets of genetic blueprints offer no clues to stopping population's cancer epidemic but do reveal stable, if low, genetic diversity

Two new complete sets of Tasmanian devil genetic blueprints hold some good news and bad news for the species. The bad news is that the marsupial’s genetic diversity is among the lowest known for any species. The good news is that the devil’s low diversity has a long history and may not be reason for as much concern as once thought.

LONG-TIME SURVIVOR Cedric, a Tasmanian devil partially immune to a fatal infectious cancer, was one of two devils whose genetic blueprints have been completely deciphered. The scar marks where a tumor was removed. Cedric was euthanized last year due to complications from the cancer. Greg Woods/Univ. of Tasmania

LOW DIVERSITY Tasmanian devils have very low genetic diversity, with only 10 DNA letters on average differing between two individuals’ mitochondrial genomes. Mitochondria supply energy to cells and carry their own DNA. W. Miller et al/PNAS

DEVIL OF A THREAT An infectious cancer called devil facial tumor disease is decimating Tasmanian devil populations. Cancer cells are transmitted by bites, usually on the face, and grow into large tumors. S. Schuster / Pennsylvania State Univ.

This low genetic diversity “does not mean the species is doomed,” says genomicist Stephan Schuster of Pennsylvania State University. “If you maintain the entire diversity this can still be a viable species.”

An international team of researchers led by Schuster and Webb Miller, also of Penn State, deciphered the genetic blueprints of Tasmanian devils named Cedric and Spirit that hail from opposite ends of Tasmania, the team reports online June 27 in the Proceedings of the National Academy of Sciences. The two devils also represent a contrast in their response to the infectious cancer that has decimated wild devil populations. The work was done as part of an effort to better understand the deadly disease and save the species.

Cedric was one of the few devils whose immune system could fight off the infectious cancer, which started in a single long-dead devil and has since swept over more than half the island. As part of efforts to study the disease, Cedric survived two attempts to infect him with the facial tumor disease, but finally succumbed to a third strain. Spirit was already infected with five tumors and was near death when she was captured. Researchers hoped that cataloging and comparing the two animals’ genomes would show why Cedric was partially immune to the fatal cancer while Spirit and so many others are not. 

The initial analysis of the two genomes doesn’t provide a clear answer, but researchers believe that further work will reveal secrets to defeating the deadly disease that may be buried in the animal’s blueprints. Scientists suspect that most devils have variants in certain genes that make them more susceptible to the tumor disease.

“The really exciting discoveries are yet to come,” says Katherine Belov, a geneticist at the University of Sydney who was not involved in the study. “We are very excited to be able to jump in and start mining this genome.”

The study reveals more details about the genetic diversity of current Tasmanian devil populations. While researchers had long known that the devils have low levels of genetic diversity, the new work shows that devil diversity is about 20 percent of that in humans. 

An analysis of DNA from museum specimens dating back to 1874 — at least 100 years before the infectious cancer first appeared — finds that Tasmanian devils have had that level of genetic diversity for a long time. The tumor has not reduced the amount of diversity in wild populations, the researchers discovered after testing 168 wild devils.

That finding strikes a hopeful note, says Anne Yoder, an evolutionary biologist who directs the Lemur Center at Duke University. “The species seemed to be doing pretty well even with that low diversity,” she says.

Of course, that was before the infectious cancer hit. Low diversity might have affected how well devils fought off the disease. Because the devils are so genetically similar to one another, their immune systems may have trouble recognizing tumor cells passed on by another devil as not their own.

Some scientists have proposed that instead of the cancer being passed directly from one animal to another, viruses transmitted during bites might cause cancer (SN: 1/30/10, p. 10). But a closer look at one of Spirit’s tumors showed that the cancer cells definitely came from another devil. The new study is “yet another confirmation that it is a clonal cancer,” says Belov.

Schuster says that even though low genetic diversity has made devils vulnerable to the cancer, it’s not an insurmountable handicap.

Breeders should use genetic tests to match up relatively diverse mating pairs of devils instead of hoping that randomly choosing animals will maintain the fragile diversity, Schuster says. He thinks that employing the right strategies can save the Tasmanian devil.

“This is probably one of the only cases where human intervention, doing all the right things, can prevent a species from going extinct,” Schuster says.

Tina Hesman Saey is the senior staff writer and reports on molecular biology. She has a Ph.D. in molecular genetics from Washington University in St. Louis and a master’s degree in science journalism from Boston University.

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