Freaky' Mouse Is Immune to Poison

A new hearty mouse has evolved resistance to warfarin and it's headed our way soon.


- Interbreeding has likely resulted in a hybrid house mouse that is resistant to a common rodent toxin.

- The findings show how hybridization can occur naturally among animals.

- Through agricultural practices and pesticide use, humans drove the process.

Mice from different countries have likely interbred, producing what researchers call a "freaky" and "superior" house mouse that is resistant to a common rodent poison.

The discovery, reported in the journal Current Biology, demonstrates how hybridization can occur naturally among animals, yielding non-sterile individuals with beneficial attributes. In this case, the result is a mouse that is resistant to warfarin, a toxic and, usually, deadly ingredient in many rodent poisons.

Hybrids often cannot reproduce, but "sometimes there is the occasional odd hybrid that has just about the right novel combination of genomes from two species that renders them, at least temporarily, superior over the pure species," lead author Michael Kohn told Discovery News.

The investigation began after Kohn's lab was asked to study mice from a German bakery. The bakery's owners had first called in a pest control specialist to deal with mice that had infested their basement. The specialist tried to kill the mice with bromadiolone, a particularly nasty version of warfarin, but the chemical didn't bother the mice at all.

Specimens were sent to Kohn's lab, where a genetic analysis took place. The researchers determined that a big chunk of the German mice's DNA was identical to that of a desert-dwelling, seed-eating Algerian mouse. This Algerian mouse had previously evolved immunity to warfarin through a process called "point mutation" -- when genes adapt through spontaneous mutations during DNA replication.

The researchers believe the Algerian mice acquired the mutation to counter a vitamin K-deficient diet. The gene that makes mice resistant to warfarin also manages vitamin K in the body.

At some point in the past, Kohn and his team believe the Algerian mice mated with European house mice, conferring their poison resistance to them. This process is called "horizontal gene transfer," and is usually only seen in microbes.

Kohn said that "we've caught evolution in the act."

He added, "The process we describe (horizontal gene transfer) introduces more variation in the genomes of populations that would otherwise, by mutation alone, be available. In that regard it potentially could speed up evolution."

Humans appear to be driving the process. The mice from different regions likely would not have met, were it not for spreading via human agricultural practices. Our use of pesticides also played a part.

"Unprofessional and widespread use of poison seems to have favored the evolution and spread of resistant mice and rats," Kohn said. "However, the novel thing reported here is that it has also enabled a potentially important process (hybridization) to turn up something advantageous that usually is not."

The genetic mutation that makes mice resistant to warfarin also causes arterial calcification and osteoporosis in humans. It remains unclear if the poison-resistant mice will also suffer from these health problems. For the time being, the mice remain "superior" over their house mice counterparts that are still vulnerable to the common rodent toxin.

Loren Rieseberg, professor of biology at the University of Indiana, Bloomington, indicated to Discovery News that he supports the new findings.

Rieseberg believes the study "furnishes exceptionally strong evidence" of hybridization among the mice. He additionally agrees "that human-mediated changes in selection pressures and dispersal patterns may frequently create conditions where introgression (hybridization) is adaptive."

"The human factor in this study is quite clear," Kohn concluded. "One of the gravest concerns to conservation of biodiversity is the inadvertent spread of invasive species across the globe. In this study, this test came in the form of our desire to extirpate so-called pest species with poisons, which we use to get rid of microbes, bugs, weeds and even some mammals."