For anyone who has to deal with rodent infestations, getting rid of the small, gnawing animals can be rough — so much so that many people immediately head for poison. However, it's not quite as easy as that. Some mice have obtained immunity to Warfarin, a common rodent poison, via a very unusual route: horizontal gene transfer.
Usually, immunity comes via the usual evolutionary route. A beneficial mutation increases the survival rate of certain individuals, who have children with the same mutation and so on. But in this case, the poison immunity mutation is coming from crossbreeding with another species of mouse, and passing on their benefits to the offspring.
Warfarin is a blood thinner used to cure humans and kill mice and rats, and the enzyme for its resistance is linked to vitamin K epoxide reductase enzyme complex (VKOR). Algerian mice (Mus spretus) come from an environment with very small amounts of natural vitamin K, making them already resistant to the poison, where house mice (M. musculus domesticus) are susceptible.
What the researchers found is that the hybrid offspring of this union came with a resistance to the poison, allowing the trait to spread, despite the hybrids being sterile approximately 50% of the time. Because people were using the poison so extensively, having a 50% chance of being able to pass on your genes but being able to survive the warfarin was more beneficial then being able to breed normally. This may be why it only took around 10 years from when the poison started being widely used in the 1950s until resistant mice started to appear.
Horizontal gene transfer has never before been recorded at this level of detail, and could substantially change the way we look at immunity - both to protect some species, and attack others.
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