Genes Found That Cue Mice To Build Predictable Underground Burrows
Brett Smith for redOrbit.com – Your Universe Online
By studying oldfield mice — colloquially known as ℠beach mice´ (Peromyscus polionotus) — as they dug through a mound of dirt in her San Diego garage, biologist Hopi Hoekstra discovered that the diligent rodents construct their burrows in extremely predictable patterns.
Working on a hunch that the mice probably inherit this behavior, she and a team of colleagues conducted a genetic analysis of the tiny rodents and found that three relatively small parts of the mouse´s genome control the burrow´s size and features.
A genetic feature that influences how an organism tries to shape its environment is known to researchers as an extended phenotype. Bird nests, beaver dams and spider webs are all examples of inherited behaviors.
“In my mind, the link between genes and behavior in natural populations and organisms is the next great frontier in biology,” Hoekstra wrote.
The project began in 2005 when Hoekstra and her colleague Jesse Weber built large sandboxes in her California garage. The two then captured mice and released them in the sand to build their burrows.
“This thing was bursting at its seams and held together with duct tape,” she said about her homemade environment. “But it worked.”
Hoekstra and Weber then created casts by removing the mice and filling the burrows with expanding foam, which hardened into solid casts once it filled the nooks and crannies of subterranean labyrinth. These casts confirmed the burrows consistent construction: an entrance tunnel, a nest, and an escape tunnel that ends just below the ground surface.
Next, the researchers began a genomic analysis by interbreeding oldfield mice with deer mice, a related species that constructs a simpler version of the burrow. The team found that the hybrid offspring of these two mice species constructed the more complex type of burrow, suggesting that the oldfield-burrow genes are the dominant genotype.
The team then bred these first generation hybrids with more deer mice and performed a genetic analysis on the second-generation animals. An analysis of the results showed three separate genetic regions that are responsible for influencing the length of the entrance tunnel. According to Hoekstra, while tunnel length is about 30 percent inherited, most of the genetic basis for the tunnel is determined by a mixture of other small genes.
Their analysis also found that one single region of the mouse genome determines whether the animals construct an escape tunnel for their underground abode — a feature exclusively found in the burrows of oldfield mice. Geneticists refer to this a “Mendelian trait” — a characteristic that is expressed when the dominant version of a gene is inherited from either one of the parents. Human examples include a cleft chin, or the ability to roll your tongue.
As a biology professor at Harvard, Hoekstra and her lab group are working to expand on the recently published study by identifying the specific genes responsible for the burrow´s construction. Each region could correspond to just one gene or several. According to Hoekstra, her team is particularly focused on a gene responsible for addictive behavior. Their hypothesis is that the two mice species have the same basic neural circuitry, but one is simply more motivated to finish the more complex architecture.
“When you watch these mice, you can´t help but think they look like they´re addicted to burrowing,” Hoekstra explained. “You drop one of these oldfield mice in one of our burrowing boxes and they immediately start digging.”