• Otávio Santiago

How the jerboa got its enormous feet

With its large ears and whiskered nose, you’d be forgiven for mistaking the jerboa for a mouse … at least from the stomach up. The animal’s legs are another story, with gigantic feet that enable it to hop like a kangaroo through hot desert environments in Africa and Asia. Now, researchers say they may have figured out how these stunning appendages evolved.

The insight comes thanks to a comparison of the “transcriptome”—essentially the sum total of all of the genes being used at a given time by an organism—from mice and jerboas, creatures separated by about 55 million years of evolution. Researchers started with an appendage that’s similar between the two: their forelimbs, or arms. By comparing the messenger RNA (mRNA) made by cartilage cells in developing mice and jerboas’ arms, the team aimed to establish a baseline set of genetic differences between the two animals that probably weren’t related to the jerboas’ big feet. (To make a protein, an active gene first makes an mRNA “transcript.”)

Then, the scientists looked at cartilage cells from the two animals’ growing feet. From those transcriptomes of mouse and jerboa feet, they were able to narrow the pool of potential genes tied to foot size down by 90%, leaving a total of 1755.

Finally, the researchers used a series of so-called network analyses to try to pinpoint which genes might have outsize effects in shaping the jerboa’s feet. Scientists have been cataloging genes, their function, and what chemical pathways they influence in the body for decades now. But nothing operates in a vacuum in the cell. Most every pathway is intertwined with others, and the end result is the product of many gene networks operating in parallel and individual genes that have many associations with others.

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