The chemical switch of genes may help explain why large mammals live longer

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When it comes to how long mammals can live, larger bodies are usually better: a typical mouse can live less than 4 years, while a bowhead whale can live up to 211 years

But within a species, this relationship may change: large dog breeds tend to have shorter life spans than smaller canines

This analysis looks at a DNA modification called methylation, which can control whether genes are turned on or off

The authors say that the methylation patterns discovered by the research team add a new layer of information to a better understanding of the differences in the DNA sequence of the genes themselves

"Why different species age at different rates is both a fascinating question and an important question," said Vardhman Rakyan, an aging researcher at Queen Mary University of London.

During DNA methylation, a molecule called a methyl group is attached to a base, usually cytosine

The MMA collected thousands of blood and other tissue samples from more than 200 mammals, from shrews to elephants, and used a chip to scan their DNA.

To explore this, Amin Hahani, a postdoctoral fellow and computational biologist in the Horvath lab, first simply looked for patterns of methylation

The evolutionary tree constructed with cytosine methylation largely reflects the phylogeny based on its gene sequence

"It is likely that these cytosine sites are part of an important pathway or cell type," Horvath said

smallerThe size of the dog is consistent with the known contradictory relationship

She said that methylation studies have shown that experiments are "what we don't know to do," such as observing whether changes in fat deposition can prevent other harmful physiological effects in big dogs

Frank Johannes, a plant epigenomics researcher at the Technical University of Munich, said that it is logical that methylation patterns change with evolution, because the process itself is controlled by genes with evolutionary sequences