A common question flowing across holiday tables trimmed with turkey this week may be "white meat or dark?" Now scientists have identified the genetic switch that governs the formation of the two types during development.

White and dark meat differ in appearance because each is made up of a distinct type of muscle fiber. Dark meat comprises so-called slow twitch muscle fibers, which are specialized for extended exertion, whereas white meat is made up of fast twitch fibers that fuel short, intense bursts of energy. That much has been known for some time. The genetic mechanism underlying the specification of one muscle type versus the other was unclear, however. Philip Ingham of the University of Sheffield and his colleagues studied muscle cells of developing zebrafish and found that a gene dubbed u-boot (ubo) plays a key role in determining what type of muscle develops by controlling the transcription factor protein known as Blimp-1. "We have seen Blimp-1 before, as it is also used to determine the type of some white blood cells, but this is the first time it has been linked to muscle development," Ingham remarks. "The find is particularly important because it is likely that the same switch is used in mammals, fish and birds."

The discovery, announced in a report published online today by the journal Nature Genetics, does more than elucidate a turkey conundrum. Notes Ingham, "this finding has implications for future research into how muscle genes are switched on and off and could provide new ways of manipulating the proportions of slow and fast twitch fibers in muscles."