Next, the researchers set out to find out what stops a chick's penis from growing while allowing a duck's to reach startling lengths. They expected to find something missing in chickens - some mysterious molecular factor that would have otherwise spurred the penises to greater lengths.
Instead, the found just the opposite. In chick embryos, penis development is halted by the release of bone morphogenetic protein 4, or Bmp4. This protein shows up along the whole length of the primitive genital swelling seen in early chick development; in ducks, it's only seen at the base of the genitals.
To make sure Bmp4 was really doing the penis-stifling deed, the researchers applied the protein to duck penises. Sure enough, development halted. Likewise, when they blocked Bmp4's expression in chick penises, the embryonic birds' phalluses continued to grow. [See video of the embryonic experiments]
It turns out that Bmp4 is a cell death factor, Cohn said. Its release encourages cells to self-destruct, turning a growing organ into a shrinking one. Cell death is normal in embryos, he said, but it's more typical to see loss of embryonic growth factors in cases where limbs regress in the womb.
"There are many paths to reach the same morphological end," Cohn said.
The new study reveals how birds lost their penises, but not why. It seems odd that birds would evolve to lose an organ so critical to reproduction, Cohn said. Evolutionary biologists have theorized that perhaps bird penises vanished because female birds preferred mates with smaller penises. In ducks and other species with phalluses, males frequently force females to copulate. By picking mates with small penises, female birds could have gained more control over the reproductive process.
Alternatively, penis loss could have been a side effect of other changes in the birds' body. Bmp proteins are responsible for the origin of feathers in birds and their loss of teeth. Bmp4, in particular, is responsible for variations in beak size and shape, Cohn said.
"It's interesting that so many of these little details of the bird body plan are associated with changes in Bmp activity," he said.
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