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How birds lost their penises

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Posted June 7, 2013
This is an embryonic chicken penis (pseudocolored red) viewed under a scanning electron microscope. Credit: C.L. Perriton and M.J. Cohn

This is an embryonic chicken penis (pseudocolored red) viewed under a scanning electron microscope. Credit: C.L. Perriton and M.J. Cohn

In animals that reproduce by internal fertilization, as humans do, you’d think a penis would be an organ you couldn’t really do without, evolutionarily speaking. Surprisingly, though, most birds do exactly that, and now researchers reporting in the Cell Press journal Current Biology on June 6 have figured out where, developmentally speaking, birds’ penises have gone.

It turns out that land fowl, which have only rudimentary penises as adults, have normally developing penises as early embryos. Later in development, however, the birds turn on a genetic program that leads their budding penises to stop growing and then wither away.

“Regulation of the balance between cell proliferation and cell death is essential for controlled growth and development,” said Martin Cohn of the Howard Hughes Medical Institute and University of Florida, Gainesville. “Too much cell division or too little cell death can lead to overgrowth or mis-regulated growth, as in cancer. If the balance is tipped in the other direction, deficient cell division or excess cell death can lead to underdevelopment or even absence of a tissue or organ.

“Our discovery shows that reduction of the penis during bird evolution occurred by activation of a normal mechanism of programmed cell death in a new location, the tip of the emerging penis.”

Read more at: Phys.org

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