Biology & Medicine

How the bird lost its penis

Study in Current Biology could have implications for understanding evolutionary loss more broadly

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Anatomical preparations of adult male external genitalia in two anseriforms (goose and duck) and two galliforms (chicken and quail). Arrows point to the reduced phalluses in galliform species. Scale bar represents 1 cm. (Source: 'Developmental Basis of Phallus Reduction during Bird Evolution,' Ana Herrera et al, Current Biology)Anatomical preparations of adult male external genitalia in two anseriforms (goose and duck) and two galliforms (chicken and quail). Arrows point to the reduced phalluses in galliform species. Scale bar represents 1 cm. (Source: "Developmental Basis of Phallus Reduction during Bird Evolution," Ana Herrera et al, Current Biology)

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 journal Current Biology, published by Elsevier's Cell Press 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 Dr. Martin Cohn of the Howard Hughes Medical Institute and the University of Florida in 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.”

A critical gene in the process is called Bmp4. In chicken development, Bmp4 switches on and the birds’ developing genitals shrink away. In more well-endowed ducks and emus, that gene stays off and their penises continue to grow.

It’s not entirely clear why chickens and other birds would have lost their penises, said Ana Herrera, lead author of the study and a graduate student in the Department of Molecular Genetics and Microbiology at Howard Hughes Medical Institute, but it may be that the loss leaves hens with greater control over their reproductive lives.

The findings could have implications for understanding evolutionary loss more broadly. (Think of snakes and their lost limbs, as another example.) They might yield some answers to medical questions as well.

“Genitalia are one of the fastest-evolving organs in animals, from mollusks to mammals,” Dr. Cohn said. “It is also the case that genitalia are affected by birth defects more than almost any other organ. Dissecting the molecular basis of the naturally occurring variation generated by evolution can lead to discoveries of new mechanisms of embryonic development, some of which are totally unexpected. This allows us to not only understand how evolution works but also gain new insights into possible causes of malformations.”

Scanning electron microscopy of chick and duck external genital development. Anterior is to the left in (C)–(F) and to the top in (G)–(N). Chick and duck embryos initiate formation of paired genital swellings (C and D), which form a single tubercle during early outgrowth (E and H). Outgrowth arrests and the genital tubercle regresses in chick embryos (I, K, andM), but genital development continues in duck embryos (J, L, and N). Hamburger-Hamilton (HH) stage numbers are shown above. Abbreviations: gs, genital swellings; gt, genital tubercle; acs, anterior cloacal swelling; pcs, posterior cloacal swelling. (Source: "Developmental Basis of Phallus Reduction during Bird Evolution," Ana Herrera et al, Current Biology)Scanning electron microscopy of chick and duck external genital development. Anterior is to the left in (C)–(F) and to the top in (G)–(N). Chick and duck embryos initiate formation of paired genital swellings (C and D), which form a single tubercle during early outgrowth (E and H). Outgrowth arrests and the genital tubercle regresses in chick embryos (I, K, andM), but genital development continues in duck embryos (J, L, and N). Hamburger-Hamilton (HH) stage numbers are shown above. Abbreviations: gs, genital swellings; gt, genital tubercle; acs, anterior cloacal swelling; pcs, posterior cloacal swelling. (Source: "Developmental Basis of Phallus Reduction during Bird Evolution," Ana Herrera et al, Current Biology)

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Read the study

Current Biology has made the study freely available online until June 13, 2013: "Developmental Basis of Phallus Reduction during Bird Evolution" (http://dx.doi.org/10.1016/j.cub.2013.04.062) [divider]

Reporting for Elsevier Connect

Mary Beth O'LearyMary Beth O’Leary is Press Officer and Associate Media Relations Manager for Cell Press (@CellPressNews), based in Cambridge, Massachusetts. She began her career at Cell Press as an Senior Editorial Assistant for the journal Cell before transitioning into a role as Marketing/Publicity Coordinator. In December, she moved into her position as Press Officer for Cell Press’s 29 journals. A graduate of the College of the Holy Cross in Worcester, Massachusetts, she studied literature and art history.

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