One of the most puzzling events in evolution is the reduction and loss of the phallus in birds. All birds reproduce by internal fertilization, but only ∼3% of birds have retained a phallus capable of intromission. A number of hypotheses have been proposed for the evolutionary mechanisms that drove phallus reduction; however, the underlying developmental mechanisms are unknown.
We investigated genital development in two sister clades of birds, Galliformes (land fowl), most of which lack an intromittent phallus, and Anseriformes (waterfowl), which have well developed phalluses; and in two outgroups, Paleognathae (emus) and Crocodilia (alligators). Galliform embryos undergo cryptic development of a genital tubercle, the precursor of the phallus, but this later undergoes apoptosis, leading to regression of the tubercle. At the molecular level, a derived pattern of Bmp4 expression was identified in chick (a galliform) genital tubercles. Inhibition of Bmp signaling in chick genitalia rescues cells from apoptosis and prevents phallus regression, whereas activation of Bmp signaling in duck (an anseriform) genitalia induces a galliform-like pattern of apoptosis. Thus, distal Bmp activity is necessary and sufficient to induce apoptosis in Galloanserae genital tubercles.
Our results indicate that evolutionary reduction of the intromittent phallus in galliform birds occurred not by disruption of outgrowth signals but by de novo activation of cell death by Bmp4 in the genital tubercle. These findings, together with discoveries implicating Bmps in evolution of beak shape, feathers, and toothlessness, suggest that modulation of Bmp gene regulation played a major role in the evolution of avian morphology.