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Related Experiment Video

Updated: May 10, 2026

Two-step Approach to Explore Early- and Late-stages of Organ Formation in the Avian Model: The Thymus and Parathyroid Glands Organogenesis Paradigm

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Developmental basis of phallus reduction during bird evolution.

Ana M Herrera¹, Simone G Shuster, Claire L Perriton

¹Howard Hughes Medical Institute, Department of Molecular Genetics and Microbiology, and the Genetics Institute, University of Florida, Gainesville, FL 32610, USA.

Current Biology : CB

|June 11, 2013

Summary

This summary is machine-generated.

The evolution of phallus loss in birds is explained by the de novo activation of cell death via Bmp4 signaling in the genital tubercle. This discovery sheds light on avian morphological evolution.

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Area of Science:

Evolutionary developmental biology
Comparative genomics
Avian morphology

Background:

The reduction and loss of the phallus in birds is a significant evolutionary puzzle, with only a small percentage retaining an intromittent organ.
While hypotheses exist for evolutionary drivers, the specific developmental mechanisms behind phallus reduction remain largely unknown.

Purpose of the Study:

To investigate the developmental and molecular mechanisms underlying phallus reduction in birds.
To compare genital development in bird clades with and without intromittent phalluses.

Main Methods:

Comparative analysis of genital development in Galliformes (lacking phallus) and Anseriformes (with phallus).
Examination of outgroups Paleognathae and Crocodilia.
Molecular investigation of Bone morphogenetic protein 4 (Bmp4) expression and signaling pathways in developing chick and duck genitalia.
Experimental manipulation of Bmp signaling to observe effects on apoptosis and phallus development.

Main Results:

Galliform embryos exhibit regression of the genital tubercle precursor via apoptosis.
A specific pattern of Bmp4 expression was identified in the genital tubercles of chicks (Galliformes).
Inhibiting Bmp signaling in chicks prevented apoptosis and phallus regression; activating it in ducks induced galliform-like apoptosis.

Conclusions:

Phallus reduction in galliform birds is attributed to the de novo activation of programmed cell death (apoptosis) mediated by Bmp4 in the genital tubercle.
Bmp4 gene regulation plays a crucial role in avian morphological evolution, as evidenced by its involvement in phallus development, beak shape, feather formation, and tooth loss.