Resurrecting embryos of the tuatara, Sphenodon punctatus, to resolve vertebrate phallus evolution.
The breadth of anatomical and functional diversity among amniote external genitalia has led to uncertainty about the evolutionary origins of the phallus. In several lineages, including the tuatara, Sphenodon punctatus, adults lack an intromittent phallus, raising the possibility that the amniote ancestor lacked external genitalia and reproduced using cloacal apposition. Accordingly, a phallus may have evolved multiple times in amniotes. However, similarities in development across amniote external genitalia suggest that the phallus may have a single evolutionary origin. To resolve the evolutionary history of amniote genitalia, we performed three-dimensional reconstruction of Victorian era tuatara embryos to look for embryological evidence of external genital initiation. Despite the absence of an intromittent phallus in adult tuataras, our observations show that tuatara embryos develop genital anlagen. This illustrates that there is a conserved developmental stage of external genital development among all amniotes and suggests a single evolutionary origin of amniote external genitalia.
Department of Molecular Genetics and Microbiology, University of Florida, PO Box 103610, Gainesville, FL 32610, USA.
Thomas J
TJ
Sanger
Department of Biology, University of Florida, PO Box 103610, Gainesville, FL 32610, USA.
Marissa L
ML
Gredler
Department of Molecular Genetics and Microbiology, University of Florida, PO Box 103610, Gainesville, FL 32610, USA Department of Biology, University of Florida, PO Box 103610, Gainesville, FL 32610, USA Howard Hughes Medical Institute, University of Florida, PO Box 103610, Gainesville, FL 32610, USA mjcohn@ufl.edu.
Martin J
MJ
Cohn
D000818:Animals; D005075:Biological Evolution; D005837:Genitalia, Male / Q000196:embryology*; D008297:Male; D038081:Organogenesis; D010413:Penis / Q000196:embryology; D010802:Phylogeny; D012104:Reptiles / Q000196:embryology*
amniote; hemipenis; macroevolution; penis