Rare reptile holds clue to penis evolution

A rare reptile is shedding light on the evolution of the penis — even though it doesn’t have one.

The tuatara, a lizardlike species in New Zealand, never grows a real phallus. Yet as an embryo, it starts forming tiny nubbins like those that turn into the great diversity of sperm-delivery organs in other mammals and reptiles, researchers at the University of Florida in Gainesville report October 28 in Biology Letters. Tuatara phallus development then stalls, but that initial burst of development supports the scenario that the phallus evolved just once in mammals and reptiles, says study coauthor Thomas Sanger.

With the tuatara (Sphenodon punctatus) on the brink of extinction, conservation managers would not permit sacrificing any of its embryos to study phallic history. But Sanger knew of some fragile, old microscope slides of tuatara embryos in the Harvard Museum of Comparative Zoology. They had come from a Victorian expert on sponge taxonomy who sojourned in New Zealand and developed a side interest in tuatara specimens provided by a friendly lighthouse keeper. 

Sanger photographed 82 of these delicate slices of an embryo. He and his colleagues then digitally cleaned away imagery of tissues that were failing with age, and combined the rest into one 3-D image. The image revealed one of the characteristic paired nubbins that other reptile and mammal embryos grow as their external genitals start to develop. In the tuatara’s closest living relatives, snakes and lizards, the nubbins grow into pairs of insertable organs. For species with a single penis — mammals, turtles, crocodilians and the few phallus-endowed birds — the two buds fuse.

“Genitalia across the animal kingdom evolve at breakneck speed — they are the fastest-evolving organ,” says evolutionary biologist Menno Schilthuizen of the Naturalis Biodiversity Center and Leiden University in the Netherlands. “Almost anything you can think of —and lots of things you cannot or would not —have arisen and sometimes disappeared again in the nether regions.”

Finding that the tuatara still retains a trace, albeit briefly, of the developmental process that starts phallus formation may resolve a long-standing debate: whether the tuatara lost an ancestral reproductive organ or its cousins independently gained a pair. The rare reptile probably lost the phallus they once shared with snakes and lizards, the researchers conclude. Other evidence suggests that most bird species also lost an ancestral phallus. The tuatara’s loss fits tidily into the scenario that some basic penis evolved just once for mammals, reptiles and the birds that reptiles evolved into.