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Scientists think they know why snakes get two penises and people only get one

From left-right: Python embryo six days after oviposition, then at six days, then at 11 days. The budding hemipenes and vestigial limb-buds can be seen at the tail end of the embryo. (Patrick Tschopp/Department of Genetics, Harvard University)

Researchers may have figured out why different species develop different kinds of genitalia. In snakes and reptiles, the genitals grow to mimic leg buds — producing twin organs. In humans, the genitals grow to mimic a tail bud — so the penis ends up as a single structure.

In a study published today in Nature, researchers focused on the difference between squamates (snakes and lizards) and amniotes (birds and mammals). In humans and other amniotes, there's only one external genital structure — the human penis, for example.

In snakes and lizards, the external genitals get a little funkier. These reptiles have paired external genitals, even though they only use one at a time during mating.

But all of these genitals spring out of the same embryonic structure, called the cloaca. This structure sends out signals to the cells around it in the embryo, telling them to turn into genitals.

Scientists have wondered why these structures — which are triggered to grow by the same genetic mechanisms during embryonic development, and have the same function in adulthood — develop so differently.

According to this new research, the location of the cloaca might be the key. This embryonic trigger (which eventually turns into the urinary and gut tracks) sends out a molecular call to the cells around it. "Genitals," it says, "make me some genitals."

So those weird reptile genitals? They might look that way because the cloaca is so close to the hind legs (or, in a snake's case, where the hind legs could have been).

"Apparently in both locations, there are pools of cells that have the ability to receive this call from the cloaca," said lead author Patrick Tschopp, a postdoctoral researcher in genetics at Harvard Medical School. "When you move the signaling center, you can recruit different cells."

Sure enough, when Tschopp and his colleagues grafted cloaca tissue next to the budding limbs of some chicken embryos, they found that the cells in the area began to grow into genitals.

So something as simple as a signal's shift in location can drastically influence an animal's evolutionary path.

"This is a great study," said Denis Duboule, chairman of the department of genetics and evolution at the University of Geneva, who wasn't involved in the research. "It's a very interesting new idea. There are these master signals during development, where cells are told to make a limb, or a pancreas. But in this case the same signal is used, and depending on where it's sent from, it will touch different cells."

Tschopp isn't sure why the cloaca locations are different in these two types of animals, but he hopes to find out. And a better understanding of how signal location affects embryo development might eventually have human medical applications.

"It's a bit farfetched, but this may eventually help us understand syndromes in humans that occur when these signals aren't given, or not at the right time, or not for long enough," Duboule said. "The truth is that when you start understanding these things, you also understand what can go wrong when they don't work properly. But that's a whole other story."