When it comes to genitalia, the shape, size, location and even quantity varies across the species -snakes and lizards have two penises, while birds and humans have one.
Now researchers have discovered not only how such external genitalia forms, they have identified why the genitals grow in different regions of the body in different animals.
During tests, a team of geneticists were even able to change the location of where the penis grew by tweaking the signals sent by cells
Snake and lizard genitalia is derived from the same tissue that gives creates hind legs, while mammalian genitalia derives from the tail bud, at the bottom of the trunk.
They may appear in different locations, but they have similar functions and genetics.
Reporting in the journal Nature, researchers from Harvard Medical School’s Department of Genetics, led by Clifford Tabin, found that a particular part of the embryo is responsible for this location difference.
The embryonic cloaca – which eventually develops into the urinary and gut tracts – sends signals that tell nearby cells and tissues to form into external genitalia.
The cloaca’s location determines which tissues receive the signal first.
In snakes and lizards, the cloaca is located closer to the lateral plate mesoderm, the same tissue that makes the paired limbs.
But, in mammals, the cloaca is closer to what’s known as the ‘tail bud’, found at the bottom of the animal’s trunk, or torso.
To confirm these findings, the researchers grafted cloaca tissue next to the limb buds in one group of chicken embryos, and beside the tail buds in a second.
The researchers found that in both cases, cells closer to the grafted cloaca responded to the signals and began forming genitals.
While mammal and reptile genitalia are not the same, in that they are formed from different tissue, Professor Tabin said, they do share a ‘deep homology’ – in that they develop from a similar genetic reaction, and are created by the same molecular signals.
‘Here we see that an evolutionary shift in the source of a signal can result in a situation where functionally analogous structures are carved out of nonhomologous substrate,’ said study author Patrick Tschopp.
‘Moreover, this might help to explain why limbs and genitalia use such similar gene regulatory programs during development.’
In a separate study, researchers from University of Florida marked a variety of cells in a chick embryo with a fluorescent marker. They were then able to follow how those cells developed.
From this, they found the cells that turned into either a penis or a clitoris start out as two groups of cells, found on opposite sides of the embryo.
As the embryo curls and forms, and it changes from a flat sheet to a 3D embryo, the cells meet.
They then form a bud and merge to become a penis.
In snakes and other reptiles, the researchers suggest these buds don’t merge in the same way, so the animals form dual penises.
They added that genitals defects in humans may be caused by the embryo not curling and closing correctly.