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article imageWhy do we have one genitalia and lizards have two?

By Tim Sandle     Nov 7, 2014 in Science
Having one set of genitalia is not always the case in the natural world. For instance, snakes and lizards have two. Why is this the case? Biologists have been looking into the issue and now have some answers.
Not only are there differences with the number of genitalia, there are differences with the location of the sexual organs too. With people, and birds, genitalia are located further down. With lizards, they are located at the level of the limbs. With snakes and lizards genitalia are derived from tissue that gives rise to hind legs.
Considering this further, biologists have concluded that embryonic cloaca, which eventually develops into the urinary and gut tracts, sends molecular signals. These signals tell neighboring cells and tissues to form into external genitalia.
It seems that the cloaca's position determines which tissues receive the signal first. In snakes and lizards, the cloaca is located in a different place to mammals; this position determines the location where the genitalia form.
To verify this, researchers grafted cloaca tissue next to the limb buds onto a group of chicken embryos; and beside the tail buds with a second group. The scientist found that cells closer to the grafted cloaca responded to the molecular signals and this determined the location where genitalia formed.
The finding may help scientists to understand why limbs and genitalia use similar gene regulatory programs during development. This could help understand human development better.
The study was carried out by Harvard Medical School's Department of Genetics and the findings have been published in the journal Nature. The research is titled “A relative shift in cloacal location repositions external genitalia in amniote evolution.”
More about Genitals, sex organs, Genitalia, Lizards, Snakes
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