Researchers looked into why different species' genitalia is varied in number and what type of tissue it arises from, despite expressing similar genes and being functionally analogous; for example, snakes have two sets while humans have only one.
The researchers determined that location was the most important factor behind whether genitalia originated from tissue that also gives rise to hind legs (like in snakes) or tail bud tissue (like in mammals), Harvard Medical School reported.
"While mammal and reptile genitalia are not homologous in that they are derived from different tissue, they do share a 'deep homology' in that they are derived from the same genetic program and induced by the same ancestral set of molecular signals," said departmental chair Clifford Tabin.
The embryonic cloaca, which eventually develops into the urinary and gut tracts, issues molecular signals that prompt neighboring cells to transform into external genitalia. The cloaca's location determines which tissue receives the signal first; where this mechanism is located can determine at what location these organs spring up.
To further confirm their findings the researchers grafted cloaca tissue next to the limb buds in a group of chicken embryos and beside the tail bud in a second group. They found in both cases the cells closer to the grafted cloaca responded to the signals and partially converted into genitalia.
"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 Patrick Tschopp, an HMS research fellow in genetics in Tabin's lab and first author on the paper. "Moreover, this might help to explain why limbs and genitalia use such similar gene regulatory programs during development."
The findings were published in a recent edition of the journal Nature and supported by the National Institutes of Health.
