Researchers in Harvard Medical School's Department of
Genetics, led by departmental chair Clifford Tabin were on a quest to find out
why and how snakes and lizards have two genitalia while birds and people have only
one.
It is noted that snakes and lizards have their paired
structures located somewhat at the level of the limbs, while ours, and the
birds', are located a bit further down. Snakes and lizard genitalia are derived
from the tissue that gives rise to hind legs, while mammalian genitalia are
derived from the tail bud. But despite such contrasts, these structures are
functionally analogous and express similar genes. Researchers found that it was
it all due to the location of cloaca.
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Python embryo at 11 days after oviposition |
The embryonic cloaca issues molecular signals that would
tell the neighboring cells and tissues to form into external genitalia. It is
the cloaca’s location that determines which tissues get the signals first. In
snakes and lizards the cloaca is located closer to the lateral plate mesoderm,
the same tissue that makes the paired limbs, receives the signal. While at the
same time in mammals it is closer to the tail bud.
To further confirm this theory the researchers grafted
cloaca tissue next to the limb buds in one group of chicken embryos and besides
the tail buds in the second group. It was found by the team of researchers that
in both the cases the cells that were grafted closer to the cloaca responded to
the signals and partially converted towards a genitalia fate. This proved that
different cell populations that have potentials respond to the cloaca signals
and contribute towards the genitalia outgrowth.
Clifford Tabin said that mammals and reptilian genitalia are
not homologous as they are derived from different tissues but at the same time
they both share homology in the sense that they are derived from the same
genetic program and induced by the same parental set of molecular signal. "Moreover,
this might help to explain why limbs and genitalia use such similar gene
regulatory programs during development," said Patrick Tschopp, an HMS
research fellow in genetics in Tabin's lab and first author on the paper. He
also added that how, “evolutionary shift in the source of a signal can result
in a situation where functionally analogous structures are carved out of
nonhomologous substrate."
This research throws a lot of light on the evolutionary
aspects of the mammalian and reptilian genitalia. Also the fact that the cloaca
initiates the genitalia outgrowth is very interesting and surprising to know. If
a more in depth research would be done I think it could reveal some more genetic
linkages between mammalian and reptilian traits.
It is very interesting to see that even though snakes and lizards genitalia and mammalian genitalia are obtained in different ways they have similar functions. Who knew the Cloaca position was the cause of this. The experiment needs more results to compare genetic linkages.
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