Tucked in the genomes of snakes, there are some components known as limb enhancers, or sequences that switch genes off or on in the creation of limbs.
That is what Douglas Menke and Carlos Infante of the University of Georgia found when they set out to understand what happened to the limbs of snakes and what role do these limb enhancers still play in the evolution of a snake embryo.
There have been previous studies assessing the importance of hundreds of limb enhancers present in the genomes of humans or mice and their crucial role in limb and external genitalia development. But none do assess their role in the development of snakes, creatures that have been crawling for millions of years. Did the limb enhancers change or have they been repurposed on the evolutionary path?
To complete their research, the scientists used the sequenced genomes of the boa constrictor, the king cobra and the Burmese python. In all three species they found 65 mammalian limb enhancers. They haven’t changed, they are easy to recognize. Yet, snakes have been crawling for 150 million years.
In order to find out what the role of the limb enhancers, Infante and Menke transplanted them to mice embryos. They acted as expected. Stitched perfectly on the mice DNA, they enabled the development of limbs and external genitalia, but played no role in the overall development of the skeleton or the mice’s brains.
In the paper featuring in the Developmental Cell journal, the researchers describe their findings. The reason why snakes have hold on to limb enhancers for so long is that these sequences play a crucial role in developing the external genitalia. They may not need them for limbs anymore, but their reproductive behavior is at their mercy. Professor of Genetics Douglas Menke stated:
“There have been many millions of snake generations since they evolved a legless body, and we would generally expect the DNA associated with limb development to fade away or mutate to do another job, but that doesn’t seem to have happened”.
With such strong evidence to pinpoint the role of noncoding DNA (the limb enhancers), the research team proposes that they are rather referred to as ‘appendage enhancers’. This is would signal their very specific role in the case of snakes.
Understanding the role of noncoding DNA known as limb enhancers is still in the incipient phases.
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