Why Don't Snakes Have Legs? Scientific Research into the Reasons Snakes Lost Their Limbs!
Why don't snakes have legs? Research has explained how snakes lost their limbs! Snakes are reptiles, and since they are reptiles, they don’t have legs. Without legs, how do we call them reptiles? Don’t many children ask this question? Snakes are very strange animals, they are living fossils on earth, they have lived for millions of years, at the same time as extinct animals, while snakes have survived. Scientists have found that ancient snakes did have legs, so why did they evolve to lose their limbs? This article gives you the answer.
Snakes and lizards belong to the order Squamata. They have several common features, but differ in one obvious aspect: snakes don’t have limbs. These suborders diverged about 140 million years ago.
The focus of this article is identifying genetic factors involved in this limb loss. ‘Julinian Gusson Roscito and Nature Communications’ collaborative article examines gene regulatory landscape associated with phenotypic loss and broad divergence in evolution regarding limbs.
The article’s other interesting focus is the degeneration of eyes in some subterranean mammals.
‘We investigated these two cases to understand a more general process, how changes in the genome during evolution lead to phenotypic changes,’ Roscito said.
Roscito is currently a research fellow at the Max Planck Institute of Molecular Cell Research and Genetics in Dresden, Germany, where he was also a postdoctoral researcher in Brazil, supported by FAPESP – São Paulo Research Foundation. Her postdoctoral scholarship was related to the project ‘New Neotropical Reptiles and Amphibians: Comparative Systematic Geography, Systematics, Paleo-Climate Modeling and Taxonomy’, with Miguel Trefaut Urbano Rodrigues as the main researcher under the BIOTA-FAPESP biodiversity characterization, conservation research plan, restoration and sustainable use.
Rodriguez is a full professor at the Institute of Biosciences of the University of São Paulo (IB-USP) and supervises Roscito’s postdoctoral research. He is also a co-author of the recently published article.
‘This study involves investigating the genomes of several vertebrates, including identification of genetic regions that have changed only in snakes or subterranean mammals, which remain unchanged in other species without limb or normal eyes,’ Roscito said.
‘In mammals with degenerate visual systems, we know that several genes have been lost, such as those related to the lens and photoreceptor cells of the retina. These genes underwent mutations during evolution. Eventually, they lost their function, meaning the ability to encode proteins. This is not what happened with snakes, they didn’t lose genes related to limb formation. More accurately, genome sequencing studies did identify a gene loss, but only in this case, our methods included not investigating genes, but investigating gene expression elements.’
Gene expression depends on the regulatory elements within genes, which are transcribed into RNA, then translated into proteins. This process is regulated by enhancer sites (CREs), which are DNA sequences near the genes they regulate. CREs control the spatiotemporal and quantitative patterns of gene expression.
Regulatory elements can activate or inhibit gene expression in a part of the body, such as a limb, and different regulatory elements can activate or inhibit the same gene in different parts of the body, for example, if a gene is lost, it is no longer expressed in two places, which can have a negative impact on the formation of the organism.
However, if only one regulatory element is lost, the expression may disappear in one part, while remaining conserved in another part,’ Roscito explained.
Tegu lizard
From a computational perspective, CREs are not as easily identifiable as genes. Genes have a characteristic grammar, with base pairs showing the start and end positions of the gene. This is not the case for CREs, so they must be identified indirectly. This identification is often based on DNA sequence conservation in many different species.
‘To detect specific sequences in snakes, it was necessary to compare the snake’s genome with the genomes of various reptiles and other fully limb-bearing vertebrates. The genome of reptiles with well-developed limbs is rare, so we sequenced and assembled the genome of the first known tetra-pod lineage species, the Tegu lizard (Salvator merianae).’ ‘The author said.’
‘Using the Tegu genome as a reference, we created genome comparisons for several species, including two snakes (python and boa), three other limbed reptiles (green lizard, bearded dragon and gecko), three birds, an alligator, three turtles, 14 mammals, one frog and a cartilaginous fish. This comparison of 29 genomes was used as the basis for all further analyses,’
Researchers identified more than 5,000 DNA regions that are considered candidate regulatory elements in several species. Then, they used the ingenious procedures described in the article to search for a CRE set that may have caused the loss of limbs in the snake’s ancestors.
‘There are studies involving a well-known regulatory factor that regulates a gene, when it is modified, it can cause limb defects. The snake has a mutation in this CRE. In a 2016 study, the mouse CRE was replaced by a species that may cause limb loss - a functional proof of this mechanism,’ Roscito said.
‘Our research expands the set of CREs. We found that some regulatory elements that regulate many genes have mutated in snakes. The signal cascade is more comprehensive.’
