Loss of limbs in lizards and snakes occurred by different evolution, says study

Adapted to the dunes of the São Francisco River, in northern Bahia, the small lizard Calyptommatus sinebrachiatus stands out for the agility with which it captures prey and flees from predators. Interestingly, it is possible that this ability is due to a characteristic not very common among lizards: it does not have legs. The species belongs to one of the 26 groups of animals that have reduced or absent limbs, including snakes.

One might imagine that limb loss occurred in the same way in both lizards and snakes, as these groups differentiated around 100 million years ago. However, in an article published in the journal Cell Reportsresearchers from Germany and Brazil show that the story is somewhat more complex.

“In previous work, where we only looked at snakes, we found a very clear signature of genomic changes associated with genes involved in limb development. In lizards, this signature is not so clear,” says Juliana Gusson Roscito, scientific coordinator at the Dresden-Concept Genome Center in Germany, who led the two studies.

In the most recent work, the group of researchers sequenced the genome of the C. sinebrachiatus and another related South American lizard, but with legs, the Tretioscincus oriximinensis.

In order to find differences that could explain the absence of legs in the lizards, the genomes were aligned with complete sequences from another legless lizard (the Asian dopasia gracilis), in addition to five species of snakes, seven lizards with legs and, in addition, mammals, birds and fish, representing the other vertebrates.

“Rather than focusing the analysis on the genes that control limb formation – which are all present and intact in both limbless lizards and snakes – we looked for changes in regions that regulate the temporal or spatial pattern of expression of these genes. We observed that regulatory regions that are conserved among limbed vertebrates have undergone several mutations in limbless species. These regions are close to the genes involved in the formation of limbs, indicating that changes in the regulation of gene expression, and not the genes themselves, may be associated with the loss of limbs in these species”, explains Roscito.

The researcher believes that it was interesting to observe that different mechanisms can contribute to the loss of limbs. In lizards, probably, multiple evolutionary paths have led to this phenomenon.

In the three lineages of limbless reptiles analyzed, the set of regulatory regions that have mutated is relatively different, indicating that there is no single evolutionary path to limb loss.

Both the current and previous work on snakes are part of a project supported by the Fundação de Amparo à Pesquisa do Estado de São Paulo (Fapesp) within the scope of the Biota Program and coordinated by Miguel Trefaut Urbano Rodrigues, professor at the Biosciences Institute of the University of São Paulo. of São Paulo (USP) and co-author of the article.

Part of the research was carried out during Roscito’s postdoctoral work at USP, and in a research internship she carried out at the Max Planck Institute for Molecular Cell Biology and Genetics, also in Dresden.

differences

Genomic analyzes of snakes and lizards revealed three of the 26 times that limbs were lost or reduced during the evolution of the order Squamata, composed of these two groups of reptiles.

“It’s not about the same regulatory regions. In snakes, the mutations are quite evident, as they lost their limbs about 100 million years ago. That’s enough time to accumulate many mutations in the genome. In these lizards, which lost their limbs between 30 and 40 million years ago, the changes are more ‘diluted’, showing that there was not enough evolutionary time for an accumulation of mutations that would make it possible to identify the possible molecular mechanism involved with the absence of legs”, account.

On top of that, adds Roscito, the mutations were not predominantly in the same regions of snake genomes. The researchers noted that the elements that regulate limblessness are specific to each reptile lineage, yet another example of so-called evolutionary convergence, when similar traits can occur through different evolutionary paths.