Lizard You give muralis nigriventris he may not grow to freakish size and smash everything in front of him, but evolution has turned this lizard into an Incredible Hulk of sorts – green skin included. P. nigriventris is something of an imposing Marvel superhero compared to other species of common wall lizards (Podarcis muralis). While the common version is relatively small and brown to greenish-brown, nigriventris the subspecies, found in central Italy, is visually impressive due to its green(er) skin with black markings, larger size and increased aggressiveness.
A team of evolutionary biologists led by Nathalie Feiner from Lund University in Sweden decided to find out which genes contributed to creation P. nigriventris so Hulkish. Like many fictional people with superpowers (but unlike the mutant Hulk), this lizard is a hybrid.
Bulky hybrids
Although common wall lizards are found from the Iberian Peninsula all the way to Asia Minor, the researchers focused on lizards from populations in central Italy (IT lineage) and the southern Alps (SA lineage). These lineages most likely diverged from a common ancestor 5-6 million years ago and then began to hybridize—individuals from different lineages mated to produce hybrid offspring.
P. nigriventris originated as a subspecies of the IT lineage. However, some of its features have been carried over from the SA lineage. This is called introgression, which occurs when genetic information is transferred from one lineage or species to another as a result of hybridization. The traits the researchers focused on were back and belly coloration, flank blue spots, body mass and head length.
Because P. nigriventris traits made their way into the SA lineage through introgression, it can be difficult to tell what is truly ancestral. It was first assumed that females selected for P nigriventris traits in men, which increased their chances of finding a partner. However, some of these features were later seen in females, so these traits are not exclusive to males competing for females.
“Various lines of evidence support the existence [these traits] in both sexes, i [the traits] remain intact during introgression into a distantly related lineage,” the researchers said in a study recently published in Science Advances.
Brownish P. muralis they are considered the ancestral phenotype. To see how nigriventris a phenotype appeared, Feiner would need to find where the traits that make up this phenotype are located in the lizard’s genome.
Green genes
What Feiner and her team discovered while analyzing lizard genomes is that the features that make P. nigriventris the genes that regulate the cells of the neural crest are determined. These multipotent stem cells, which arise during the early embryonic stage, can turn into different types of more specialized cells throughout the embryo, contributing to everything from heart valves to color.
One such gene, Rbm10, regulates the proliferation of neural crest cells, i.e. their growth and division, while others, Tfcp2l1, regulates how they differentiate into more specialized cells as the embryo develops. One more, Gpc3, it is needed by neural crest cells to migrate to different locations in the embryo where they integrate into tissues.
But where was the difference that would ultimately change the color nigriventris phenotype? The answer was in the region of the chromosome that contains the gene, known as Rab18, which also helps with neural crest cell migration. Rab18 and Acbd5 both have a role in pigment production. It could be related to green and black P. nigriventris.
“Gene flow analyzes revealed that this [chromosomal] a region likely introgressed from the IT lineage into strongly expressing SA lineage populations nigriventris phenotype,” the researchers said in the same study.
While the exact factors that caused these green and black P. nigriventris monsters (at least monsters next to their own P. muralis relatives) to develop different colors to adapt to changes in their environment are still unknown, and where their appearance came from is no longer a mystery.
At least the Hulk-ified lizards don’t have to be exposed to extreme radiation to get their size, color and strength, unlike Bruce Banner himself.
Scientific progress, 2024. DOI: 10.1126/sciadv.adk9315