First Lizard Genome Sequence Could Unveil Secrets of Evolution: Scientists

The green anole lizard, also known as Anolis carolinensis, is the first non-bird species of reptile to have its genome sequenced and assembled.

Researchers at the Broad Institute of MIT and Harvard in Cambridge, Mass., tapped into the relatively unexplored genetic landscape of reptiles to produce the first genome sequence of a lizard. The anole lizard, an agile and active creature, is a native of the Southeastern United States.

Researchers said the new clues that have emerged from the full sequencing of the lizard's genome may offer some insight into how the genomes of humans, mammals, and other reptiles have evolved.

The findings are reported online in the Aug. 31 issue of the journal Nature.

Sometimes you need to be at a certain distance in order to learn about how the human genome evolved, said Jessica Alföldi, co-first author of the paper and a research scientist in the vertebrate genome biology group at the Broad Institute. You have to look out further than you were looking previously.

Broad researchers have assembled and analyzed more than 20 mammalian genomes to include those of some of our closest relatives. However, the genetic makeup of reptiles needed deeper exploration. Mammals and reptiles parted ways around 320 million years ago.

People have been sequencing animals from different parts of the vertebrate tree, but lizards had not been previously sampled, said Kerstin Lindblad-Toh, scientific director of vertebrate genome biology at the Broad and senior author of the Nature paper, in a statement. This was an important branch to look at.

Lizards are more closely related to birds than to any of the other organisms whose genomes have been sequenced in full. Like mammals, birds and lizards are amniotes, meaning they aren't restricted to laying eggs in water.

About 400 species of anole lizards have fanned out across the islands of the Caribbean, North America, Central America, and South America. This has made them an appealing model for studying evolution. Researchers know much about the lizard's biology and behavior, but their genomic information may have been a critical missing piece for understanding how the lizards became so diverse.

The team found the sex chromosomes of the lizard, which is something researchers were only able to hypothesize about in the past. Like mammals, green anoles seem to have XX and XY chromosomes. However, in birds males have two identical sex chromosomes called ZZ and females have two different ones known as ZW. The lizard's X chromosome turned out to be one of its many microchromosomes.

By sampling the genomes of more than 90 species, the researchers were able to make a preliminary map of how these species evolved to colonize the islands they inhabit. The researchers were also able to create a parts list of proteins found in green anole eggs. They then compared that with those found in eggs from chickens and found that both bird and lizard egg genes are evolving rapidly.

Researchers also found many genes in the anoles genome that's associated with color vision. Anoles rely on this to identify choice mates.

Anoles are rich in ecology and morphology and have just the right amount of diversity to make them interesting yet tractable to study, said Jonathan Losos, an author of the paper and professor at Harvard University. But a big stumbling block in studying them has been that they have not been great organisms for classical genetic study. The genome is going to revolutionize our ability to study that aspect of their evolutionary diversification.

Researchers said one of the questions this newly sequenced genome may help resolve has to do with the origin of conserved, non-coding elements in the human genome. These regions don't have protein-coding genes but are thought to have critical roles since they have remained unchanged for millennia. They said in humans, many of these so called jumping genes have lost their jumping ability, but in anole lizards, they continue to hop.