320 million years ago, mammals and reptiles reached an evolutionary parting of the ways. We've now sequenced a lizard genome for the first time ever, and it's vastly different from our own...but in a few crucial ways, it's shockingly similar.
The green anole lizard of the southeastern United States is the first non-bird species of reptile to have its genome sequenced. While we've previously sequenced nearly two dozen mammalian genomes, reptiles - and lizards in particular - have remained largely ignored. Like birds and mammals, reptiles are special among vertebrates because they don't have to lay their eggs in water, which means they're fundamentally tied to the land in a way that amphibians and fish rather obviously are not.
There are over 400 species of anole lizards found in nearly every corner of the Americas, making them one of the most intriguing groups to study in terms of their evolutionary history. But this isn't just about lizard evolution - the anole lizard may have already shed some much-needed light on a mysterious part of the human genome. Within our genome are several key sequences that are non-coding - in other words, they don't serve any immediate function - but have remained largely unchanged in our genome for many thousands of years.
What they might be are the husks of special DNA sequences known as transposons. These can only be described as "jumping DNA", able to actually move through the genomes and copy and paste themselves elsewhere. Transposons can give any genome that carries them great agility and resilience in dealing with unexpected environmental challenges.
It appears humans can do just fine without them, as they've been inactive for a long, long time, but these transposons are alive and leaping inside the green anole lizard genome. The researchers took these transposon sequences and matched them to nearly a hundred different non-coding sequences inside the human genome, which would indicate that they share a common history. The exact mechanics of why they became non-coding is still a mystery, but at least we now know a key step in their evolutionary journey.
The green anole genome is also pretty remarkable in its own right. We can confirm that lizards possess the same sex chromosomes as we and other mammals do - males have the XY combination while females have XX. Birds, for instance, have a completely different setup featuring entirely different sex chromosomes, as male birds carry what's known as the ZZ pair and females have the ZW pair.
The lizard also possesses an unusual feature known as microchromosomes - these are pint-sized version of ordinary chromosomes that reptiles, amphibians, and fish all sometimes possess, but are entirely absent in mammals. In fact, the green anole X chromosome is one of these microchromosomes.
The researchers were also able to compare the proteins found in the eggs of the green anoles with thoses in chicken eggs. It appears that, as far as egg genes are concerned, reptiles are in a constant state of evolutionary flux, with the proteins revealing clear signs of rapid evolutionary change. The lizard also had a huge number of genes devoted to vision, which fits with their very strong vision - indeed, some of these anole lizards can see into the ultraviolet spectrum.