The secret of the green-blooded lizards

There are several lizard species in New Guinea with green blood, instead of the usual red. The lime green colour is due to high concentrations of bile pigments in their bloodstreams, and are toxic in other vertebrates. Now, a genomic analysis of the lizards reveals this remarkable physiology may have arisen in the group four separate times in their evolutionary history.

The bile pigments, known as biliverdin and bilirubin, are toxic by-products of red blood cell catabolism – the process by which enzymes breakdown large molecules into smaller components. In humans, and, indeed, all other vertebrates, chronic accumulation of these pigments in the blood causes jaundice.

But this small group of lizards, in the genus Prasinohaema, have such high concentrations in their blood that their muscles, flesh and bones are bright green. Yet they suffer no ill effects. Understanding the evolution and mechanism of such an unusual physiology may provide insights into jaundice and related diseases in humans. In adults, the condition indicates serious underlying disease, usually related to the liver or gall bladder.

An earlier study from the University of Texas, US, found that Prasinohaema plasma contains a biliverdin concentration approximately 40 times greater than that found in jaundiced humans. How the lizards avoid developing the condition is unknown.

In the latest study, a team led by Zachary Rodriguez from Louisiana State University, US, has brought us a step closer to unpacking the mystery, with research revealing that green blood evolved independently in the Prasinohaema genus four times. 

The lizards are skinks, members of the very diverse Scincadae family. In terms of body shape and habitat they bear little resemblance to each other – and are only classified as belonging to a single genus because of the colour of their blood. Until now, their evolutionary relationship has been unclear.

Rodriguez and colleagues analysed the genome data and conducted a phylogenetic and ancestral state character reconstruction in 24 individual lizards from six species in the genus, along with 95 related Australasian lizards with normal red blood.

The team’s analyses indicate four independent origins of green blood from a single red-blooded ancestor. The researchers say that the discovery of multiple origins demonstrates the “surprising evolutionary dynamism of green blood”.

Now that a thorough analysis of the data has been done, the stage is set, as it were, for further analysis into the role natural selection may have played in shaping this curious trait, as well as understanding the genetic and biochemical basis for the lizards’ remarkable lack of jaundice.

The study is published in the journal Science Advances.