Zoology Notes 004: Chameleons contain crystals

A panther chameleon in its relaxed state. But how does it manage to change colour so quickly? Photograph: Michel C. Milinkovitch

It’s all about guanine nanocrystals.

What?

Guanine nanocrystals.

Many chameleons are capable of impressive changes in colour. Up until now, most people assumed that this was probably due to the movement of pigment-filled melanosomes within cells. But research reveals a different mechanism altogether, one that involves the interaction between photons and two superimposed layers of skin cells containing nanoscopic crystals of the purine guanine.

A male panther chameleon (Fulcifer pardalis) gets excited (by showing him another male). The original video has been stabilised and accelerated 8 times. A photo of the first frame of the sequence, the chameleon’s original state, is shown in the bottom right. Nature Communications

A mature male panther chameleon from the north of Madagascar, for instance, can undergo rapid changes in skin colour when trying to impress a female or intimidate another male, greens turning to yellow, blues becoming whiter and reds brighter, all in a matter of minutes. “These colours are generated without pigments,” says Michel Milinkovitch, professor of genetics and evolution at the University of Geneva and the senior author of a paper out today in Nature Communications.

Milinkovitch and his colleagues have demonstrated the presence of two layers of light-reflecting cells in the chameleon’s skin, a superficial layer with a regular lattice of guanine crystals in the cytoplasm and a deeper layer with a less organised array of larger crystals. The chameleon’s optical wizardry results from subtle shifts in the orientation of crystals in the upper layer and the combined effect that both layers have on light.

This is what the superficial layer of nanocrystals looks like under an electron microscope. When the chameleon is in its relaxed state (left) the guanine crystals lie in a triangular formation. When the chameleon gets excited (right) the lattice spreads into something more like a square grid. The scale bar is 200 nm. Nature Communications

It is not yet known how the chameleon achieves these structural changes in its skin. One possibility is that swelling or shrinking the cells results in a transformation of the guanine lattice. Alternatively, the crystals could be organised on cytokeletal elements within the cells, says Milinkovitch.

The two layers of specialised cells are present in all the chameleons studied to date. “If this is confirmed in all chameleons, it would suggest that the ancestor of all chameleons had colour-change abilities,” says Milinkovitch. For those species that appear to have lost this ability, the prediction is that the guanine lattice will be less regular and/or less flexible, he says. “Our qualitative preliminary analyses go in that direction.”