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Many cephalopods exhibit remarkable dermal iridescence, a component of their complex, dynamic camouflage and communication. In the species Euprymna scolopes, the lightorgan iridescence is static and is due to reflectin protein-based platelets assembled into lamellar thin-film reflectors called iridosomes, contained within iridescent cells called iridocytes. Squid in the family Loliginidae appear to be unique in which the dermis possesses a dynamic iridescent component with reflective, coloured structures that are assembled and disassembled under the control of the muscarinic cholinergic system and the associated neurotransmitter acetylcholine (ACh). Here we present the sequences and characterization of three new members of the reflectin family associated with the dynamically changeable iridescence in Loligo and not found in static Euprymna iridophores. In addition, we show that application of genistein, a protein tyrosine kinase inhibitor, suppresses ACh-and calciuminduced iridescence in Loligo. We further demonstrate that two of these novel reflectins are extensively phosphorylated in concert with the activation of iridescence by exogenous ACh. This phosphorylation and the correlated iridescence can be blocked with genistein. Our results suggest that tyrosine phosphorylation of reflectin proteins is involved in the regulation of dynamic iridescence in Loligo.

Squids have used their tunable iridescence for camouflage and communication for millions of years; materials scientists have more recently looked to them for inspiration to develop new "biologically inspired" adaptive optics. Iridocyte cells produce iridescence through constructive interference of light with intracellular Bragg reflectors. The cell's dynamic control over the apparent lattice constant and dielectric contrast of these multilayer stacks yields the corresponding optical control of brightness and color across the visible spectrum. Here, we resolve remaining uncertainties in iridocyte cell structure and determine how this unusual morphology enables the cell's tunable reflectance. We show that the plasma membrane periodically invaginates deep into the iridocyte to form a potential Bragg reflector consisting of an array of narrow, parallel channels that segregate the resulting high refractive index, cytoplasmic protein-containing lamellae from the lowindex channels that are continuous with the extracellular space. In response to control by a neurotransmitter, the iridocytes reversibly imbibe or expel water commensurate with changes in reflection intensity and wavelength. These results allow us to propose a comprehensive mechanism of adaptive iridescence in these cells from stimulation to color production. Applications of these findings may contribute to the development of unique classes of tunable photonic materials. Doryteuthis opalescens | iridophore | structural colorA lthough structural color is widespread across both the animal and plant kingdoms (1-4), there are very few cases in which the photonic structures are tunable and adaptive (5-7). Many cephalopods exhibit iridescence, but only a few squid species can tune this iridescence for adaptive camouflage and communication (8,9) by modulating the periodicity of multilayer reflectors (7) in specialized cells classically called iridocytes.[Earlier workers have referred to these cells variously as iridocytes, iridophores, or reflective cells. We use here the unambiguous convention of contemporary cell biology, referring to them as iridocytes (literally "iridescent cells"), with no specific photonic mechanism implied.] For this reason, the tunable photonics of squids have long been a source of intrigue and inspiration to materials scientists (10-12). Although it is known that the neurotransmitter acetylcholine (ACh) activates a signal-transduction cascade to drive the changes in periodicity of the reflectors (7, 13), and the nerve cells delivering this signal recently have been identified (14), details of the molecular mechanisms and cellular architectures governing the biophotonic processes themselves have remained elusive.Morphology of cephalopod iridocytes has traditionally been interrogated by light microscopy ( Fig. 1 A and B) and transmission electron microscopy (TEM) (Fig. 1 C and D), revealing the membrane-bound subcellular lamellae that constitute a Bragg reflector. Whereas these analyses by optical imaging are fundamentally limited because t...

Background: ACh-induced phosphorylation drives assembly of reflectins, dynamically tuning iridescence from subcellular Bragg reflectors in squid iridocytes. Results: Reflectin sequences and phosphorylation sites are characterized from iridocytes with different photonic behaviors. Conclusion: Differences in reflectin structures and phosphorylation determine the emergent photonic behavior of reflective squid tissues. Significance: Biomolecular mechanisms of adaptive iridescence provide new insights into protein-dependent energy transduction and approaches to tunable optical materials.