Uric Acid Spherulites in the Reflector Layer of Firefly Light Organ

Abstract

BackgroundIn firefly light organs, reflector layer is a specialized tissue which is believed to play a key role for increasing the bioluminescence intensity through reflection. However, the nature of this unique tissue remains elusive. In this report, we investigated the role, fine structure and nature of the reflector layer in the light organ of adult Luciola cerata.Principal FindingsOur results indicated that the reflector layer is capable of reflecting bioluminescence, and contains abundant uric acid. Electron microscopy (EM) demonstrated that the cytosol of the reflector layer's cells is filled with densely packed spherical granules, which should be the uric acid granules. These granules are highly regular in size (∼700 nm in diameter), and exhibit a radial internal structure. X-ray diffraction (XRD) analyses revealed that an intense single peak pattern with a d-spacing value of 0.320 nm is specifically detected in the light organ, and is highly similar to the diffraction peak pattern and d-spacing value of needle-formed crystals of monosodium urate monohydrate. However, the molar ratio evaluation of uric acid to various cations (K+, Na+, Ca2+ and Mg2+) in the light organ deduced that only a few uric acid molecules were in the form of urate salts. Thus, non-salt uric acid should be the source of the diffraction signal detected in the light organ.ConclusionsIn the light organ, the intense single peak diffraction signal might come from a unique needle-like uric acid form, which is different from other known structures of non-salt uric acid form. The finding of a radial structure in the granules of reflector layer implies that the spherical uric acid granules might be formed by the radial arrangement of needle-formed packing matter.

Sharp intense line in the bioluminescence emission of the firefly

“…The photogenic layer houses the light reaction, while the reflector layer is a specialized tissue which is believed to play a key role for increasing the bioluminescence intensity through reflection. Very recently, it was reported that the cytosol of the reflector layer is filled with densely packed spherical granules, which should be uric acid granules [22]. Diameters of these granules are of the order of the wavelength of the emitted light, and it is tempting to speculate that the emitted light waves are trapped by multiple scattering by them.…”

Ultrastructure on the Light Organ of Tropical Synchronize Firefly, Pteroptyx tener

“…This layer is believed to function as a reflector for the luminescent produced in the photogenic layer. This was proven to be true by studies done by Goh et al (2013) that showed this layer was capable of reflecting bioluminescence and increased the light intensity. They also believed that the densely packed tiny granules of uric acid in this layer had allowed the tissues to act as a physical barrier for bioluminescence activity.…”

“…The cells in this layer are considered acidic resulting from eosin failed to stain the cell cytoplasm. According to Goh et al (2013), the granules were uric acid granules and insect uric acid can produce a white pigment. This might explain why the dorsal layer has white and opaque color in living tissues.…”

Photonic Crystal Characterization of the Cuticles of Chrysina chrysargyrea and Chrysina optima Jewel Scarab Beetles

“…There are many reports of the occurrence of uric acid in insect cuticles. For example, it is present in the white stripes of the armyworm Pseudaletia separata [39] and in the reflecting cuticle of the light organ of fireflies [40]. However, exists some controversy with regard to Caveney’s chemical extraction experiments performed in the 1970s that may not support this claim [7,41].…”