Amorphous calcium phosphate in the pupal cuticle of Bactrocera dorsalis Hendel (Diptera: Tephritidae): A new discovery for reconsidering the mineralization of the insect cuticle. (November 2019) - Rong, Jingjing
Graphical abstract: Highlights: Amorphous calcium phosphate was found in the pupal cuticle of Bactrocera dorsalis. This amorphous calcium phosphate has a composition similar to that of CaHPO4 ·2H2 O. The mineralization of ACP is upregulated when the tanning progress is restrained. Abstract: It is now widely accepted that Hexapoda emerged from Crustacea. Compared to the ubiquitous calcified exoskeleton in crustaceans, a mineralized cuticle in insects is extremely rare. Catecholamine-driven protein cross-links play a leading role in the sclerotization of insect cuticle. In this study, mineralization was discovered in the pupal cuticle of Bactrocera dorsalis (Diptera: Tephritidae), a common pest of fruit farms. We mainly profiled the features of mineralized pupal cuticles from B. dorsalis and its white mutant B. dorsalis wh and unmineralized cuticle from Musca domestica using high-resolution field emission scanning electron microscopy (SEM) and transmission electron microscopy (TEM) combined with structural analysis involving infrared (IR) spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and synchrotron X-ray absorption near-edge structure (XANES) spectroscopy. We also compared the thermodynamic and mechanical properties of different pupal cuticles. The results showed that the pupal cuticles of B. dorsalis contain a phase of stable amorphous calcium phosphate (ACP) with a high level of magnesium, which is mainly distributed in the exocuticle and assists in the formation of a graded, stiffened cuticle structure. Unexpectedly, this ACP possesses a very low Ca/P ratio and has a composition similar to that of CaHPO4 ·2H2 O. The degree of mineralization in the pupal cuticle of B. dorsalis wh (approximately 22 wt%) is significantly greater than that of wild-type B. dorsalis (approximately 12 wt%), which indicates that there may be a connection between the biomineralization and tyrosine-mediated tanning pathways. These findings provide new evidence for the mineralization of the insect cuticle, which may shed new light on the evolutionary mechanism underlying the divergence of cuticle sclerotization between insects and crustaceans.