Figure.8. The inferred CPLV transduction cascade process of O. oratoria compound eye. Note: LCPL: left-rotation circularly polarized light. GDP: guanosine diphosphate. GPK: G protein-coupled receptor kinase. T: transducing. P: phosphorylation. GTP: guanosine triphosphate. cGMP: cyclic guanosine monophosphate. PDE: phosphodiesterase. cGMP-PDE: cyclic guanosine monophosphate-specific phosphodiesterase. Arr: arrestin protein. H+: hydrogen ion. Ca2+: calcium ion. K+: potassium ion. Cl-: chloride ion. The yellow arrow represents the inferred CPLV transduction cascade process. The black arrow represents the efflux/influx of ion.
4.4 CPLV depends on the shape of microvilli at the retinular cells
The microvilli are the material basis for ensuring the highly sensitive and sharp frequency-selective response to mechanoelectrical transduction (MET) generated by various stimuli (Lelli et al., 2016). A previous study confirmed that light stimulation can cause a series of changes in the ultrastructure of the compound eye, including the decrease of the diameter of the rhabdom, the disordered arrangement of microvilli, the thickening of the cytoplasm around the rhabdom, and the appearance of many organelles (i.e. multivesicular bodies, vesicular lamellar bodies, lamellar bodies and lysosomes) (Luo et al., 2006). In the present study, two genes (MYO III and MYO XV ) related to the regulation of microvilli structure were found to be involved in the light adaptation of O. oratoria compound eye. In fact, myosins encoded by the MYO genes are important molecular motors that can convert the energy generated by ATP hydrolysis into kinetic energy, and have been revealed to contribute to various cellular activities of hearing and vision (Cao et al., 2011; Lelli et al., 2016). Myosins (myosin-3 and myosin-15) encoded by MYO III and MYO XV belong to the unconventional myosins, which are mainly involved in cell movement and intracellular transport of various substances (Cao et al., 2011). Circumstantially, myosin-3 protein was first discovered inDrosophila compound eye photoreceptor cells, and its N-terminal kinase domain can activate multiple types of light signaling elements to initiate downstream cascades that help organisms recognize light (Shieh and Zhu, 1996; Salles et al., 2009). Myosin-15 at the tip of microvilli has also been shown to be involved in the regulation of microvilli growth, and the destruction of myosin-15 often results in abnormal shortening and non-hierarchical distribution of microvilli (Delprat et al., 2005; Manor et al., 2011). Therefore, the up-regulation ofMYO genes and myosin proteins ensures the structural integrity and correct arrangement of microvilli at the retinal cells, and promotes the conversion of LCPL optical signals into effective electrical signals in the O. oratoria compound eye. Another interesting discovering about the MYO genes is that MYO genes are involved in an ingenious automatic-gain control in the compound eye, in which theMYO genes activated by bright light can pulls pigment particles to the rhabdoms to help compound eyes adapt to bright light (Lin-Jones et al., 2009; Peña et al., 2016). Meanwhile, light-induced Ca2+ influx is highly effective in stimulating the motor activation of MYO (Satoh et al., 2008; Shen et al., 2016). It is well known that many lower animals that lack pupillary reflexes have developed pigment granules movements in response to changes in light conditions (Zang and Neuhauss, 2021). Specifically, pigment granules in the O. oratoriacompound eye in the darkness were concentrated at the basal part of pigment cells. With LCPL exposure, the pigment granules in the O. oratoria compound eye will migrate to the fully light adapted position, forming a protective mechanism like “sunglasses” (Ali, 1971). Not surprisingly, MYO III and MYO XV were significantly upregulated in LCPL scenario, which may regulate the structural integrity and arrangement of microvilli in retinular cells, thereby helping O. oratoria compound eye to recognize LCPL.