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.