3.8 Divergent expression of opsin genes
Opsin expression plays a key role in facilitating fish ecological
adaption and evolution (Hofmann & Carleton 2009). To determine
functional changes in the expanded genes, we performed expression
analysis of opsin via RNA-seq of the retina tissue of C.
undulatus . A total of 44,028,650 clean reads were obtained, and 3,849
genes showed higher expression (FPKM > 15), accounting for
17.3% of the total retina-expressed genes. Genes highly expressed in
the retina include genes encoding retinal dehydrogenase 5 (FPKM: 15.34),
which catalyzes the final step in the biosynthesis of 11-cis
retinaldehyde to produce a light-sensitive chromophore (Skorczyk- Werneret al. 2015). Genes encoding retinal dehydrogenase 9 (FPKM:
26.98) are capable of converting 9-cis retinal to corresponding retinol
with high efficiency. In contrast, the gene encoding retinol
dehydrogenase 10 (FPKM: 18.63) converts all-trans-retinol to
all-trans-retinal, which plays a profound role in chromophore generation
at the level of rhodopsin (Tian et al. 2013). The cluster
analysis of opsin genes showed divergent expression (Fig. 9).SWS2 a, SWS2 d, and Rh1 b are expressed in the retina,
whereas SWS1 , SWS2 c, Rh1 a, and Rh2 b were
expressed in other areas instead of the retina, suggesting functional
changes after gene duplication, and acquisition of novel functions
(Porath-Krause et al. 2016).
It is interesting to note that LWS1 was not expressed in any
tissue. In many fishes, LWS1 and SWS2 spectrally reside in
a head-to-tail pattern (Mackin et al. 2019), while in beetles,SWS2 was lost recently, and the sensitivity of SWS2 to
blue wavelengths was restored by LWS1 extra copies (Sharkeyet al. 2017). It has been reported that labrid species have shown
diverse expression of opsin genes in adaption of variable visual
sensitivities to drive phenotypic diversity and behavioral ecologies
(Phillips et al. 2016). A recent study found that the expression
of the LWS1/SWS2 gene is a stochastic event, and exhibits a
switch between opsin genes, even copies of which are triggered by the
endocrine signal thyroid hormone (Mackin et al. 2019). This
mechanism of opsin expression is useful to provide alternative adaption
of visual cues in the developmental stages of organisms, the
environment, and behavior. The alternative expression of opsin genes may
be used to adapt to different photic environments for juveniles growing
into C. undulatus adults, which live in different habitats
(Sadovy et al. 2003), and opsin expression variation is also
highly correlated to feeding strategy in damsels with herbivorous
feeders (Stieb et al. 2017). Furthermore, C. undulatus is
a protogynous hermaphrodite, changing sex from female to male around 8-9
years of age (Sadovy et al. 2003), and environmental stimuli,
such as a unique mating behavior during spawning aggregations, is a
primary trigger of sex change (Todd et al. 2019). Opsin gene
expression divergence is believed to be responsible for detecting and
discriminating between mating partners (Sandkam et al. 2017). Our
results showed that the opsin gene expression pattern plays an important
role in visual plasticity, development, behavior, and reproduction.