Expression of the olfactory receptor genes
Transcriptome sequencing using RNA-seq was performed on the nasal mucosa
of the putative olfactory organ (R-001, R-006, R-046), the olfactory
bulb (R-008), and the external skin (R-214 and R-215) of common minke
whales. The FPKM values of β-actin were as follows: R-001, 983; R-006,
285; R-046, 549; R-008, 371; R-214, 161; R-215, 143 (Table 2),
indicating successful RNA extraction from all the samples. In this
study, 81 intact, 12 pseudo, and 266 truncated genes were annotated. The
maximum expression level (as a percentage of FPKM for β-actin) of intact
ORs in negative controls (the olfactory bulb, R-008, and external skin,
R-214 and R-215) was 0.75 in R-008 (Fig. 3). The average expression of
intact ORs with non-zero expression across all samples was 1.014. This
expression level was used as the criterion for significant expression in
this study.
Among the nasal mucosa samples, R-006 exhibit the presence of 22
significantly expressed intact ORs, while five of them were also
expressed in R-001. OMP, on the other hand, was exclusively expressed in
R-006, with an expression level greater than 1.014. In contrast, R-046,
obtained from the anterior portion of the nasal chamber, did not
demonstrate significant expression of ORs or OMP. Out of the 81
annotated intact ORs, all the genes expressed in the nasal mucosa
samples belonged to Class-2 OR. Although four copies of intact Class-1
OR were annotated, none of them were expressed in the six samples
investigated. The significantly expressed intact ORs did not form a
cluster in the phylogenetic tree (Fig. 4). It is worth noting that some
pseudogenes were expressed in the nasal mucosa, and one pseudogene was
exclusively expressed in the external skin, as per our investigation.