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Recent genome wide in silico analyses discovered a new family (type 2 or family H) of odorant receptors (ORs) in teleost fish and frogs. However, since there is no evidence of the expression of these novel OR genes in olfactory sensory neurons (OSN), it remains unknown if type 2 ORs (OR2) function as odorant receptors. In this study, we examined expression of OR2 genes in the frog Xenopus tropicalis. The overall gene expression pattern is highly complex and differs depending on the gene and developmental stage. RT-PCR analysis in larvae showed that all of the OR2η genes we identified were expressed in the peripheral olfactory system and some were detected in the brain and skin. Whole mount in situ hybridization of the larval olfactory cavity confirmed that at least two OR2η genes so far tested are expressed in the OSN. Because tadpoles are aquatic animals, OR2η genes are probably involved in aquatic olfaction. In adults, OR2η genes are expressed in the nose, brain, and testes to different degrees depending on the genes. OR2η expression in the olfactory system is restricted to the medium cavity, which participates in the detection of water-soluble odorants, suggesting that OR2ηs function as receptors for water-soluble odorants. Moreover, the fact that several OR2ηs are significantly expressed in non-olfactory organs suggests unknown roles in a range of biological processes other than putative odorant receptor functions.
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22509266
???displayArticle.pmcLink???PMC3324471 ???displayArticle.link???PLoS One
Figure 1. Phylogenetic tree of X. tropicalis OR1 and OR2 genes. Amino acid sequences of all XtOR2 and all XtOR1 class I (OR(I)), four XtOR1
class II ORs (OR(II)c), and three melanocortin receptors (MCRs), were used for the phylogenetic analysis.
doi:10.1371/journal.pone.0033922.g001
Figure 2. XtOR2 gene expression in various organs in stage 55 larvae and full-grown adults. The primer set for class II OR amplified multiple class II OR genes. PCR cycles were adjusted to obtain adequate amounts of the products (35 cycles for OR2 and class I ORs, and 30 cycles for rpL8 and class II ORs).
doi:10.1371/journal.pone.0033922.g002
Figure 3. OR gene expression in adult olfactory cavities. A: Schematic illustration of the nasal cavities of the adult frog. The MC is filled in
water and the PC is open to air. The air flow goes though the PC to the lung. B: The expression of OR2gs and some OR1s (class I and II) in adult nasal
cavities. Quantitative PCR was done for each OR gene and normalized by using the OMP gene, which is expressed in every mature OSN. Most class I
ORs were preferentially expressed in the MC. Class I ORa5 was exceptionally expressed at a significant level in the PC as well as the MC. Bars represent
standard deviation (n = 3). Note that the expression level of OBP and class II (mix) OR was much higher than each single OR2g and class I OR gene
(different scale). Gene ID: OR1a2: ENSXETG00000024801.1, OR1a4: fgenesh1_pg.C_scaffold_1078000009, OR1a5: e_gw1.2098.6.1, OR1b3:
fgenesh1_pg.C_scaffold_976000003, OR1b5: e_gw1.976.19.1, OR1d2: e_gw1.799.9.1, OR1e4: e_gw1.799.69.1.
doi:10.1371/journal.pone.0033922.g003
Figure 4. Whole mounts in situ hybridization for the XtOR2g4
and XtOR2g5 gene in the olfactory cavity of stage 47 tadpoles.
XtOR2g RNA and OMP were detected by Cyanine 3 (red) and
fluorescein (green), respectively, and the nucleus was stained with
Dapi (blue). A: A confocal image of the olfactory cavity. A: XtOR2g5
expression (red). B: Enlargement of dotted square region in panel A. C:
XtOR2g4 expression (red). D: A z-stack of all optical planes. XtOR2g4
positive cells (red). OC; olfactory cavity, OE; olfactory epithelium, VNO;
veromonasal organ.
doi:10.1371/journal.pone.0033922.g004
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