Development of olfactory organs and gene expression patterns of representative olfactory receptors in blunt snout bream (Megalobrama amblycephala)

The olfactory organs development of blunt snout bream (Megalobrama amblycephala) was investigated using histological and morphological methods. The results revealed that the olfactory organs of M. amblycephala were positioned on the back and in front of the eyes. The olfactory epithelium (OE) was oval-shaped near the base of the olfactory cavity. The olfactory placodes were parallelly arranged in the cranio-caudal direction. There were concave marks on the olfactory fossa’s periphery when fish were hatching out. Afterwards, an elongated olfactory placode was formed in the olfactory fossa’s core. The arrangement of the primary olfactory placodes of the unilateral OE were from loose to tight with M. amblycephala growth. The height and number of the olfactory placodes, as well as the overall surface area of the OE gradually expanded and stabilized in the adult stage. qRT-PCR were used to studied the expression patterns of representative ORs. The results showed that ORs expression differed in different stages and tissues of M. amblycephala. ORs Beta-2, 9, 10 and 11 were highly expressed at several stages of embryonic development, including the blastocyst stage, blastopore closure stage and olfactory lamellae stage (P<0.01). Beta-2, 9, 10, 11 and Epsilon-7 were weakly expressed at 3 to 15 dpf, in the larval and juvenile stages, but highly expressed at 30 to 60 dpf (P<0.01). Beta-2, 10, 11 and Epsilon-10, 13 were strongly expressed in the OE of 4 periods from juvenile to adult stages (including 3, 6, 12 and 24-month-old) (P<0.05). Beta-2, 9, 10, 11 and Epsilon-7, 10, 13 were also highly expressed in the OB (olfactory bulb) in 3-month-old M. amblycephala (P<0.01), but not expressed in the 6 to 24-month-old stage. Beta-2, 11 expression levels were low in the brain at the 4 age periods (P<0.05). These findings provide theoretic evidence for further exploration of the functions of ORs in fish, and deepen the research on fish olfactory recognition.