Olfaction in birds: differential embryonic expression of nine putative odorant receptor genes in the avian olfactory system

We have isolated nine putative odorant receptor genes from the chick, named COR1 to COR9, that belong to the large multigene family of olfactory G protein-coupled receptors found in the fish, rat, mouse, dog, and human. By combining genomic DNA blot analysis, low stringency library screenings, and several PCR analyses, we were able to detect approximately 20 COR genes in the chick genome highly related to COR1-9. By in situ hybridization of newborn and adult, COR expression was detected only in the olfactory epithelium, and exhibited a random spatial distribution. During development, COR expression was observed as early as embryonic stage E5. Different levels of gene expression were observed for the COR1-9 genes: at E5, COR1-6 expression was high compared to the expression of COR7, COR8, and COR9. Surprisingly, at E5, a row of COR1-6 positive cells probably associated with the olfactory nerve extended outside the olfactory placode, reaching the anterior pole of the developing forebrain. These results suggest that, in addition to their role as putative odorant receptors, some COR may play a role in the development of the avian olfactory system.