Olfactory receptor subgenome and expression in a highly olfactory procellariiform seabird

Procellariiform seabirds are known for their well-developed olfactory capabilities, reflected by their large olfactory bulb to brain ratio and olfactory-mediated behaviors. Many species in this clade use olfactory cues for foraging and navigation, and some species can recognize individual-specific odors. Their genomes and transcriptomes may yield important clues about how the olfactory receptor (OR) subgenome was shaped by natural and sexual selection. In this study, we assembled a high-quality Leach9s storm petrel (Oceanodroma leucorhoa) genome to facilitate characterization of the OR repertoire. We also surveyed expressed OR genes through transcriptome analysis of the olfactory epithelium - to our knowledge, the first avian study to interrogate OR diversity in this way. We detected a large number (~61) of intact OR genes, and identified OR genes under positive selection. In addition, we estimated that this species has the lowest proportion (~60%) of pseudogenes compared to other waterbirds studied thus far. We show that the traditional annotation-based genome mining method underestimates OR gene number (214) as compared to copy number analysis using depth-of-coverage analysis, which estimated a total of 492 OR genes. By examining OR expression pattern in this species, we identified highly expressed OR genes, and OR genes that were differentially expressed between age groups, providing valuable insight into the development of olfactory capabilities in this and other avian species. Our genomic evidence is consistent with the Leach9s storm petrel9s well-developed olfactory sense, a key sensory foundation for its pelagic …