Genome-Wide Identification and Characterization of Olfactory Receptor Genes in Silver Sillago ( Sillago sihama ).

Olfactory receptor (OR) genes are essential in the specific recognition of diverse stimuli in fish. In this study, a total of 141 OR genes were identified in silver sillago ( Sillago sihama ), a marine fish sensitive to environmental stimuli, including 112 intact genes, 26 truncated genes, and three pseudogenes. A phylogenetic tree analysis elucidated that the OR genes of S. sihama were classified into six groups, of which β, γ, δ, ε, and ζ groups belonged to type I, and the η group belonged to type II. The type I OR genes contained almost all conserved motifs ( n = 62), while type II OR genes mainly retained conserved motifs 7(3), 1, 10, 4, and 2 ( n = 39). OR genes were mainly distributed on LG1, LG9, LG11, and LG12. Of all OR genes, 36.23% (50 genes) showed significant expansion in S. sihama . Ka/Ks analysis demonstrated that 227 sites were under purifying selection, while 12 sites were under positive selection, including eight genes in the OR2A12 gene subfamily. Sixty-one genes (44.20%) displayed differential expression under hypoxic stress. The identified OR genes explored the mechanism of environmental stress and ecological adaptation of S. sihama , and provided valuable genomic resources for further research on the olfaction of teleosts.