Abstract
The sense of smell, or olfaction, is fundamental in the life of animals. However, penguins (Aves: Sphenisciformes) possess relatively small olfactory bulbs compared with most other waterbirds such as Procellariiformes and Gaviiformes. To test whether penguins have a reduced reliance on olfaction, we analyzed the draft genome sequences of the two penguins, which diverged at the origin of the order Sphenisciformes; we also examined six closely related species with available genomes, and identified 29 one-to-one orthologous olfactory receptor genes (i.e. ORs) that are putatively functionally conserved and important across the eight birds. To survey the 29 one-to-one orthologous ORs in penguins and their relatives, we newly generated 34 sequences that are missing from the draft genomes. Through the analysis of totaling 378 OR sequences, we found that, of these functionally important ORs common to other waterbirds, penguins have a significantly greater percentage of OR pseudogenes than other waterbirds, suggesting a reduction of olfactory capability. The penguin-specific reduction of olfactory capability arose in the common ancestor of penguins between 23 and 60 Ma, which may have resulted from the aquatic specializations for underwater vision. Our study provides genetic evidence for a possible reduction of reliance on olfaction in penguins.
Highlights
Animals are believed to have five basic senses: sight, hearing, taste, touch, and smell[1]
We analyzed the draft genome sequences from the two penguins, which diverged at the origin of the order Sphenisciformes and represented the two major clades of the penguin species tree[35]; we examined six closely related outgroup species with available genomes (Fig. 1), and identified 29 one-to-one orthologous olfactory receptor genes that are putatively functionally conserved and important across the eight birds
The low number of intact olfactory receptors (ORs) in birds suggests a reduced reliance on olfaction, which is consistent with the common view that most birds are primarily visual animals[38], because the reduction of ORs was coincident with the occurrence of better color vision in primates[13]
Summary
Animals are believed to have five basic senses: sight, hearing, taste, touch, and smell[1]. Penguins, along with other waterbirds (or aquatic birds), were identified to carry a significantly greater number of olfactory receptor genes, as compared with vocal-learning birds in a recent genomic analysis[15], which suggested that penguins rely heavily on olfaction. We hypothesize that penguins may have a reduced number of OR genes as compared to other waterbirds To test this hypothesis, we analyzed the draft genome sequences from the two penguins, which diverged at the origin of the order Sphenisciformes and represented the two major clades of the penguin species tree[35]; we examined six closely related outgroup species with available genomes (Fig. 1), and identified 29 one-to-one orthologous olfactory receptor genes that are putatively functionally conserved and important across the eight birds. With additional sequencing of these orthologous genes, we found that penguins have a significantly higher percentage of pseudogenes than other waterbirds, they still retain many intact and putatively functional genes
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