Evolution of avian olfaction

The sense of smell enables animals to e.g. locate food, to navigate
or to avoid predators. In vertebrates, odorants are detected by
olfactory receptors (ORs) that are expressed in the olfactory
epithelium on olfactory sensory neurons. In this study, I
investigated OR gene repertoires in both closely and distantly
related bird species. Using polymerase chain reaction (PCR) with
degenerate primers designed to amplify OR genes, I showed that the
majority of OR genes sequenced (~ 84%) were potentially functional
in nine bird species from seven different orders. A nonparametric
statistical technique was used to estimate the total number of OR
genes in avian genomes. The total number of OR genes was
surprisingly large and varied up to six fold between species
(range: 106 – 667 OR genes). The total number of OR genes but not
the proportion of potentially functional OR genes was positively
correlated with the relative size of the olfactory bulb, which is
considered an anatomical correlate of olfactory capability. A
Southern Blot approach in combination with a PCR based approach
revealed that two nocturnal bird species that heavily rely on
olfactory cues, have evolved a larger OR gene repertoire than their
diurnal, closest living relatives. Thus, it is likely that
ecological niche adaptations (e.g. adaptations related to daily
activity patterns) have shaped avian OR gene repertoires.
Phylogenetic trees derived from predicted OR protein sequences
revealed that a large, expanded OR gene clade, termed group-γ-c, is
present in all bird genomes examined in this study. This clade
seems to be a shared characteristic of all bird genomes. Further, I
showed that positive selection has driven the molecular evolution
of avian group-γ-c OR genes. Positively selected sites encoded
residues within transmembrane regions that most likely interact
with odour molecules and thus might influence OR receptor
functioning. Interestingly, OR gene transcripts have been detected
in testis and sperm of both mammals and fish, suggesting that OR
genes are also involved in sperm-egg communication. Using reverse
transcription (RT)-PCR with degenerate primers specific for OR
genes, and subsequent cloning, I showed that several OR gene
transcripts are present in chicken (Gallus gallus domesticus)
testes and that they belong to the class-γ OR gene clade. Finally,
a database search in the red jungle fowl (Gallus gallus) genome
revealed that trace amine-associated receptors (TAARs) - a second
family of chemosensory receptors primarily expressed in the
olfactory epithelium that detect amine-based odour cues - are also
encoded in avian genomes. The findings in this thesis contribute to
our understanding of the evolution of avian OR genes. The estimated
OR gene repertoire sizes, and the proportion of presumably
functional OR genes, strongly suggest that avian olfactory ability
is well developed and much more important than previously thought.