Natural variation in a Drosophila melanogaster odorant receptor locus
Katherine Heather Shaw
10.4225/03/5876db348e6fa
https://bridges.monash.edu/articles/thesis/Natural_variation_in_a_Drosophila_melanogaster_odorant_receptor_locus/4541707
Olfaction plays a
large role in an insect’s life. Adults use their sense of smell to locate the
best mates and food, as well as the oviposition sites that will give their
progeny the best head start in life. An insect’s ability to detect odorants is
determined by olfactory receptor neurons (ORNs), which express members of three
olfactory receptor families. The odorant receptors (<i>Ors</i>) are the largest of
these three families. Despite their extensive study since their discovery in
1999, little is known about how the ligand-binding properties of the insect <i>Ors</i>
are defined. <br>
<br>
The <i>Ors</i> are a large multigene family thought to have originated
from a single ancestral gene. An interesting example of more recent evolution
in this family is the case of <i>Or22a</i> and <i>Or22b</i> in <i>Drosophila melanogaster</i>. These
receptors share 78% amino acid identity and it has been shown that Or22a
determines the response of the ab3A ORN in the Canton S laboratory strain,
while Or22b appears to be non-functional in this strain. This locus has
undergone a deletion event within <i>D. melanogaster</i>, generating a single
chimaeric gene called <i>Or22ab</i> that consists of the first exon of <i>Or22a</i> and the
remaining three exons from <i>Or22b</i>. This chimaera is found with high frequency in
the north of Australia and is completely absent from the south of Australia,
where the <i>Or22a </i>and <i>Or22b</i> variant is fully penetrant. This clinal variation indicates
that this locus might be under selection. <br>
<br>
In this thesis the relationship between ORN response and Or
sequence was evaluated for the naturally occurring receptor variants at the
<i>Or22</i> locus. Through <i>in vivo</i> rescue experiments in the ab3A neuron it is shown
here that this receptor variation leads to variation in ab3A response
properties. In order to better understand the spatial variation in allele
frequency at the <i>Or22</i> locus in Australia, <i>Drosophila</i> preference behaviours to
key odorants were evaluated. From this it was found that some of the ab3A
phenotypic variation correlated with altered olfactory behaviour. This
behavioural variation is therefore potentially linked to the geographical
distribution of the allele frequencies. <br>
<br>
The genetic changes underlying the ab3A phenotypic changes
were also analysed, and it was found that one amino acid residue in the second
extracellular loop in Or22b, residue 194, is important for overall
functionality, while two other amino acid residues, 92 and 201 in the second
and fourth transmembrane domains, are important for odorant specificity. To
better understand the evolution of this locus the ab3A response variants in
<i>Drosophila</i> were also compared to those of closely-related species, which have
varying ab3A responses. This showed that the ab3A phenotype seen with a
functional Or22b is likely the ancestral phenotype. Taken together the findings
of this thesis suggest an important role for variation in odour specificity of
the ab3A neuron in the evolution of odour-mediated behaviours in Drosophila,
and determine the genetic basis of this variation in odour specificity. The
results of this thesis show how the naturally occurring variation in ab3A
response in <i>Drosophila</i> is directly linked to variation at the <i>Or22</i> locus. The
variation at the <i>Or22</i> locus provided an opportunity to better understand how
these receptors have evolved and how they function. This direct linkage of
receptor variation to natural neuronal phenotype variation makes this locus
useful for evaluating how insect Ors bind ligands.
2017-01-12 01:26:10
Drosophila
Olfaction
Genetics
Behaviour
Electrophysiology
Neuroscience