Or83b, also known as Orco (short for "odorant receptor co-receptor"), is an

odorant receptor Olfactory receptors (ORs), also known as odorant receptors, are chemoreceptors expressed in the cell membranes of olfactory receptor neurons and are responsible for the detection of odorants (for example, compounds that have an odor) which give ri ...

and the corresponding gene that encodes it. The odorant receptor Or83b is not exclusively expressed in insects. Though its actual function is still a mystery, the broadly expressed Or83b has been conserved across highly divergent insect populations across 250 million years of evolution.

Function

There are two contending models for potentially explaining Or83b

function Function or functionality may refer to: Computing * Function key, a type of key on computer keyboards * Function model, a structured representation of processes in a system * Function object or functor or functionoid, a concept of object-oriente ...

. One proposes that it could interact with specific

odor An odor (American English) or odour (English in the Commonwealth of Nations, Commonwealth English; American and British English spelling differences#-our, -or, see spelling differences) is caused by one or more volatilized chemical compounds ...

stimuli independently of other odorant receptors (ORs). The other proposes that it could act with other ORs to mediate responses to all odors. Or83b

mutation In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, mi ...

disrupts behavioral and

electrophysiological Electrophysiology (from Greek , ''ēlektron'', "amber" etymology of "electron"">Electron#Etymology">etymology of "electron" , ''physis'', "nature, origin"; and , ''-logia'') is the branch of physiology that studies the electrical properties of bi ...

responses to many

odorants An aroma compound, also known as an odorant, aroma, fragrance or flavoring, is a chemical compound that has a smell or odor. For an individual chemical or class of chemical compounds to impart a smell or fragrance, it must be sufficiently vol ...

, which supports the second model that OR plays a general rather than specific role in

olfaction The sense of smell, or olfaction, is the special sense through which smells (or odors) are perceived. The sense of smell has many functions, including detecting desirable foods, hazards, and pheromones, and plays a role in taste. In humans, it ...

. When mutating the Or83b gene, larval ''

Drosophila ''Drosophila'' () is a genus of flies, belonging to the family Drosophilidae, whose members are often called "small fruit flies" or (less frequently) pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species ...

'' do not travel towards an area of

ethyl acetate Ethyl acetate ( systematically ethyl ethanoate, commonly abbreviated EtOAc, ETAC or EA) is the organic compound with the formula , simplified to . This colorless liquid has a characteristic sweet smell (similar to pear drops) and is used in glues ...

which is an important odorant related to rotting fruit.

Functional conservation

The OR genes appear to be a single lineage nested within the gustatory receptor (GR) family. Or83b, however, is divergent from other OR proteins and appears most similar to the usual GRs. This conservation suggests that Or83b serves a function unlike that of other

chemoreceptors A chemoreceptor, also known as chemosensor, is a specialized sensory receptor which transduces a chemical substance (endogenous or induced) to generate a biological signal. This signal may be in the form of an action potential, if the chemorecept ...

. By cloning orthologs of Or83b, performing ''in situ'' hybridization of the clones, and by creating transgenic flies with orthologs of mosquitos, medflies, and moths, the function of Or83b was determined to be conserved across highly divergent insect populations. Or83b has homologs in other insect species. Since Or83b responds not to specific odors but to odors in general the Or83b receptor must respond to a feature of other ORs that it has coevolved with. That insects have used only a single protein for odor detection suggests that Or83b functions in insects in a way that cannot be diversified.

Practical implications

Since insects play a large role in spreading infectious blood borne diseases between humans, understanding how they locate their human

hosts A host is a person responsible for guests at an event or for providing hospitality during it. Host may also refer to: Places *Host, Pennsylvania, a village in Berks County People *Jim Host (born 1937), American businessman *Michel Host ( ...

with their olfactory systems could lead to inhibiting their method of locating hosts. The potential strategy involved would be to create small molecules that mimic the Or83b mutation and inhibit the receptor and disperse them, since it would be impractical to mutate the gene for the receptor in an entire population of disease-spreading insects. In some species of insect, drawing blood meals is a behavior not normally witnessed that could be brought on by a sudden lack of avoidance of vertebrate odor. Food preference in insects is not always about attraction to certain odors but the lack of repellant response to the odor. The fruit-piercing moth (''Calyptrata thalictri'') has been known to draw blood from mammalian hosts when the number of a particular group of olfactory sensing neurons that generally produce a repellent response to vertebrate volatiles is reduced. This halts their usually innate repulsive behavior to vertebrate odors and increases the chance that they will attempt to draw blood from a host. Because this is a change not in Or83b but in a population of olfactory sensory neurons, dispersing molecules to inhibit Or83b would not stop the behavior and a new strategy would have to develop.

References

{{reflist Insect genes Drosophila melanogaster genes Olfactory receptors