Odorant receptor copy number change, co-expression, and positive selection establish peripheral coding differences between fly species

1.Despite numerous examples of chemoreceptor gene family expansions and contractions, how these changes relate to modifications in the neural circuitry in which they are expressed remains unclear. Drosophilas Odorant receptor (Or) family is ideal for addressing this question because a large majority of Ors are uniquely expressed in single olfactory sensory neuron (OSN) types. Between-species changes in Or copy number, therefore, may indicate diversification/reduction of OSN populations. To test this, we investigated a rapidly duplicated/deleted subfamily (named Or67a) in Drosophila melanogaster and its sister species (D. simulans, D. sechellia, and D. mauritiana). We found that the common ancestor had three Or67a paralogs that had already diverged adaptively in their odor-evoked responses. Following their speciation, two Or67a paralogs were lost independently in D. melanogaster and D. sechellia, with ongoing positive selection acting on the intact genes. Instead of the expected singular expression of each diverged Ors, the three D. simulans Or67a paralogs are co-expressed. Thus, while neuroanatomy is conserved between these species, independent selection on co-expressed receptors has contributed to species-specific peripheral coding of olfactory information. This work reveals a model of adaptive change previously not considered for olfactory evolution and raises the possibility that similar processes may be operating among the largely uninvestigated cases of Or co-expression.