Periodic Reporting for period 1 - CAOCD (Comparative functional and neuroanatomical analyses of olfactory circuit in drosophilids)
Berichtszeitraum: 2020-11-01 bis 2022-10-31
Each animal species has adapted its behaviors to its ecological niche through the course of evolution, presumably through changes in the nervous system. The current proposal aims to understand how nervous systems and behaviour evolve by using the fruit fly Drosophila sechellia, which is a powerful model system for comparative neurobiology. D.sechellia is closely-related to D. melanogaster – one of the best-understood and experimentally accessible model nervous system – but has adapted to a very different ecological niche. In contrast to the cosmopolitan generalist D. melanogaster, D. sechellia is endemic to the Seychelles archipelago, where it feeds, breeds and lays eggs on a single host plant, Morinda citrifolia. Morinda fruit emits characteristic volatile cues that attracts D. sechellia but repel D. melanogaster. Previous work in the host lab has identified methyl hexanoate as a cue which is detected by the odorant receptor OR22a and mediates long-range attraction. Importantly, both the physiological sensitivity and the number of these two classes of olfactory sensory neurons (OSNs) are increased in D. sechellia compared to D. melanogaster. However, how the peripheral signal is represented in the second-order interneurons (i.e. projection neurons; PNs), which carry olfactory information from the primary olfactory center (antennal lobe; AL) to higher brain centers (mushroom body and lateral horn), has not been characterized, precluding an appreciation of how these olfactory circuits evoke different types of behavior and the nature of evolutionary adaptations in the central brain.
By comparative calcium imaging of olfactory neurons in D. melanogaster and D. sechellia, I found that OSN sensitization is correlated to PN sensitization while OSN number increase itself is not. Rather, OSN number increase is correlated to sustainment of PN activity in response to dynamic odour inputs. PAGFP labelling revealed that the PN number was not different between species despite the increase in their cognate OSN number, suggesting an increased pooling of OSN inputs per PN. Optogenetic activation of Or22a OSNs elicited attractive behaviour in both species, but the duration of which was more sustained in D. sechellia. Taken together, these results imply that evolutionary increase of OSN number underlies robust tracking of odour plumes by sustaining the PN activity through increased pooling, thereby promoting host adaptation in D. sechellia.
The evolution of the nervous system is a topic of high public interest, as it is closely related to the question on the emergence of human brain and intelligence. The current proposal aimed to understand how the central olfactory circuitry has evolved to give rise to species-specific behaviors, by developing and applying neuroanatomical and neurophysiological analyses in a non-model organism, D. sechellia. The results from this project would be of general interest not only in the field of neuroscience but also evolution, ecology, and genetics.
By comparative calcium imaging of olfactory neurons in D. melanogaster and D. sechellia, I found that OSN sensitization is correlated to PN sensitization while OSN number increase itself is not. Rather, OSN number increase is correlated to sustainment of PN activity in response to dynamic odour inputs. PAGFP labelling revealed that the PN number was not different between species despite the increase in their cognate OSN number, suggesting an increased pooling of OSN inputs per PN. Optogenetic activation of Or22a OSNs elicited attractive behaviour in both species, but the duration of which was more sustained in D. sechellia. Taken together, these results imply that evolutionary increase of OSN number underlies robust tracking of odour plumes by sustaining the PN activity through increased pooling, thereby promoting host adaptation in D. sechellia.
The evolution of the nervous system is a topic of high public interest, as it is closely related to the question on the emergence of human brain and intelligence. The current proposal aimed to understand how the central olfactory circuitry has evolved to give rise to species-specific behaviors, by developing and applying neuroanatomical and neurophysiological analyses in a non-model organism, D. sechellia. The results from this project would be of general interest not only in the field of neuroscience but also evolution, ecology, and genetics.
I first performed calcium imaging from OSNs and PNs in D. melanogaster and D. sechellia. The initial hypothesis was that both OSN sensitization and OSN number increase contribute to sensitization of their cognate PNs. Jowever, I found that OSN number increase by itself is not sufficient to sensitize the cognate PNs. Rather, OSN number increase contributes to sustaining the PN response to dynamic odour.
Next, I performed photoacticatable GFP (PAGFP) labelling of Or85c/b PNs in both species and found that PN number is conserved in sharp contrast to their cognate OSNs.
Finally, to address the behavioral significance of OSN number increase, I developed a tethered fly assay, in which the fly can navigate in an virtual environment while sensory inputs (e.g. odour) are provided in a time-controllable manner. Through measurement of behavioural response to odor and optogenetic activation, I found the attraction behavior to be more robust in D. sechellia compared to D. melangaster.
During the tenure of the fellowship, I presented my work externally at various scientific conferences in Europe and USA.
The results are now being documented together with the host supervisor, pending some additional data that are now being collected. Once the manuscript is ready for submission to a journal, this will be posted at a preprint server for rapid communication to the scientific community. No website has been developed for the project
Next, I performed photoacticatable GFP (PAGFP) labelling of Or85c/b PNs in both species and found that PN number is conserved in sharp contrast to their cognate OSNs.
Finally, to address the behavioral significance of OSN number increase, I developed a tethered fly assay, in which the fly can navigate in an virtual environment while sensory inputs (e.g. odour) are provided in a time-controllable manner. Through measurement of behavioural response to odor and optogenetic activation, I found the attraction behavior to be more robust in D. sechellia compared to D. melangaster.
During the tenure of the fellowship, I presented my work externally at various scientific conferences in Europe and USA.
The results are now being documented together with the host supervisor, pending some additional data that are now being collected. Once the manuscript is ready for submission to a journal, this will be posted at a preprint server for rapid communication to the scientific community. No website has been developed for the project
The results obtained so far are significantly different from the expected outcome outlined at the beginning of the project. The initial hypothesis was that the two modes of peripheral evolution (OSN sensitization and OSN number increase) lead to a similar consequence of central processing and odour-guided behaviour, namely the physiological and behavioural sensitization. In contrast, the results imply that while OSN sensitization promotes PN sensitization, OSN number increase leads to sustainment of PN activity and attractive behaviour. These two modes of peripheral changes thus work in great synergy to promote host attraction in D. sechellia.
In general, sensory neuron number greatly varies between animal species as well as between sensory modalities. However, the impact of sensory neuron number changes on central circuit and behaviour had not been well investigated. The results from the current project proposes a new concept in which sensory neuron number increase may lead to sustainment of the activity of second-order neurons as well as the behaviour that is elicited by the activation of this sensory pathway. This work would stimulate the field of sensory neurobiology as well as the evolution of nervous system and behaviour.
While the evolution of the nervous system is a topic of high public interest, the modality of which is very poorly understood at the moment. This work provides a basic yet novel observation that may be one of the general principles on the mechanism and consequences of nervous system evolution.
In general, sensory neuron number greatly varies between animal species as well as between sensory modalities. However, the impact of sensory neuron number changes on central circuit and behaviour had not been well investigated. The results from the current project proposes a new concept in which sensory neuron number increase may lead to sustainment of the activity of second-order neurons as well as the behaviour that is elicited by the activation of this sensory pathway. This work would stimulate the field of sensory neurobiology as well as the evolution of nervous system and behaviour.
While the evolution of the nervous system is a topic of high public interest, the modality of which is very poorly understood at the moment. This work provides a basic yet novel observation that may be one of the general principles on the mechanism and consequences of nervous system evolution.