Olfactory receptor and circuit evolution promote host specialization.

The evolution of animal behaviour is poorly understood1,2. Despite numerous correlations between interspecific divergence in behaviour and nervous system structure and function, demonstrations of the genetic basis of these behavioural differences remain rare3-5. Here we develop a neurogenetic model, Drosophila sechellia, a species that displays marked differences in behaviour compared to its close cousin Drosophila melanogaster6,7, which are linked to its extreme specialization on noni fruit (Morinda citrifolia)8-16. Using calcium imaging, we identify olfactory pathways in D. sechellia that detect volatiles emitted by the noni host. Our mutational analysis indicates roles for different olfactory receptors in long- and short-range attraction to noni, and our cross-species allele-transfer experiments demonstrate that the tuning of one of these receptors is important for species-specific host-seeking. We identify the molecular determinants of this functional change, and characterize their evolutionary origin and behavioural importance. We perform circuit tracing in the D. sechellia brain, and find that receptor adaptations are accompanied by increased sensory pooling onto interneurons as well as species-specific central projection patterns. This work reveals an accumulation of molecular, physiological and anatomical traits that are linked to behavioural divergence between species, and defines a model for investigating speciation and the evolution of the nervous system.

Random convergence of olfactory inputs in the Drosophila mushroom body.

The mushroom body in the fruitfly Drosophila melanogaster is an associative brain centre that translates odour representations into learned behavioural responses. Kenyon cells, the intrinsic neurons of the mushroom body, integrate input from olfactory glomeruli to encode odours as sparse distributed patterns of neural activity. We have developed anatomic tracing techniques to identify the glomerular origin of the inputs that converge onto 200 individual Kenyon cells. Here we show that each Kenyon cell integrates input from a different and apparently random combination of glomeruli. The glomerular inputs to individual Kenyon cells show no discernible organization with respect to their odour tuning, anatomic features or developmental origins. Moreover, different classes of Kenyon cells do not seem to preferentially integrate inputs from specific combinations of glomeruli. This organization of glomerular connections to the mushroom body could allow the fly to contextualize novel sensory experiences, a feature consistent with the role of this brain centre in mediating learned olfactory associations and behaviours.

Neuroscience: The Secret of Sauce Béarnaise Syndrome Is in the Circuit.

During conditioned food aversion - a.k.a. sauce béarnaise syndrome - the ingestion of a spoiled food item leads to a lasting aversion towards cues reminiscent of the item. A new study finds that, in Drosophila, taste aversion depends on the immune system and the mushroom body.

BAPTI and BAPTISM birthdating of neurons in zebrafish.

This protocol describes how the photoconvertible protein Kaede can be used to determine the birthdates of neurons in live zebrafish. The methods used are birthdating analysis by photoconverted fluorescent protein tracing in vivo (BAPTI) and BAPTI combined with subpopulation markers (BAPTISM). Because Kaede can be converted from green to red fluorescence at any developmental time point, it serves as a temporal landmark for cell birth. When it is used in combination with subpopulation markers, the eventual fate of a cell can be correlated with its birthdate. We describe how we used this method to study the development of trigeminal sensory neurons and discuss how the technique can be extended to the study of other organs.

In vivo birthdating by BAPTISM reveals that trigeminal sensory neuron diversity depends on early neurogenesis.

Among sensory systems, the somatic sense is exceptional in its ability to detect a wide range of chemical, mechanical and thermal stimuli. How this sensory diversity is established during development remains largely elusive. We devised a method (BAPTISM) that uses the photoconvertible fluorescent protein Kaede to simultaneously analyze birthdate and cell fate in live zebrafish embryos. We found that trigeminal sensory ganglia are formed from early-born and late-born neurons. Early-born neurons give rise to multiple classes of sensory neurons that express different ion channels. By contrast, late-born neurons are restricted in their fate and do not form chemosensory neurons expressing the ion channel TrpA1b. Accordingly, larvae lacking early-born neurons do not respond to the TrpA1b agonist allyl isothiocyanate. These results indicate that the multimodal specification and function of trigeminal sensory ganglia depends on the timing of neurogenesis.