Évolution et adaptation des systèmes sensoriels

How do changes in cell fate affect the function and performance of neural systems, in particular to adapt visual and olfactory systems to their environment? For example, butterflies have improved color vision compared to other insects such as Drosophila. I will discuss how this is achieved by duplicating the (R7) photoreceptor cell: Each of the two R7s makes a stochastic choice to express the transcription factor Spineless, leading to three stochastic types of ommatidia (Spineless on/on, on/off or off/off), instead of the two found in Drosophila (Spineless on or off). We used CRISPR to decipher the function of Spineless in butterflies and showed that it controls the fate of the ommatidia. This shows that evolution can shape the visual system to adapt it to specific environmental conditions by modifying a more ancestral gene network that determines photoreceptor fate.
The opposite change occurs in the housefly Musca that has improved motion vision at the expense of color vision, again through changes in specification of cell fate. In a region of the male eye that is dedicated to chasing females (the “love spot”), R7 color photoreceptors express the broad spectrum Rhodopsin normally used in motion vision. They are also rewired to connect to the motion processing centers for additional sensitivity. We will discuss the molecular basis for this fate transformation and modification of axon targeting.
We also studied how ants depend extensively on their dramatically expanded olfactory system rather on vision to detect pheromones involved in complex social interactions. We generated a mutant in the orco (odorant co-receptor) gene that renders ants unable to detect pheromones. This disrupted their social interactions, and unlike Drosophila, loss of Orco dramatically altered the development of their olfactory system.
Therefore, we can show how developmental systems can be modified for specific functions and how the incredible diversity of neural fates found in the visual and olfactory systems are properly specified during development.