Development and Divergence of INSECT OLFACTORY SYSTEMs
Insects rely on their sense of smell in various aspects of their life: finding food, finding mating partners, and locating sources of danger. Naturally, different insect species feed on different kinds of food, aim at attracting only mating partners of their species, and face very specific dangers in their environment. Therefore, olfactory systems of different insect species were subject to evolutionary pressure and change.
The insect olfactory system is very similar to the mammalian olfactory system. The olfactory system of Drosophila melanogaster is significantly simpler. This relative simplicity together with the genetic tractability makes Drosophila melanogaster an ideal model to identify the genetic mechanisms of specification and targeting of olfactory neurons. Individual classes of neurons that express the same olfactory receptor innervate the same structures in the brain. For instance, neurons expressing the CO2 receptors (Gr21a/Gr63a) target to the so-called V-glomerulus, while some food odor detecting olfactory neurons innervate medial glomeruli. Previously, we have identified mutations that disrupt the stereotypical pattern of synaptic connections of olfactory neurons in an extensive genetic screen. We are now addressing the exact molecular and cellular function of some of these genes in the formation and function of the olfactory system.
The CO2 sensory system
The detection of CO2 plays a specific role in the life of many insect species. Moths might use it to locate plants to deposit their eggs. In honey bees, high levels of CO2 induce a collective fanning behavior to ventilate the nest. Here, CO2 is interpreted as a social cue. The malaria vector mosquito is attracted to CO2 and uses it to find human hosts. In contrast, Drosophila melanogaster flies detect CO2 as part of the so-called stress odor, and thus are strongly repelled by it.
Recently, we have identified a small genetic network including the microRNA gene, miR-279, and the homeobox transcription factor Prospero in Drosophila melanogaster. Loss of function of miR-279 or Prospero results in the formation of CO2 neurons that show hybrid characteristics of CO2 neurons in mosquito and fly. This suggests that a single microRNA could have been involved in the evolutionary divergence of the olfactory systems of mosquito and fly.
We are addressing how gene networks have changed to give rise to the olfactory system of different insects. We look at behavioral and structural divergence between species, and by using Drosophila genetics, we aim at finding the responsible genes that were critical for development and evolution of these differences.