As you may recall from Chapter 18, a knockout experiment is a genetic technique in which one of an organism’s genes is made inoperable (“knocked out”) to see what effect its elimination has on the organism’s phenotype. Knocking out genes involved in sensory pathways can have pronounced effects on behavior. One example is a gene for a specific olfactory receptor in mice.
As was discussed in Key Concept 45.2, mice have two olfactory organs: the nasal olfactory epithelium common to all mammals, and a small organ in the nasal passages called the vomeronasal organ, or VNO (Figure 52.3). Catherine Dulac at Harvard University discovered that pheromone receptors were expressed in that organ. (Pheromones are signaling molecules that are released into the environment and are used for communication between individuals of the same species.) Dulac hypothesized that when sex pheromones produced by female mice bind to the receptors in the male’s VNO, they stimulate mating behavior. To test this hypothesis, Dulac created a genetically engineered male mouse in which a gene for VNO receptor signaling was knocked out. Contrary to the hypothesis, the knockout males in fact did pursue and mate with females placed in their cages. However, they also pursued and tried to mate with males placed in their cages. Normally a male mouse reacts aggressively to a strange male, but the knockout male could not discriminate between males and females placed in his cage. Thus properly functioning VNO receptors appear to be essential not for sexual attraction, but for sex identification. You can imagine how selection working on this one gene could modify the intensity of male–