Pheromones in MiceBiology 342 Fall 2007
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Reed College |
PhylogenyThe study of phylogeny examines the origins and evolution of a set of species to determine ancestral relatedness. Here, we discuss the phylogenetic history of chemical cueing of behaviors across species to better understand the development of the VNO pathway. Two Main Olfactory SystemsThere are two main olfactory systems in vertebrates, the main olfactory system (MOS) and the vomeronasal system (VNO). MOS is receptive to volatile cues, whereas the VNO is receptive to nonvolatile pheromones. The evolution of the VNO has sparked much debate, as it is a sensory organ present in most vertebrates but does not seem to be present in primates or humans [Liman & Innan, 2003].VNO Dependence and Independence between VertebratesFurther, it has also been discovered that lordosis in female pigs and suckling behaviors in newborn rabbits are unaffected by removal of VNO. However, rodents and mice behaviors are notably changed after removal of VNO. This dichotomous processing of pheromones displays a divergence amongst mammals, as some species process pheromone detection in the MOS and the VNO, whereas other species rely heavily on the VNO for pheromone detection [Leypold et al., 2002].VNO in Aquatic Species was Emergent and Advantageous in Terrestial SpeciesOne of the earliest theories of VNO development posited that the organ developed as an adaptation to terrestrial life [Dulac & Torello, 2003], however, studies soon identified VNO pathways in aquatic species and may indicate that the VNO developed in aquatic species and proved to be more advantageous to terrestrial species [Dulac & Torello, 2003]. Current studies done with TRPC2 (a gene that is required for VNO function in mice and may play a role in the pheromone signaling cascade) may indicate that the selective pressure for terrestrial species to maintain the VNO may have relaxed during primate evolution [Liman & Innan, 2003]. Evolution of the TRPC2 Gene (a gene responsible for VNO function)It is currently understood that the VNO in primates may have last been functional in the common ancestor of New World and Old World monkeys and apes. Further, it has been proposed that during primate evolution, other signaling pathways, such as the development of color vision, may have relaxed selective pressure on the pheromone- signaling pathway in the VNO [Liman & Innan, 2003].Mutation of the TRPC2 gene began in common ancestor of Old World monkey Figure A-B. [Liman & Innan, 2003] Evolution of the V1R Gene (a pheromone receptor gene)V1R and V2R are two pheromone receptor families that are present in the VNO and know to affect VNO function. Studies have shown a divergence in these receptors in six mammals: the mouse, rat, human, cow, dog, and opossum. Although humans and rodents may be closely linked genetically, the pheromone receptor expressions differ greatly between species [Grus, W.E. et al, 2005].Loss of V1R ancestral genes due to pseudogenes The loss of V1R gene and VNO function are compensated This phylogenetic tree displays the variation in V1R genes throughout different mammals [Grus et al., 2005]. |