1 6 2 Sensory Systems Chemoreception BIO 361T Fall 2014 1 Many vertebrate can distinguish more than 10x as many odorants apart as they have odorant receptor genes Propose a proximate explanation for how this is possible Vertebrate olfaction is a combinatorics problem Each smell is actually a combination of several odorants so this results in a combination of neural activation Further each olfactory neuron expresses only one gene but each receptor can recognize more than one odorant A given odorant excites multiple olfactory neurons to different degrees CNS integrates combination that is unique to each odorant 2 The olfactory receptor proteins represent the largest gene family present in mammals with roughly 1 000 members 3 of the mouse genome In mice 20 of these genes have become pseudogenes they have accrued random mutations to the point of being non functional This is compared to 60 in humans and 100 in toothed whales e g orcas sperm whales and dolphins Propose an ultimate explanation for this variation Relaxed selection nothing to smell via respiratory inspiration in the ocean so no consequence to accruing mutations in genes for olfactory receptor proteins 3 Many insects have chemoreceptors in various head appendages that are extremely sensitive to CO2 Propose an ultimate explanation for these chemoreceptors why might these be positively selected for in some insects To find respiring animals like in blood feeders 4 Given how gustation operates in vertebrates give a proximate explanation for how there can be zones for particular tastes on the tongue Although each taste receptor in vertebrates expresses genes that allow them to be activated by every taste modality each cell usually has a primary taste it responds to like a preferred modality in most sensory receptor cells in general possibly due to differences in distribution of receptor proteins along the membrane Some preferential processing seems to also occur in the fibers of the taste nerves Taste receptors and gustducin have recently been found expressed in many other regions including the gut pancreas and testis Why don t we perceive taste from these areas They connect to other parts of the brain where perception occurs Could be related to glucose sensing and appetite regulation sperm guidance and division 5 Insects have as many as 68 gustatory receptor genes In Drosophila sweet taste neurons express genes for two sugar receptors bitter taste neurons express genes for at least nine different receptors However the honeybee genome appears to contain only 10 gustatory receptor genes in total Why do you think this is Eat sweet nectar and can detect via other means no need to warn against noxious bitter tastes 2 6 Most mammals excrete small peptides and volatile compounds in their urine that are used as pheromones the existence and importance of which are controversial in humans For our brain size our VNO is extremely small and possibly non functional Some of the pheromones excreted by other mammals indicate genetic dissimilarity enabling them to avoid mating with kin These pheromones are related to a set of genes called the major histocompatibility complex MHC See the graphs below which show an experiment conducted on humans to test whether we are also able to make such olfactory distinctions In this experiment groups of men and women had some of their MHC loci genotyped Men were given a shirt to wear to bed for two nights then the women were asked to sniff the shirts and rate the pleasantness of the odor Women were separated into those that were on oral hormonal contraceptives and those that weren t Men and women were paired as having similar MHC genotypes or dissimilar genotypes Note A p value of less than 0 05 is conventionally considered indicative of a statistically significant difference between the two bars being compared Wedekind et al 1995 a Why do you think the women were divided into these two groups May interfere with ability to choose mates properly or olfaction b Did MHC similarity affect perceived odor intensity in women on contraceptives Explain the difference if there was any c Or not on contraceptives Explain the difference if there was any d Did MHC similarity affect perceived odor pleasantness in women on contraceptives Explain the difference if there was any Preferred similar MHC genotype Suggests that perhaps during pregnancy mate preferences change and or women prefer family over a genetically dissimilar mate e Or not on contraceptives Explain the difference if there was any Preferred dissimilar MHC genotype Diversity of MHC is also suggested to be important in pregnancy success both becoming pregnant and carrying to term Dissimilar genotypes also rated as smelling reminiscent of a former sexual partner suggesting that MHC diversity does actually play a role in partner choice Studies following up on this one are mixed Potential confounding factors there are many stage in menstrual cycle pre existing preference that is associated with choosing to be on the pill relationship status relationship history treatment and presentation of odors order effects any other inherent differences between the two groups of women Many of these are controlled for in Roberts et al 2008 who found an effect of contraceptive use but not a reversal of mate preference However many other studies support a reversal of mate preference
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