Thursday, April 9, 2009
Smell
As Carroll points out on pgs127-129, many animals have a powerful sense of smell “used to find food, identify mates and offspring, and detect danger” (127). Describe how the different odors are detected and discriminated. Also talk about the olfactory gene and how it works, compare human’s olfactory genes to mice’s. And if possible, answer the questions of why have humans abandoned the use of such large fraction of odor receptors. In your response include the theory of evolution and relaxed selection.
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ReplyDeleteIn 2004, Linda Buck and Richard Axel were awarded the Nobel Prize in Physiology or Medicine for their research of the olfactory genes and the genetics behind smell. Different odors are detected by various olfactory receptors; their specificity comes from the fact that each olfactory receptor is produced by a single sensory neuron. The nose contains about 3 cm. of olfactory epithelium which contains the olfactory receptors. When a chemical “scent” binds with its specific olfactory receptor, an electrical impulse travels across a single axon to the olfactory bulbs in the inferior part of the brain. This action potential is sent directly to the cerebral cortex, specifically the limbic system (this is why there is a large emotional association with smell).
ReplyDeleteAbout half of humans’ olfactory receptor genes are fossilized and subsequently, cannot create specific olfactory receptors. Receptors coded by the olfactory V1r genes show the most dramatic difference in the sense of smell in humans compared to that of mice; mice have 160 functional V1r receptors while human have about 5 functional V1r receptors out of 200 in our genome. Additional research has been done with primates and other mammals and it has been shown that there is a strong correlation between the proportion of fossilized olfactory receptor genes and the evolution of full color vision. The evolution of trichromatic vision in humans and some primates allowed these organisms to rely on visual cues to aid in mating, hunting, and detecting danger, and has decreased their reliance on smell. These organisms’ olfactory genes were under relaxed selection, allowing many more mutations and consequently, the lack of function.
http://physiologyonline.physiology.org/cgi/content/full/13/1/1
http://en.wikipedia.org/wiki/Olfactory_bulb
In the book, Carroll states that the olfactory receptor genes are the largest family of genes in mammal genomes. Each gene is able to detect different odors. Odors are detected by the receptors from the chemical "scent" they give off. The chemicals are read and odor discrimination takes place when the neurons send messages to the brain. This tells mice whether it’s something safe to eat or a threat.
ReplyDeleteIn humans, half of our olfactory genes are fossilized and no longer can make functional receptors. One class of receptors encoded by V1r genes is studied in humans and in mice. Mice have about 1400 olfactory receptor genes out of their genome which has 25000 genes. The mouse has about 160 functional v1R receptor genes and humans only have 5 functional V1r genes but we contain 200 of these genes in our genome. Mice have to watch for predators like cats, which give of specific chemical odors, which cause the mice to flee. Mice also use their sense of smell to ensure that something is okay to eat. Humans over time have left these activities, because we buy our food from grocery stores and we have no predators to look for constantly.
One reason that humans may no longer need a great sense of smell is because of technologies. Most people do not hunt, which our ancestors did often to obtain food. Also, in our world today we use our sight more often. With technology increasing throughout the years, we can see better then ever before (glasses, contacts, LASIK, etc.) also hearing aids help us hear things more clearly. Today, we rely on our sight and hearing more then our smell. So our sense of smell over time has become fossilize because it is not being used as often. But, someone who is blind may have a greater sense of smell because they have learned to pay close attention to details in smells and they recognize the smells that many of us ignore.
http://www.leffingwell.com/odorback.htm
Many of the human olfactory genes have become fossilized due to the development of trichromatic vision, which has allowed us to find prey and resources well enough without an exceptionably good sense of smell. Other animals that have bad senses of smell or have totally lost their sense of smell have compensated in other ways. One example of a group of animals with poor senses of smell is birds. Birds have very poor abilities to use smell compared to most mammals; this is probably because their ability to fly has allowed for the relaxed selection of olfactory genes and eventually the fossilization of many of these genes. Because they can fly, birds have better abilities to find resources, food, and mates, and they are also able to avoid predators more easily with good senses of smell.
ReplyDeleteIn addition, cetaceans (including whales and dolphins) have no senses of smell at all. These animals are able to compensate for this in many ways. First of all, whales and dolphins have very fine senses of taste. Scientists have found that dolphins, like humans, are able to distinguish the four basic taste stimuli (sour, sweet, salty, and bitter), unlike most other animals. Also, beluga whales can “taste” blood of other belugas in nearby waters, which allows them to flee in time and avoid predators. Cetaceans also have very good senses of touch, as shown when whales and dolphins often rub against each other and stroke each other with their fins or flippers. In fact, humans have been able to train dolphins to do trick using touch alone as a positive reinforcement.
These examples show that smell is not necessary in some organisms, although it is vital for many terrestrial animals such as mammals. Dogs have noses that can be a hundred thousand to a million times more sensitive than a human’s nose, which reflects their reliance on smell. Dogs such as Basset Hounds are bred to track and hunt rabbits and other small animals. The necessity to track other animal’s scents such as in their feces was vital to their survival in the wild; however, since they have become domesticated, their reliance on smell is much less since they are fed by their owners. Therefore, relaxed selection may occur in their olfactory genes as well, and dogs might eventually have a sharp decrease in their senses of smell.
Sources:
http://en.wikipedia.org/wiki/Olfaction
http://science.jrank.org/pages/1356/Cetaceans-Sensory-perception.html
In addition to Ryan and Kimberly’s response, I would like to elaborate more on how the process of smelling works. We are able to smell because specialized receptor cells of the olfactory epithelium detect and recognize smells. First, smells are tiny molecules of chemicals from things like food or flowers. As the air passes through the nasal cavity and through a thick layer of mucous to the olfactory bulb and each smell molecule fits into a nerve cell like puzzle pieces. Then, the cells send signals to the brain through the olfactory nerve. The brain interprets those molecules as different smells.
ReplyDeleteSense of smelling is very important for any organisms because it is closely related to sense of taste. Most of our sense of taste is really about our sense of smell. Before we take a bite of our food, we sniff, while we are chewing, the smell molecules from the ground-up food inside our mouth float upwards to our nose. This is a reason why we can’t taste anything when we have cold. It is because of the smell that we feel so much joy in eating food. However, the most of the process of how we detect smell and differentiate smells remain a mystery.
Bettina Malnic and Paul A.Godfrey have studied the human olfactory receptor gene family for many years and they conducted a comprehensive analysis of the composition of the human olfactory receptors gene family, its chromosomal organization, its structure. They first sought to identify the full repertoire of human olfactory receptors. They searched for sequences encoding olfactory receptors in the 93% of the human genome available in databases. Then, they retrieved DNA sequence in the area of each match and examined the protein it encoded. As a result, theses studies identified 646 human olfactory receptor genes. Also, they found out that the human olfactory receptors can be divided into 172 subfamilies according to their structure and their functions. Bettina and Paul concluded that humans have 636 olfactory genes, 339 of which are intact and therefore likely to encode functional odorant receptors in the nose. They also determined that according to the structure and the location of these olfactory receptors, they detect and process different smells.
As you can see, there are a lot to learn about the process of smelling. Now, to answer the final question, we need to discuss the evolution of color vision in humans and primates. I will use the example of Howler monkeys. On page 140, Sean Carroll explains that Howler monkeys have lost their olfactory genes occurred as the evolution of color vision happened: “The evolution of color vision, leaf-feeding habits, and loss of olfactory genes in howler monkeys occurred “(140). As we evolved to have color visions, primates didn’t have to rely on their sense of smell anymore. They can just look at the color of the food or predators to identify them. Then, the natural selection relaxed on our sense of smell and our olfactory genes kept on fossilized. This is a great example of evolution. As organisms evolve, they are just fine tuning their genes to match and survive in their given environment. As we evolve, we may lose some sense but we may gain some new skills to survive and thrive in capricious environment.
http://www.pnas.org/content/101/8/2584.full
http://www.coolquiz.com/trivia/explain/docs/smell.asp
http://yucky.discovery.com/flash/body/pg000150.html