Thursday, March 18, 2010

OR Genes

If the ability to recognize different smells is useful to an organism, why do you think humans have about 400 hundred olfactory receptor genes that are permanently inactivated? What advantages could this have on the human race? Also, Coyne says that "we carry this genetic baggage because it was needed in our distant ancestors who relied for survival on a keen sense of smell" (71). Since evolution is driven by natural selection, wouldn't it be beneficial to stop DNA replication of this unnecessary "baggage"? What could be some reasons as to why our bodies continue to replicate 800 OR genes knowing that 400 of them are inactive?


  1. Humans are visual animals. We have one of the most complex eyes in the animal kingdom. As we evolved this eye, we started to evolve a less heightened sense of smell. I theorize that our weaker sense of smell evolved because of how inefficient it was to have fully functional eye and nose. Our eyes made it possible to rely less on our smell and I'm guessing that having only 400 olfactory receptor genes active is the most efficient number of OR genes to have active with our highly evolved eyes.
    Also, unlike what is commonly believed, our sense of smell is still very strong. According to The Mystery of Smell: The Vivid World of Odors by Maya Pines, human mothers can recognize the smell of their baby, and our sense of smell is strongly tied with our sense of memory. That is because smells can tell us what we cannot see. If our house has a gas leak, we'd never be able to see the gas. The next time we'd turn on a fire, the house would go KABOOM!! Our sense of smell is very important for these types of purposes. According to The Smell Report, female noses recognize pheromones in male sweat. It is a natural aphrodisiac!
    The 400 unused OR genes are now dead genes, one of the greatest pieces of evidence for evolution. According to Coyne himself, our DNA has a ton of these pseudogenes. Why would a designer leave these genes when it is inefficient to replicate them? These dead genes exist because it would be catastrophic if DNA was moved around and cut up during the process of evolution. Our synthesis of proteins from DNA requires very specific locations of DNA sequences and I believe that taking out even a dead gene would cause a organism to not develop. The 400 OR genes that are inactive cannot be just cut out. They are now part of the vast part of DNA that is not used for protein creation.

  2. As Rzepecki stated, Coyne states that there are about 400 inactive OR genes. As one developed from an ancestor that relied on all the sensory stimulus in order to survive, it is not surprising to find the 400 olfactory receptor genes. Similar to Shtivelberg's idea, humans gradually adapted to having a better sense of visual reliance rather than olfactory reliance. This seems true because although olfactory is the only (that I know of) sensory system that does not go through the thalamus of the brain and "strongly tied with our sense of memory," olfactory stimulation, in a sense has wide variety of difficulties. Although the strong points are retaining the memory and distinguishment of objects, most of the stimuli cannot be fully distinguished. According to the known book Psychology: Themes and Variations, humans can smell up to 10,000 different odors but have a hard time recognizing the oders. This may have been advantageous to the common ancestors who had more reliance in olfactory senses for reliance in survival. In addition, the 'baggage' of the OR genes cannot be specially said to have just cut out. The reason is because these inactive genes may have roles that people haven't figured out yet. Because DNA research is recently developed, majority of the human DNA functions are unknown. This baggage of DNA may have acted as introns, or inactive genes that functions in alternative splicing of the mRNA. These introns allow mutations of important genes from occuring. By filling the DNA with introns, there will be more frequent mutations in the inactive olfactory receptor(OR) genes than the active olfactory receptor genes. Also the alternative splicing may occur so that the introns may act as exons. It is hard to say that the 400 dead OR genes are considerably and completely useless. All in all, the 400 inactive OR genes give the humans an effective body system because not only OR genes but if every single gene in our system, dead or alive, were transcribed and translated, there would be too many consumption of ATP, a human energy molecule, for people to effectly survive.

  3. As discussed by the previous two posts, the 400 OR genes that are permanently inactivated are probably like so because humans have evolved to become much more visually based organisms rather than olfactory. After researching a little bit, I had one additional suggestion as to why this may have happened. Apparently, “odorant [olfactory] receptors (ORs) function in the guidance of axons of olfactory sensory neurons (OSNs) to glomeruli in the olfactory bulb” (Feinstein, Mombaerts). So if humans evolved to make the transport of OR axons more efficient, using less OR genes, then it’s possible that some of these may have been silenced from lack of purpose in transport of axons in addition to the shift to a visual system. Nevertheless, that is probably due to the fact that humans shifted away from an olfactory-based recognition system to a visually-based one.
    In terms of selective advantage, this would have the obvious purpose of reducing the amount of energy needed to express and utilize genes, providing more energy for other biochemical pathways in the body. For instance, “estimated demands at the axonal terminal and the dendritic release side are 15,400 ATP per vesicle” (Nawroth et. al.). With a little bit of math, considering the 400 some genes that were turned off that performed the above stated function, that is about 6,160,000 ATP molecules ess used in recognizing odors, and that is only for the transport, completely disregarding the total amount of ATP necessary for every other part of olfactory recognition. Although that may be a marginal amount of ATP that is created in each cell in the body, the amount of times that we actually use our abilities to smell would mean a hefty load of extra ATP usage. Clearly, this would lessen the load of making more ATP for unnecessary OR gene expression and provide a selective advantage to silence them.
    I agree that it may be evolutionary baggage that doesn’t have much but a vestigial use in our bodies, but Eric does make a good point: it isn’t as simple as cutting out some 400 genes. Liondrummer touched on alternative splicing as one of the reasons, and I completely agree; but in addition to that, I think that the underlying reason as to why it’s not possible to cut them out lies in the process of DNA replication in the S Phase of mitosis. Every time a cell divides, a little bit of its DNA is lost and a 3’ stand is left overhanging. Fortunately, this 3’ strand consists of telomeres, which provide protection to the important sequences that are encoded in our DNA. In order to cut out those useless genes, as you propose, DNA replication would have to continue to persist until the ends of the strands reach the OR genes and begin to degrade. By the time that happens, however, other crucial genes would have been degraded, rendering this approach not only “un-evolutionary” but not physically possible.
    Finally, I think that Liondrummer correctly points out that in terms of alternative splicing, different sequences may use parts of the OR gene sequences, meaning those sequences are still important even though the OR genes themselves may be silenced. In addition, it’s possible that devolution could occur, allowing these OF genes to be used once again. For instance, Coyne discusses how whales “devolved” back into the water rather than following the pattern from water to land (Coyne 47-52). Similarly, these OR genes can “devolve” and shift from a vision-based recognition body to a olfactory-based recognition body.,f1000m,isrctn

    Jerry Coyne: Why Evolution is True

    Niel Campbell and Jane Reece: Biology: AP Edition, 8e