Thursday, April 7, 2011
Smelly!
On page 145, Shubin begins the interesting discussion of how jawless fish have very small numbers of odor genes. This is very odd considering that us mammals have over a thousand genes dedicated to odor. How did this "jump" in number of odor genes occur? Also, Shubin talks of how researchers found that us humans have roughly 300 odor genes that have been rendered useless by mutations that cannot be fixed. How did this occur? Explain. Shreeraj Patel shreeraj.patel1@gmail.com
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Our sense of smell, like all other senses, is an adaptation. As time passes, more and more revisions may occur if an adaptation benefits an organism. If we look at a phylogenetic tree animals such as lampreys and hagfishes (the surviving jawless fishes) were one of the first chordates recorded in evolutionary history. (Campbell 703) Since the smell receptor genes are only found in the vertebrate, it would make sense that the jawless fishes have so few of them. This has to do with "continuity and change." The jawless fishes started with a sparse number of olfactory receptors (most likely due to a rare case where a genetic mutation led to beneficial proteins). Over time, small mutations and rearrangements of the olfactory components led to more and more diverse receptors. Through natural selection, receptors that were vital to survival became permanent while receptors that were inessential eventually disappeared from the overall population. Also, eventually after our ancestors began to live more separate from water, water-based olfactory receptors disappeared for the same reason: they were no longer necessary. (Shubin 145) However, why would humans still have useless mutations while other animals, such as dogs, have thousands more of fully functional receptors? The answer lies in our sight. As primates began to change from monochromatic vision to our current trichromatic vision, we partially gave up our sense of smell in the process. Most likely, as more genes began to code for better vision, they interfered with the amino acids coding for the functionality of the 300 now useless receptors. (http://www.ncbi.nlm.nih.gov/pmc/articles/PMC314465/?tool=pmcentrez)
ReplyDeleteOne reason why certain animals, such as mammals, have so many odor genes is many of them have Trace amine-associated receptors. TAAR seem to be "evolutionarily much younger than the related OR and ORA/V1R olfactory receptor families, which are present already in lamprey, a jawless vertebrate." (Hussain, 2008). The switch from aquatic to terrestrial habitats may be a reason for, or a result of, the increase in the number of odor genes. Evidence of "a late evolutionary origin of the TAAR gene family after the divergence of jawed and jawless vertebrates" may also ascertain that the TAAR gene wasn't as prevalent in the aquatic population as it was in the terrestrial population that stemmed from it, and terrestrial selective pressures may have even favored TAAR genes that made them more common in terrestrial locations. Specifically, Class III of the TAAR gene family, which is characterized by the loss of the aminergic ligand motif, extensive recent gene duplications, and 4 independent intron gain/loss events, "is likely to be under unusually strong positive Darwinian selection." (Hussain, 2008).
ReplyDeleteThere may have been a jump in the number of odor genes as animals evolved, but there seems to be a second stage of evolution that caused the use of odor genes to decrease across evolutionary history. As we learned in the genetics unit, "the bulk of most eukaryotic genomes consists of DNA sequences that neither code for proteins nor are transcribed to produce known RNAs; this noncoding DNA was often described in the past as junk DNA." (Campbell, 434). The fact that all organisms have DNA and much of the DNA is unexpressed is substantial proof of an evolutionary theme to the relationships between organisms. A reason why humans have roughly 300 odor genes that have become "pseudogenes" is because mutations have resulting in those genes not being expressed anymore. One reason why this tends to occur is because "gene-related regulatory sequences and introns account for 24% of the human genome", these causing certain genes to never be expressed (Campbell, 434). In TAAR genes, "Two independent intron gains in class III represent extraordinary evolutionary dynamics, considering the virtual absence of intron gains during vertebrate evolution." The most likely cause of odor genes to become pseudogenes is because positive selection in terrestrial environments made the use of these genes unnecessary, so introns suppressed their expression over time to save energy. Needless to say, many odor genes are expressed in other organisms, such as dogs, which further proves our evolutionary relationship to them, and may even support the idea that dogs might have evolved before us humans. What do you think?
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2657432/?tool=pmcentrez
ReplyDeleteTroy Glickstern
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