Saturday, March 24, 2012

Polydactyly, or Having More Than Five Digits on the Distal Portion of a Limb


            Having an abnormal number of digits occurs more frequently than you might think, 1 in every 500 births, and it’s been going on for hundreds of years. The most famous case in history is that of Anne Boleyn. It was rumored she always wore gloves because she was hiding a sixth finger. This was seen as a sign of a witch, and it was one of the charges read against her before her beheading (amongst the charges of adultery and incest). The fact that an extra appendage was used in 1536 to identify evil meant there were prior occurrences of the same trait. Would an extra finger be more helpful or more cumbersome? Which way would natural selection act?
            Surprisingly, polydactyly is an autosomal dominant trait, but the allele frequency is now so low that it is a rare condition. Another theory suggests that polydactyly is the result of mutations in a variety of developmental genes in an individual (i.e. not heredity). One gene thought to be affected is the LMBR1 gene, or limb region 1 protein homolog. It produces a protein used in regulating developmental gene expression of the sonic hedgehog gene. This gene makes proteins for the hedgehog signaling pathway in mammals that plays a role in organogenesis, especially the growth of digits on the distal parts of limbs. If the LMBR1 gene is mutated, it can interrupt expression of the sonic hedgehog, affecting limb patterning.
Although classified as an extra finger, the digit often doesn’t have a joint, but rather is an abnormal fork from an established digit, thus making it a non-functional protrusion of soft tissue and some bone. Thus, it is often removed at birth. With no joint, the extra digit is useless. With a joint, however, it might be useful, and one might think natural selection would favor this extra, functional finger. Yet, the occurrence of a functional joint is rarer than an extra flab of flesh. Perhaps this is due to a developmental gene needing to be kept “on” longer than normal, and the longer it’s kept “on” by a mutated expression gene, the closer the extra digit gets to a functioning digit. Since normal development calls for this expression to cease early on, expression will shut down before a fully functional additional digit can be formed, leading to a non-functional appendage more often than not.
            At this point, we would think natural selection would select against a useless flap of skin. Without a joint, it might be more of a hindrance to daily life. Humans helped selection with their ignorant interpretation of additional digits. Physical anomalies were seen as physical manifestations of evil, so oftentimes, these people would be shunned or put to death, both acting as crude forms of sexual selection because that “deformed” person would not get the chance to spread his/her genes to posterity.
            In human populations, polydactyly is seen frequently among small communities in India and the Old Order Amish in Pennsylvania. In the Amish population, polydactyly isn’t an independent occurrence, but a symptom of Ellis-van Creveld syndrome (a type of dwarfism), a genetically inherited disorder. There is such a high occurrence of this disorder (amongst others) in the Amish population because they tend to marry within their community for generations, preventing new alleles from entering the gene pool. This combined with genetic drift (random fixation or deletion of alleles over time in an isolated population) and the founder effect decreases genetic variation in this population.
            This condition, however, is not unique to humans. It can be seen in small mammals, such as dogs, mice, cats and moles. Talpid moles were thought to have six digits on each foot, but in actuality, it is an outgrowth of the radial sesamoid bone, or a wrist bone. Like the human flaps, this “digit” although it has bone, does not have a joint, so it can’t be a real finger. The development of the extra digit occurs during development after the five true fingers develop, giving another possibility to the development of the anomaly in humans. Natural selection has put positive pressure on this type of mole because the sixth digit assists in digging, so the talpid mole is quite common. In humans, however, we don’t need to dig and there is no inherent advantage to an extra digit, so it appears natural selection is working in reverse compared to its operations with moles.
            Our fascination with extra digits lead to the exhumation of Anne Boleyn in the reign of Queen Victoria (late 1800s). Anne, in fact, did not have polydactyly. It was just another weak accusation to depose her so Henry VIII could move onto another woman. Although the origins of polydactyly are not specifically known, it’s very much around, since it’s hard to get rid of traits/alleles (if, indeed it is hereditary) with low frequencies.

Sources:
·         http://rsbl.royalsocietypublishing.org/content/early/2011/06/29/rsbl.2011.0494.abstract, Mitgutsch, Christian et al., “Circumventing the polydactyly ‘constraint’: the mole’s ‘thumb’”
·         Heus HC, Hing A, van Baren MJ, Joosse M, Breedveld GJ, Wang JC, Burgess A, Donnis-Keller H, Berglund C, Zguricas J, Scherer SW, Rommens JM, Oostra BA, Heutink P (Aug 1999). "A physical and transcriptional map of the preaxial polydactyly locus on chromosome 7q36". Genomics 57 (3): 342–51. http://www.sciencedirect.com/science/article/pii/S0888754399957968
·         Ianakiev P, van Baren MJ, Daly MJ, Toledo SP, Cavalcanti MG, Neto JC, Silveira EL, Freire-Maia A, Heutink P, Kilpatrick MW, Tsipouras P (Jan 2001). "Acheiropodia is caused by a genomic deletion in C7orf2, the human orthologue of the Lmbr1 gene". Am J Hum Genet 68 (1): 38–45. www.sciencedirect.com/science/article/pii/S0002929707624707
·         http://www.dailymail.co.uk/news/article-2018470/Akshat-Saxena-Indian-boy-born-34-fingers-toes-breaks-world-record.html “On-year-old Indian boy breaks world record after being born with thirty-four fingers and toes.”


5 comments:

  1. I really like the direction that out posts are going! Great Job Everyone! Amazing.

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  2. Interesting post, I had no idea that extra fingers lacked joints. So they do have soft bones and the nerve connections, correct? I wonder when this development occurs- during gastrulation. blastula formation, or even later.

    Also, connecting back to my previous post "Evolution of Fingers," I wonder why some species developed polydactyly, like pandas.

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  3. This post reminded me of six-toed cats which are kind of a coveted trait in the pet market---Ernest Hemingway was famous for "collecting" cats with 6 toes.
    I thin the cost vs. benefit argument of extra digits would be more interesting if the digits had joints.

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  4. In response to Nate's post, oftentimes, the extra finger on humans is just flesh and nerves, no bones. That's why it is described as a "stump" on the hand or foot. As for development, I'm not sure when it develops, or if this development is uniform throughout all mammals/vertebrates. In the mole, the extra digit develops after the formation of the extreme digits (phalanges) from the "wrist bone." I don't think the extra finger develops from the carpals in humans, put rather from the splitting of phalanges in development. The gene however, for the distal digits, oftentimes doesn't stay "turned" on long enough for a complete divergence from a single digit into two. I hope that made sense. It seems, from observational (not experimental..haha for those that went to class on Friday) studies, that polydactyly is most prevalent in mammals. As to why, I've no idea.

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  5. We've been talking so much about hitchhiking and gene linkage in class that it makes me wonder which genes neighbored the gene for polydactyly. I wonder if it was in gene equilibrium with any other particular phenotypic traits during the "witch hunt" era, and if that might have affected the way people with polydactyly were viewed back then.

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