Friday, April 1, 2011
my genes, your genes, their genes, more genes!
In Chapter 6, Neil Shubin discusses Hox genes and their role in determining anterior/posterior/ventral/dorsal sections of organisms. Humans have Hoxa, Hoxb, Hoxc, and Hoxd when organisms such as jellyfish and fruit flies have less Hox genes. What determines the number of Hox genes in an organism in terms of complexity and symmetry? Would a bilateral or a radial symmetry organism have more complex Hox genes? Provide and example of both.
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The various forms of the Hox gene have arisen from the copying of an original Hox gene; the original gene was copied, and those were again copied to form clusters of the Hox gene that we see in humans and other organisms. The copying of the gene is found to occur when species evolve dramatically, giving rise to a new cluster of the gene.
ReplyDeleteThere is currently no link between the complexity of an organism and the number of hox genes the organism possesses. Possessing multiple Hox genes could possibly signify that an organism has evolved many times, thus resulting in multiple clusters Hox genes. The evolution of more complex traits, such as heads and spinal cords, arose from the duplication of a Hox cluster. So yes, some more complex organisms have more Hox genes, but this is not always the case. Hox genes do control traits such as symmetry, but this is a result of how the gene is expressed in the chromosomes, not the complexity of the organism.
Information found in a review of Christiane Nusslein-Volhard's book Coming to Life
http://neurophilosophy.wordpress.com/2006/08/09/the-role-of-hox-genes-in-development/
Most organisms, in fact, have very similar Hox genes. The differences that cause the devlopmental differences are caused by the Hox gene targeting differeng Hox-encoded proteins. These proteins are what regulate and turn genes on or off, thus creating diferent traits in organisms possessing very similar Hox genes.
Information from the National Center for Biotechnology Information
http://www.ncbi.nlm.nih.gov/books/NBK9978/
Anna Leng -(annaissbananas@gmail.com)
Early in embryonic development, organisms of diverse species look indistinguishable. Even after they have completed existence as a simple ball of cells (morula and blastula), limbs and heads can hardly distinguish one species from another until a little later in development. It is because of homeoboxes, or hox genes, that organisms develop their unique features. The control these gene have on development is remarkable. Examining these genes uncovered that "single mutations creating new body parts and eliminating others, these genes were acting as master switches, turning on and off arrays of other genes involved in body shape, and controlling the number, pattern, position, and fusion of segments and appendages." (Zihlman, 2001). Hox genes even seem to draw evolutionary connections between vertebrates and invertebrates because it has been declared that "all animals have the same body plan.. Because the main nerve cord is in the front part of insects and in the back part of vertebrates, vertebrates are essentially upside-down invertebrates." (Zihlman, 2001).
ReplyDeleteThe idea that vertebrates are just upside-down invertebrates seems to ascertain an example of a possible mutation that resulted in a new species, and eventually a new phylum, of animals. This may have come into being when "Hox genes evidently duplicated twice during the evolution of invertebrates into vertebrates, just as there were multiple duplications of the globin gene."(Zihlman, 2001). Even when Hox genes do not function correctly, gerastic changes can occur and have enormous amounts of control over the fate of organisms.
The scope to which Hox genes may be difficult to understand, so lets think of an example. The genes in mice and humans "are very similar in number and chromosomal arrangement...only about 40 genes out of a total of about 100,000 control most of the development,architecture, and appearance of the body plan of complex mammalian species."(Zihlman, 2001).
Differences in the number of Hox genes does not necessarily mean differences in complexity. Hox genes control much of the development of species, but the differences in individual genes between seemingly disparate organisms plays a much greater role in differences in complexity. Organisms as diverse as flies and humans have similar numbers of Hox genes, but it is obvious that humans are much more complex. It has been known that "the conservation or diversity of homeobox genes is responsible for the similarity and variability
of some of the morphological or physiological characters among different organisms" (Nam, 2005). Conversely, it was concluded that "both gains and losses of homeobox genes were important for the evolutionary change of phenotypic characters in bilateral animals." (Nam, 2005). With such an astounding impact on the similarities and differences between organisms, Hox genes may be one of the greatest influences on the divergence of organisms into species and a key factor in evolution.
http://biology.ucsd.edu/classes/bild10.WI08/documents/AnimalBodyPlanHoxcoloring.pdf
ReplyDeletehttps://homes.bio.psu.edu/people/faculty/nei/lab/2005-nam-nei.pdf
Troy Glickstern
cleverstar8@comcast.net