Anna Leng (annaissbananas@gmail.com)
Thursday, March 17, 2011
The Hox Box
In chapter six, Shubin explains how bodies are built. He explains that Hox genes control mutations by "establishing proportions of our bodies...changes in them bring about changes in the ways our bodies are put together" (110). Hox genes are common among all animals, but the number of genes present differ from species to species. Why do you suppose this is? Is it of any benefit to have more or less Hox genes? Clearly, such genes play a vital role in assembling our body plan, but how much will a mutation involving the Hox gene impact an organism's ability to survive and reproduce? What would happen if the gene did not impact an organism's ability to survive?
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This question has shown up a number of times already, but I am more than happy to answer it again. The reason why Hox genes occur in varying numbers across different species is because they are vulnerable to the continuity and change of environmental pressures.
ReplyDeleteFrom a study of Hox genes, it was proposed that "there were at least about 88 homeobox genes in the common ancestor of bilateral animals." (Nam, 2005). Though, organisms with common ancestors were proposed to have eventually diverged in the number of Hox genes they had because, of the genes studies "About 50–60 genes of them have left at least one descendant gene in each of the 11 species studied, suggesting that about 30–40 genes were lost in a lineage-specific manner. Although similar numbers of ancestral genes have survived in each species, vertebrate lineages gained many more genes by duplication than invertebrate lineages, resulting in more than 200 homeobox genes in vertebrates and about 100 in invertebrates." (Nam, 2005). It is because mutations, or even the phenomenon of natural selection at work, that certain genes changed over time and their progeny appeared across future generations of species.
It may be easy to assume that an increasing number of Hox genes may contribute to increased complexity in physical structure, but this structure to function relationship does not seem very crucial. Hox genes control much of the development of species, but the differences in individual genes, rather than an overall number of 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). However, 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).
To answer Anna's next question about the sheer influence of a mutation in hox genes and its impact on the overall ability of an organism to survive, we should review a few key points about evolution. Mutations that affect an organisms ability to survive and reproduce negatively would eventually fade over time because they are not well adapted for life. If the mutations have no benefit, then they will probably survive in small numbers because competition with organisms that are better adapted would "lead to local elimination of the inferior competitor, an outcome called competitive exclusion." (Campbell, 1199). Lastly, if an advantage is provided by a mutation, the population would boom and be on the winning side of competitive exclusion. 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). I said this before, and I will say this again, I believe Hox genes are probably the most important force of continuity and change that drives the course of evolution over time.
http://biology.ucsd.edu/classes/bild10.WI08/documents/AnimalBodyPlanHoxcoloring.pdf
https://homes.bio.psu.edu/people/faculty/nei/lab/2005-nam-nei.pdf
Troy Glickstern
cleverstar8@comcast.net