The structure of the eye differs in complexity even if we just examine the phylum of mollusks. What factors would induce such a difference? One could address the information we learned about mollusks in the invertebrate unit.
Saturday, April 4, 2009
Pax-6 and Eye Structure
Different eyes have developed in different species. The Pax-6 gene plays a role in the eye development of many organisms such as the mouse, human, squid, and ribbon worm. What is Pax-6 and what is its function? What would occur if there was a disruption in the Pax-6 gene of a human? Why would a mouse Pax-6 gene be able to also "induce fly eye tissue" (196)?
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Some pre-info:
ReplyDeleteOfficial Symbol: PAX-6
Official Full Name: Paired Box 6
Gene Type: Protein Coding
Organism: Homo Sapiens
Summary:
"This gene encodes paired box gene 6, one of many human homologs of the Drosophila melanogaster gene prd. In addition to the hallmark feature of this gene family, a conserved paired box domain, the encoded protein also contains a homeo box domain. Both domains are known to bind DNA, and function as regulators of gene transcription. This gene is expressed in the developing nervous system, and in developing eyes."
Source: http://www.ncbi.nlm.nih.gov/sites/entrez?db=gene&cmd=retrieve&list_uids=5080
The Pax-6 gene is a homeobox gene; it is a "~180-nucleotide sequence within a homeotic gene encoding the part of the protein that binds to the DNA of the genes regulated by the protein," (Campbell Glossary). A homeotic gene is a gene "that control[s] the overall body plan of animals by controlling the developmental fate of groups of cells," (Campbell Glossary). The Pax-6 gene controls the development of the eye and parts of the nervous system. If there was a disruption in the Pax-6 gene of a human, the human would not be able to see because his/her eyes would not have been properly formed (and he/she would suffer from neurological disorders because the Pax-6 gene also controls the developmental fate of the nervous systems in some respects). A mouse Pax-6 gene would also be able to "induce fly eye tissue," (Carroll 196) because the function and sequence of the gene has not changed over 500 million years. Each organism uses the same genetic code, the same building blocks if you will, as we have learned (heritable information). The Pax-6 gene is no different - it is made up of genetic code, nucleotides, and sugars, that make up organisms. "The widespread role of Pax-6 in eye development must be due to historical reasons. That is, a common ancestor of these animals used Pax-6 in the development of some, perhaps very primitive eye. All the marvelous and more complex eyes that evolved in this ancestor's descendants would then be built on this foundation," (Carroll 196). Since the gene is in essence the same throughout many organisms, it is possible to replace a Pax-6 gene from one organism with that of another organism's because they will have the same function and relatively the same sequence.
Factors that would induce differences in eye complexity include a driving force: natural selection. When an organism needs to scrounge for food, it develops a sense in order to find it. Natural selection eliminates those organisms which cannot find their food because they will die off and not survive/reproduce (sexual selection may also take a place in choosing the healthiest/fittest mates). Eventually, organisms that need to search for greater quantities of food may have mutations in their genes causing the production of opsins forming an eye. The development of the eye is promoted by natural selection because it enables organisms who possess it to have greater success in capturing prey - thus surviving and reproducing. "From simple beginnings, incremental changes that improve eyes' functional properties would be incorporated. Indeed, if we look at just one phylum of animals, the mollusks, we find a wide variety of eyes that represent different grades of complexity (figure 8.5)," (Carroll 199). The evolution of the eye is described by figure 8.5 on page 201 in "Making of the Fittest" with a cup-shaped eye filling with fluid and bending inward finally becoming covered with a lens - a complex eye. Many cephalopods have well-trained eyes, "squids and octopuses also have well-developed sense organs," (Campbell 659).
And thus, the Pax-6 gene relates to the biological theme of heritable information because the surviving gene has been passe down for 500+ million years and carries the same function - to develop eye tissue. The Pax-6 gene also relates to the biological theme of evolution in that natural selection and predation are possible factors that drove the development of eye complexity.