Shubin asks the question, "how can eyes that look so different-such as those of worms, flies, and mice-be closely related?" When Mildred Hoge was recording mutations in fruit flies, she found a fly that had no eyes whatsoever. She also found that she could breed a whole line of eyeless flies, and a similar mutation was discovered in mice soon after. In humans, individuals that are missing large pieces of their eyes are said to have aniridia. Between these very different creatures, geneticists were finding similar kinds of mutants. They found that the genes responsible to eyelessness had very similar DNA structures and sequences in all three creatures, and after manipulating the normal version of the gene, Walter Gehring and his team were able to turn on the eyeless gene anywhere on the body, or even on the body of a different species, and have an extra fly eye form.
What is this "eyeless" gene called? What were some of the technologies that geneticists probably used to isolate the DNA sequence and turn the gene on anywhere they wanted on the body? How was the discovery of this particular gene important to evolution and relating the creatures that were tested? Also, explain the importance of the eye and its evolutionary history.
Hannah Kay (hgkay@aol.com)
In each of the three animals tested, each were given a different term based on the process behind each of the "eyeless" conditions. For fruit flies, which was contributed to one of the first discovery of this gene was named simply eyeless gene. However, in a later study, with a mouse, this gene was named "small eye" gene caused by mutation in one of the two genes leading to underdeveloped eyes while mutation in both results in the dissappearance of the eyes completely. The human version of the gene is named aniridia and mutation in one of these genes causes malfunctioning or defect of iris, lens, cornea, and retina. (http://www.sciencemag.org/content/267/5205/1766.extract)
ReplyDeleteDifferent technologies that geneticists used may include the PCR used to replicate isolated DNA to testifiable amounts. Another technology includes electrophoresis used to separate different DNA fragments based on DNA endonuclease that is used to cut the DNA strand and distinguishing them. Furthermore, geneticists can regulate the activation and deactivation of genes testing various known transcription factors on the DNA which works to convert genes into RNA which enables the activation of the gene or prohibit it. (http://www.thetech.org/genetics/ask.php?id=63)
Finally, discovery of this gene in all three different types of animal lead scientists to question or reassess their prior hypothesis on their theory that the eyes of each organism underwent 40 or more independent evolutions. This theory was based on the idea of optics and the abundance of ways that animals used the properties of optics in order to formulate image. However, when analyzed from the gene aspect and not physical aspect, Shubin writes that the DNA responsible for eyelessness are actually "similar in structure and sequence" for all three animals. This allowed the scientists to hypothesize that rather than stemming from each individual ancestors, the eyes of different animals came from a single, distant ancestor.
Through this experiment, Walter Gehring also discovered the appearance of transdetermination where insect's imaginal disks or embryonic tissues form a different structure than what they were orginially instructed to become. Gehring found that although certain disks were supposed to have developed into a wing developed into eyes or other body parts. Gehring hypothesized that this maybe due to activation of wrong genes and certain stages of developement (http://www.sciencemag.org/content/267/5205/1766.extract) This may account for the appearance of eyes at different places of the flies body based on the positioning of the gene on different parts of the DNA that Shubin discusses on pg 156.
Kevin Jeon:bboybyung@gmail.com