Tuesday, April 7, 2009
Nocturnal Species
On page 124, Carroll discusses the absense of the opsin gene in higher primates. Many times, these species evolve to become colorblind or nocturnal. Discuss Carroll's findings and do some research of your own to find examples and reasons for this. Also, research the SWS opsin gene and the mutations in the genes that may cause nocturnality. There has been a clear link between nocturnality and SWS opsin. Research this and discuss the examples that Carroll uses while bringing in your own.
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ReplyDeleteOn page 123, Carroll states that if an opsin is no longer needed in an organism, natural selection will become relaxed. When natural selection is relaxed, there is no mechanism that prevents the opsin genes from changing due to mutations. Because of these organisms’ nocturnal lifestyles, their opsin genes are vulnerable to mutation and became a dispensable part of their genome. Owl monkeys are a nocturnal primate species and upon examination of it’s SWS opsin gene, there is a mutation that changes a “TCG” into a “TGA”, a stop triplet that signals for the end of translation, resulting in a dysfunctional gene. Carroll also demonstrates the link between nocturnality and the SWS opsin in nocturnal prosimians; the slow loris and bush babies both have a deletion in the beginning of their SWS opsin gene that was inherited from a common ancestor.
ReplyDeleteThe SWS opsin gene is a short wavelength opsin that is used to detect the color violet and ultraviolet in many mammals. Jeremy E. Nevin of the Smithsonian Tropical Research Institute found that grey squirrels’ (Sciurus carolinensis) SWS opsin genes have a peak absorption spectra in the violet range of the light spectrum. He compared the grey squirrel’s SWS opsin amino acid sequence to that of a UV sensitive SWS opsin of a mouse and found that, at one position, phenylalanine(Phe) had been substituted with (Tyr). To test whether this substitution led to the lack of functionality of the SWS opsin in the grey squirrels, Nevin constructed a grey squirrel opsin sequence in which the Tyr was replaced with Phe. The peak absorption of the grey squirrel shifted into the ultraviolet range of light and this demonstrates that the substitution is indeed responsible for the lack of functioning SWS opsins in grey squirrels. The reason that nocturnal animals lack a functioning SWS opsin is because they must use all available photons of light to get sufficient “spatial and temporal resolution”; that is, because they are most active in darkness, these organisms must rely more on position and time to identify objects rather than color. The lack of a functioning SWS opsin gene allows these organisms to focus on the aspects that their visual systems rely most heavily upon.
http://jeb.biologists.org/cgi/reprint/209/11/v.pdf
Many primates became nocturnal when the dinosaurs ruled the earth. They were hiding in the caves because they would get hunted by dinosaurs when they walk around in the daylight. Ancient primates will come out at dark to find food. This is why there is a lack of development of opsin gene in higher primates. The traditional view of primate evolution holds that the last common ancestor of living primates was nocturnal and that nocturnality has continued uninterrupted. However, According to Ying Tan and Anne D, there are three major arguments to this view. First, the vast majority of extant prosimians are nocturnal, second, the tapetum Lucidum, which is a reflecting layer that enhances the ability of the eye to collect light, is found in many strepsirhines, and third, the orbital convergence that is diagnostic of the primate clad is believed to be the result of nocturnal visual predator lifestyle.
ReplyDeleteTo provide clear evidence, they studied the opsin genes in prosimians. They know that under nocturnality either the short wavelength or long wavelength opsin will experience relaxed selective constraints and may become nonfunctional. They give examples of loss of the Short wavelength opsin in the raccoon and the Kinkajou. They tested 4 different nocturnal species. According to traditional view they should expect similar levels of functional relaxation in the short wavelength opsin gene in all nocturnal prolimian lineages because they evolve from common ancestor. However, their data show drastically different levels of functional relaxation in the short wavelength gene which shows that they are not from common ancestor. They conclude that the loss of Short wavelength opsin gene can easily occur through random mutation.
As we can see from Ying Tan’s research, mutation plays a big part in evolution. As species evolve, they are just fine tuning to the surrounding nature and some random mutations gets favored by natural selection and that gene will get passed on to the next generation to survive and thrive.
http://www.pnas.org/content/102/41/14712.full.pdf