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Lecture 11

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University of Toronto St. George
Martin Ralph

Lecture 11: Biological Clocks and their Adaptive Significance During the term, we have discussed numerous ways in which the clock is used by different organisms. It should be apparent that these organisms benefit from the ability to use the clock. Organisms may be able to perform these activities without a clock, but the results would not be as reliable for them. One of the major assertions in this field is that organisms benefit because the clock optimizes organization and the ability to anticipate regular changes in the environment. However the direct evidence for this is sparse. Circadian systems and clocks in general have existed for a long time. They have retained their original uses as well as developed new ways of using these clocks. The original purpose of the clock is likely organization and anticipation. Currently adaptations of clocks functions for orientation, migration, coordination of reproduction and seasonal changes. Organization: o rhythm mutations and competition (Ouyang et al., 1998)  Ouyangs work with Cyanobacteria that take organisms and placing them in an environment to assess their competitive ability to be able to pass on their genes onto the next generation. o learning impairments and reduced longevity o SCN transplants that improve rhythms result in an increased longevity in hamsters (Hurd et al., 1998) o internal desynchrony and disease None of these are actual proves that rhythmiticity is adaptive. Because we see biological rhythms in all living things, circadian rhythms in particular, we assume that circadian rhythms confer some sort of advantages on these organisms. And in many cases the clock can be removed and nothing seem to happen, but this may be attributed to the fact that the definition of adaptations is not well defined. Experiment 1: Cyanobacteria expressions of bioluminescence show that Wild type has circadian clock of 24h. What is it about this organism that requires a biological clock? What we know that these organism are sensitive to a variety of toxic situations such as UV, salinity. Why is there a need for gene regulator activity when they cannot escape the light? Looked at how they behaved in nature where competition existed, hence different mutants of Cyanobacteria were placed in completion with the others. example: 28 hour vs. 24 hour in constant light After 70 generations, the mutations do not affect the ability of the organism to survive or reproduce. However, when there is a light cycle is 24, the percentage of the 28hour Cyanobacteria quickly drops to zero. Meanwhile they will out-compete the wild type when the light cycle is 30hours. The winner in each case is the mutation that matches the zeitgeber of the light cycle. This suggests that clocks produce an adaptive advantage. NOT SOLID PROOF: The problem is that it is an artificial situation as there is not competition. It is likely that individuals are not sync with the external cycle which affects their internal regulatory cycles. This causes DNA damage that cannot be repaired. It also does not say why they do not have a clock in the first place. Experiment 2: o The tau mutation causes rhythm disruptions and decrease longevity in hamsters. The temporal pattern of behavior is essentially the same in constant dark regardless of mutations or wild type. However, differences are seen when animals are placed in light cycles. As usual wild type syncs nicely, while mutant hamsters require constant resetting of the clock. There is also a fragmentation of patterns in the tau hamsters this means that the light cycle is doing something to the animals even though the amount of activity is the same. The amount of hours spent active is 4 hours longer than those of wild type. The animals with fragmented patterns did not live as long. The 20 hours mutants are not significantly affected by the 24 hour life cycle. o The fragmentation and disorganization of the clock is likely what creates the risk for earlier death. This disorganization is expressed as behavior. Is there a way to reverse this? o SCN transplants improve rhythms and produce an increase in longevity in hamsters. All hamsters regardless of genotype their activity becomes erratic when they get old. The transplantation of a new SNC (clock) from a fetal hamster produced improved rhythms and produced increase longevity. There is a restoration of the fragmented pattern to a certain extent. Total activity goes up while the total fragmented activity goes down even without the removal of the old SCN. In other words, one organism that has fragmented behavior due to the light cycle have shorter lives. Two animals that have been predicted to not live for much longer can be rejuvenated to a certain extend and will live much longer. However, this only tells us that the clock is important but it does not show why it is adaptive. This transplant links the host SCN with a certain factor. Anticipation: o optimal use of biochemical processes o increased predation with SCN lesions (Decoursey et al., 2000) or decreased life span o preparation for nutrient availability o avoidance of danger Clocks tell us the time of day and how long until we need to get to something - as timing mechanisms. All organisms seem to have them. However, it is possible to get around without the presence of clocks. There are other cues that help us tell time but it does become a problem when the clock is wrong. When they go wrong we still assume they are correct and ignore other cues which cause disorganization. This stops us from anticipating correctly and disrupts organizational levels. This is why clocks tend to be linked with disease. Or
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