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.
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
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.
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
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.
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.
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