Module 3. Evolution and Adaptive Significance of Circadian Rhythms and Clocks

local features

temperature, precipitation, topography

global features

enduring, stable and nontrivial

what kingdoms are considered the most ancient and primitive organisms?

cyanobacteria and archaea

multicellcular eukaryotic organisms

• exhibit true circadian rhythms

• examples include:

  • fungi (neurospora crassa)
    plants (arabidopsis thaliana)

  • range of animals (nematodes, fruit flies, zebrafish, reptiles, birds, mice, humans

unicellular eukarayotic organisms

• bona fide circadian rhythms

  • examples: green algae

prokaryotic organisms

• most ancient life forms on earth

• lack cell nuclei and membrane-bound organelles

• examples: human gut bacteria, cyanobacteria, extremophile archae

what are two of the biological challenges faced by early life that led to evolution of circadian clocks?

• need to coordinate biochemistry with daily cycles of solar radiation and temperature (external coordination)

• coordinate biochemical processes with each other (internal coordination)

escape from light hypothesis

circadian clocks shifted cellular processes to the night to shield them from UV radiation

How is sunlight a danger?

• strips electrons from atoms and molecules

• affects DNA replication, gene transcription and many other biochemical reactions

great oxidation event

• atmospheric oxygen levels rapidly increased due to the success of cynaobacteria

• evolution of aerobic organisms and extinction of anaerobic life forms

aerobic metabolism

generates molecular byproducts that have an unpaired electron in their outer shell

escape from oxygen hypothesis

• photosynthesis depends on light

• photosynthesis engages in oxygen0drive metabolic processes

• production of antioxidants in the day when oxygen molecules accumulate and then turn off production at night

nitrogen fixation

process by which cells convert environmental nitrogen to ammonia

example of internal biochemical coordination

nitrogenase enzyme is moved to production during the night because nitrogen fixation and photosynthesis cannot occur at the same time

what is one of the most important functions of clocks?

enabling organisms to anticipate the change from day to night and back

eclosion

when the adult emerges from a pupal case

food-anticipatory activity (FAA)

memory inferred from the emergence of activity prior to mealtime

• requires food to be presented at the same time of day for a several days

alarm clock mode

• bee example

• instructs the bee when to wake up to go forage but also to not be too far from the hive after sunset

wristwatch mode

example: bees can remember at least 9 daily mealtimes linked to unique places

• it is continuously consulted, sort of like a wristwatch to know what time it is and to be at the right place at the right time

time-place learning or time-place association

animals show that they can discriminate time of day by learning to go to a particular place at a particular time of day

waggle dance

a form of communication that bees use to communicate the location of nectar source based on the angle with respects to the angle of the sun

worker (naked mole rate)

• smaller in size

• specialized for digging tunnels

• occasional light exposure

disperser

• much larger size and greater body fat

• life purpose is to leave the colony, surface and join another colony

• ensure occasional outbreeding

fledge

to leave their nest

swamping

strategy of predator avoidance

• examples: fledge in large groups or in the evening

temporal niche segregation

use of circadian clock to limit activity to day or night in order to avoid competition with other species or with dominant individuals of the same species

photoperiodism

ability to measure the length of the day, as a means to keep track of the time of year and adjust behavior and physiology in advance of seasonal changes in the weather

habitat selection

depending on when they need to be safe during the night and day

• example: caves or underground burrows

extended physiological states

extending wake and sleep states since taking regular naps in the outside world can be dangerous

is the circadian rest-activity cycle necessary for life

no

what are the two observations that indicate growth and survival in plants and animals are best

• they have a functional circadian clock that matches environmental cycles

• environment is rhythmic rather than constant