Living and Adapting to Its Own Habitat - Building the Most Complex Structure on Earth - page 210

Clocks and Calendars for Timing Phenotypic Changes
Not all changes in the environment are unpredictable. There are two major periodical
changes in the environment that result from geophysical factors, the rotation of the
Earth around its own axis and around the Sun and, to a lesser extent, the rotation of
the moon around the Earth. The first two determine diurnal (from Latin dies denot-
ing day and urnus denoting time) and annual seasonal cycles, respectively.
Diurnal and annual cycles involve climatic changes that in moderate climates are
drastic enough to pose serious physiological problems. Living organisms have had
to cope with these problems from the dawn of life on Earth. In order to adapt their
physiology to these drastic changes in the temperature, irradiation, humidity, and
predation related to the day-night and annual cycles, living organisms evolved circa-
dian (from Latin circa denoting around, roughly) clocks and circannual “calendars,”
which synchronize their physiology and behavior to these cycles.
The evolution of these timing devices allows living organisms to avoid trouble-
some surprises, predict the approach of unfavorable conditions in the environment
and, in anticipation, develop morphological, physiological, or life history adaptations.
At the most basic level, adaptation of the phenotype to daily and seasonally
changing conditions requires the daily and seasonal adjustment of temporal patterns
of gene expression. Biological systems needed to perceive time, so they evolved a
“sense” of time by “inventing” a biological clock, a timing device that would allow
them to forecast and prepare for approaching changes in the environment. This ena-
bled them to program and harmonize their physiology and behavior with cyclically
changing conditions in the environment. The pressure for biological clocks has been
so strong that they evolved with the photosynthetic prokaryotes, cyanobacteria, one
of the most ancient forms of which evolved probably 3.5 billion years ago ( Ditty
et al., 2003 ).
How important is the advent of the biological clock to the evolution of living
systems is shown by its presence in cyanobacteria, the oldest of extant unicellulars.
These prokaryote cells perform two basic, but incompatible processes: photosynthe-
sis and nitrogen fixation. The enzyme nitrogenase, which fixes atmospheric nitrogen,
is sensitive to the oxygen generated during photosynthesis. Given that in unicellu-
lar organisms, these processes cannot be spatially separated, the bacteria temporally
separate photosynthesis and nitrogen fixation, by undertaking the first in the day
and the second at night ( Kondo and Ishiura, 2000 ). These simple prokaryotes also
use the circadian rhythms for cell division ( Sweeney and Borgese, 1989 ). Molecular
mechanisms of circadian clocks in the living world are, to a great extent, conserved
( Paranjpe and Sharma, 2005 ).
Where is the timing device in unicellulars located? Unlike metazoans, where
the central clock is located in hypothalamic neurons, it is impossible to answer
this question for unicellulars. There is abundant information on changes in gene
expression related to the function of the biological clock in unicellulars. It is sug-
gested that the cytoskeleton may play a role in the circadian rhythm since both are
closely related with the cell cycle ( Shweiki, 1999 ). Indeed, migration of organelles