Living and Adapting to Its Own Habitat - Building the Most Complex Structure on Earth - page 212

Circadian rhythms are crucial in plant life. Matching biological rhythms to
environmental periods is positively related to photosynthetic output ( McClung,
2006 ). In the small flowering plant, Arabidopsis thaliana , for example, plants with
circadian clocks better matching the environment contain more chlorophyll, fix more
carbon, grow faster, and survive more often than other plants ( Dodd et al., 2005 ).
Expression of about 10-15% of genes in this plant is regulated by the plant's central
circadian mechanism ( McClung, 2008 ).
Differential exposure to light (many cells are not exposed at all) and temperature
of different cells in multicellulars makes the diurnal harmonization of their function
impossible. An evolutionary pressure for harmonizing functions of all cells led to
the evolution of the central neural clock, adjusting and synchronizing the clocks of
all cells, throughout the animal body, from insects to birds and mammals and verte-
brates in general.
The Drosophila brain contains ~100,000 neurons. About 150 express the canoni-
cal clock machinery. These neurons control and regulate the circadian rhythms of
the fly's physiology and behavior. Based on their anatomical location, these clock
neurons are divided into seven groups ( Nitabach and Taghert, 2008 ). Four are in
the lateral brain and include large ventrolateral neurons (l-LNvs), small ventrolat-
eral neurons (s-LNvs), dorsolateral neurons (LNds), and lateral posterior neurons
(LPNs). Almost all of the l-LNvs and s-LNvs express the neuropeptide pigment-
dispersing factor (PDF), and they are the only PDF-expressing neurons in the fly brain.
Light information is conveyed to Drosophila clock neurons via three pathways:
retinal photoreceptors of compound eyes, brain photoreceptive neurons Hofbauer-
Buchner, and the photopigment cryptochrome (CRY) protein. The first two path-
ways transmit light information to the clock neurons through synaptic connections
( Nitabach and Taghert, 2008 ).
Circadian rhythm is driven by transcription factors clock (CLK) and cycle (CYC)
that induce expression of period (PER) and timeless (TIM) and other genes. The
products of these genes, mRNAs and proteins, fluctuate in functional levels, owing
to post-translational modifications ( Figure 4.8 ). The proteins PER and TIM suppress
the expression of genes clk and cyc , leading to the depletion of PER and TIM and,
consequently, to the resumption of clk and cyc expression in 24 h cycles ( Nitabach
and Taghert, 2008 ).
Although the Drosophila has circadian clocks in many tissues, the central neu-
ronal clock is required for some peripheral clocks. The central and peripheral clocks
communicate with each other at least at the physiological and behavioral levels. The
circadian mechanism in Drosophila is involved in “hundreds of behavioral and met-
abolic events that are adjusted on a daily basis” ( Bradshaw and Holzapfel, 2010 ).
Flies with normal circadian rhythms live longer than flies with disturbed rhythms
( Kumar et al., 2005 ). Mating clock-deficient flies result in 40% less progeny com-
pared to normal flies ( Paranjpe and Sharma, 2005 ).
Looking at the figure, there is no explanation for the initial (and for the latter,
for that matter) expression of the clk and cyc genes. These genes are present in all
Drosophila cells but expressed only by the clock neurons in response to electrical
signals coming to the clock neurons from photoreceptors. Why do not other cells