Biology Reference
In-Depth Information
measure time (
Kyriacou 1990, Takahashi 1992
). Such circadian rhythms have
several characteristics:
1. The clocks that regulate such behavior usually are “free running” in
constant environments and are not simple responses to changes in light or
temperature.
2. Although the rhythms are free running, an initial environmental signal
is required to start the clock. Among the cues that “set” the clock are
alternating light and dark cycles, high and low temperature cycles, or short
pulses of light.
3. The circadian rhythm is relatively insensitive to changes in temperature
(temperature compensated).
4. Altering the cues that entrain the clock can reset the clock.
Drosophila melanogaster
reared in constant darkness exhibit circadian loco-
motor activity rhythms as adults. However, the rhythms of the individual flies in
these populations are not synchronized with one another (
Sehgal et al. 1992
).
Rhythms can be synchronized if dark-reared flies are exposed to light treatments
as first-instar larvae. Light treatments occurring before hatching of the first-
instar larvae fail to synchronize adult locomotor activity rhythms, indicating the
clock functions continuously from the time larvae hatch until they reach adult-
hood. The rhythm can be advanced, delayed, or unchanged, depending on the
phase of the cycle at which the cue is given.
The circadian rhythm has an approximate periodicity of 24 hours. Molecular-
genetic analyses of
Drosophila
clock mutants provided fundamental understand-
ing of the mechanisms of the circadian clock. Rapid advances have been made in
understanding the molecular aspects of circadian clocks in a variety of organisms
(
Hogenesch and Ueda 2011
). Circadian rhythms are found in all organisms and
probably evolved early. Common genetic elements are present in
Drosophila
,
Neurospora
, mammals, and cyanobacteria (
Lakin-Thomas 2000, Loudon et al. 2000,
Allada et al. 2001, Harmer et al. 2001, Merrow and Roenneberg 2001, Williams
and Sehgal 2001
). In
Drosophila
, the genes
period
+
,
timeless
+
,
Clock
+
,
cycle
+
,
doubletime
+
, and
cryptochrome
+
are involved in the circadian clock (
Lakin-Thomas
2000, Emery et al. 2000
,
Table 11.2
).
Numerous reviews have compared the molecular, genetic, and neurological
components of biological rhythms, reflecting the excitement of the scientific com-
munity in understanding the molecular basis of this complex behavior (
Hall 1995,
1998a; Iwasaki and Thomas 1997; Dunlap 1998, 1999; Young 1998; Ishida et al.
1999; Giebultowicz 2000; Lakin-Thomas 2000; Wager-Smith and Kay 2000; Allada
et al. 2001; Harmer et al. 2001; Williams and Sehgal 2001
). The numerous reviews
