In identifying the behaviors that are regulated by the circadian clock, researchers have
set four main criteria for an oscillation to be
defined as truly circadian (Johnson et al. 2004).
First, the period of the rhythm must be at or
near 24 h, thereby coinciding with the external
24 h day/night cycle on Earth. Second, the
rhythm must be entrainable, meaning that it is
responsive to external cues, such as light, which
can reset its phase relative to the current environment. For example, when traveling to a different time zone, a person will gradually begin
to wake up at the local dawn time as a result of
their internal clock adjusting to local lighting
cues, rather than continue to wake up during
the dawn phase of their previous time zone.
Third, the rhythm must be able to persist without external cues. This characteristic demonstrates that the rhythm is an endogenous,
anticipatory process and not simply a response
to changes in the environment. Finally, the
rhythm must be able to maintain a consistent
period under different environmental conditions, a property called compensation. Since
biochemical reactions speed up at higher temperatures, if the circadian clock were not buffered, then it would yield a shorter period and
would not be a good pacemaker. While there
are numerous biological oscillations in organisms, only those rhythms that meet these four
criteria are considered circadian (Johnson et al.
2004).
As circadian clocks appear to have evolved
independently in only a few cases, there is a
high degree of similarity between the mechanisms that time the clocks of fungi and other
higher eukaryotes, e.g., mammals and the fruit
fly Drosophila, making N. crassa an excellent
model system in which to study these clock
mechanisms (Crosthwaite et al. 1997; Dunlap
1999; Bell-Pedersen et al. 2005; Dunlap and
Loros 2017). Much of the foundational research
into the molecular mechanism underlying circadian clocks in higher eukaryotes occurred in
the filamentous fungus N. crassa (Dunlap 2008;
Dunlap and Loros 2017; Loros 2019). A mutant
in N. crassa, which displayed a “banding” developmental growth pattern on race tubes was
Fig. 4.1 Race tubes and the bd mutation enable period
and phase analysis in Neurospora crassa. A race tube is
a tool used to measure the overt phenotypic rhythm of
banding strains of N. crassa. A hollow glass tube is bent
upward at both ends and filled with molten agar growth
media, as shown at top. Once the media has cooled and
solidified, an N. crassa strain with a band (bd) background is inoculated at one end and then grows laterally down the tube. As the bd strain grows, it will
periodically (in time with the circadian cycle) form
aerial hyphae and conidia, shown here as the orange
clumps. The race tube is marked once every 24 h at the
growth front. The periodicity of the aerial hyphal formations is directly tied to the period of the clock. Race
tubes can be scanned and densitometric analysis of
these race tube images can be used to measure the
time between successive bands to predict the period
of the strain, as shown at bottom. Image courtesy of
Joshua Thomas
78
M. S. Jankowski et al.
set four main criteria for an oscillation to be
defined as truly circadian (Johnson et al. 2004).
First, the period of the rhythm must be at or
near 24 h, thereby coinciding with the external
24 h day/night cycle on Earth. Second, the
rhythm must be entrainable, meaning that it is
responsive to external cues, such as light, which
can reset its phase relative to the current environment. For example, when traveling to a different time zone, a person will gradually begin
to wake up at the local dawn time as a result of
their internal clock adjusting to local lighting
cues, rather than continue to wake up during
the dawn phase of their previous time zone.
Third, the rhythm must be able to persist without external cues. This characteristic demonstrates that the rhythm is an endogenous,
anticipatory process and not simply a response
to changes in the environment. Finally, the
rhythm must be able to maintain a consistent
period under different environmental conditions, a property called compensation. Since
biochemical reactions speed up at higher temperatures, if the circadian clock were not buffered, then it would yield a shorter period and
would not be a good pacemaker. While there
are numerous biological oscillations in organisms, only those rhythms that meet these four
criteria are considered circadian (Johnson et al.
2004).
As circadian clocks appear to have evolved
independently in only a few cases, there is a
high degree of similarity between the mechanisms that time the clocks of fungi and other
higher eukaryotes, e.g., mammals and the fruit
fly Drosophila, making N. crassa an excellent
model system in which to study these clock
mechanisms (Crosthwaite et al. 1997; Dunlap
1999; Bell-Pedersen et al. 2005; Dunlap and
Loros 2017). Much of the foundational research
into the molecular mechanism underlying circadian clocks in higher eukaryotes occurred in
the filamentous fungus N. crassa (Dunlap 2008;
Dunlap and Loros 2017; Loros 2019). A mutant
in N. crassa, which displayed a “banding” developmental growth pattern on race tubes was
Fig. 4.1 Race tubes and the bd mutation enable period
and phase analysis in Neurospora crassa. A race tube is
a tool used to measure the overt phenotypic rhythm of
banding strains of N. crassa. A hollow glass tube is bent
upward at both ends and filled with molten agar growth
media, as shown at top. Once the media has cooled and
solidified, an N. crassa strain with a band (bd) background is inoculated at one end and then grows laterally down the tube. As the bd strain grows, it will
periodically (in time with the circadian cycle) form
aerial hyphae and conidia, shown here as the orange
clumps. The race tube is marked once every 24 h at the
growth front. The periodicity of the aerial hyphal formations is directly tied to the period of the clock. Race
tubes can be scanned and densitometric analysis of
these race tube images can be used to measure the
time between successive bands to predict the period
of the strain, as shown at bottom. Image courtesy of
Joshua Thomas
78
M. S. Jankowski et al.